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Competition and coexistence in plant communities: intraspecific competition is stronger than interspecific competition

Affiliations.

1 Department of Wildland Resources and the Ecology Center, Utah State University, Logan, UT, 84322, USA.
2 School of Natural Resources and Environment, University of Florida, Gainesville, FL, 32611, USA.
PMID: 29938882
DOI: 10.1111/ele.13098

Theory predicts that intraspecific competition should be stronger than interspecific competition for any pair of stably coexisting species, yet previous literature reviews found little support for this pattern. We screened over 5400 publications and identified 39 studies that quantified phenomenological intraspecific and interspecific interactions in terrestrial plant communities. Of the 67% of species pairs in which both intra- and interspecific effects were negative (competitive), intraspecific competition was, on average, four to five-fold stronger than interspecific competition. Of the remaining pairs, 93% featured intraspecific competition and interspecific facilitation, a situation that stabilises coexistence. The difference between intra- and interspecific effects tended to be larger in observational than experimental data sets, in field than greenhouse studies, and in studies that quantified population growth over the full life cycle rather than single fitness components. Our results imply that processes promoting stable coexistence at local scales are common and consequential across terrestrial plant communities.

Keywords: Biodiversity; Lotka-Volterra; competition coefficient; forests; grasslands; meta-analysis.

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Article Contents

Introduction, theories of competition, intraspecific competition, interspecific competition, dynamic models of plant growth and competition, factors influencing the outcome of competition, practical application of crop–weed competition models, acknowledgements, the theory and application of plant competition models: an agronomic perspective.

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SARAH E. PARK, LAURENCE R. BENJAMIN, ANDREW R. WATKINSON, The Theory and Application of Plant Competition Models: an Agronomic Perspective, Annals of Botany , Volume 92, Issue 6, December 2003, Pages 741–748, https://doi.org/10.1093/aob/mcg204

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Many studies of plant competition have been directed towards understanding how plants respond to density in monocultures and how the presence of weeds affects yield in crops. In this Botanical Briefing, the development and current understanding of plant competition is reviewed, with particular emphasis being placed on the theory of plant competition and the development and application of mathematical models to crop–weed competition and the dynamics of weeds in crops. By consolidating the results of past research in this manner, it is hoped to offer a context in which researchers can consider the potential directions for future research in competition studies and its application to integrated weed management.

Received: 6 March 2003; Returned for revision: 29 May 2003; Accepted: 2 September 2003 Published electronically: 23 October 2003

The decline in weed seed numbers in arable fields (approx. 95%) over the course of the 20th century is witness to the success of weed control measures ( Robinson and Sutherland, 2002 ). This success in weed control has resulted primarily from the extensive use of herbicides, changes in crop rotations and a range of cultivation methods. However, the sustained use of a range of agro‐chemicals, in recent years, has resulted in an increase in the number of herbicide‐resistant weed species ( Powles and Shaner, 2001 ), a shift in the weed flora of arable fields from one of mixed weeds to one dominated by grass weeds ( Barr et al ., 1993 ) and increasing environmental and public health concerns over the use of pesticides ( Conway and Pretty, 1991 ).

As a consequence, there is an increasing interest in methods of weed control that allow a reduction in the use of herbicides. This is reflected in the increased interest in non‐chemical methods of weed control ( Weiner et al ., 2001 ), organic farming ( Lampkin, 2003 ) and the use of intercropping ( Vandermeer, 1989 ). Recently, it has also been argued ( Dewar et al ., 2003 ) that the use of genetically modified herbicide‐tolerant crops with glyphosate and glufosinate herbicides may allow a more flexible, knowledge‐based management to weed control, permitting higher weed populations early in the season than is possible in conventional systems to promote biodiversity. If such systems are to be employed, however, it is essential that the impact of delayed control on the competitive balance between weeds and crop is fully understood, if yields are not to be reduced.

Clearly, the efficacy of using agronomic practices to manage weed populations will be improved by a comprehensive understanding of the mechanisms of competition. Mathematical models to describe the process of plant competition have developed concurrently with our increasing empirical understanding. The structure of models has reflected the prevailing approach to weed management. Earlier research was focused on the calculation of yield loss as a result of weed competition and an empirical modelling approach. A more recent interest in managing competition, through increased knowledge of the ecology and biology of competing species, has resulted in an increase in the use and development of more mechanistic‐based and dynamic population models for weeds. Used as either a tool for research or as a method for prediction, the mathematical model is an essential and integral part of the study of plant competition.

The aim of this Botanical Briefing is, first, to provide an overview of our current understanding of the mechanisms of competition at the individual plant level and, secondly, to review the development of mathematical models of plant competition, particularly in their application to the management of agricultural weeds. It is the aim to provide an overview of the broad spectrum of approaches that have been adopted within the study of plant competition as it relates to weed management. The focus here is on the quantification of intraspecific and interspecific competition in the crop–weed environment, and on the impact of competition on plant performance within the growing crop and weed population dynamics. By consolidating the results of past research in this manner, it is hoped to offer a context in which researchers can consider the potential directions for future research in competition studies and its application to weed management.

Whilst definitions of competition abound, they can typically be divided into two categories, those that focus on mechanisms and resource acquisition (e.g. Tilman, 1982 ; Grime, 2001 ) and those that focus on the reduction in fitness brought about by a shared requirement for a resource in limited supply ( Silvertown and Charlesworth, 2001 ). In crop–weed competition studies, it is the mechanistic modelling approach that highlights the importance of the acquisition and pre‐emption of resources in the competitive process. In contrast, it is the definitions of competition that focus on fitness that have influenced the development of phenomenological models of crop–weed competition and the quantification of yield loss and the dynamics of weeds in agro‐ecosystems.

Much of the present understanding of intraspecific competition in plant populations is credited to a series of papers written in the 1950s and 60s by a group of Japanese researchers ( Yoda et al ., 1963 ). In summary, the papers identified three principal effects resulting from intraspecific competition in monocultures: a competition–density effect (decrease in mean size of surviving plants with increasing density); alteration in the size structure of the population (size hierarchy development); and density‐dependent mortality (self‐thinning). The process of self‐thinning is not described here (see Yoda et al ., 1963 ) as plants in an agronomic environment rarely reach the combinations of weights and densities where self‐thinning would be expected to occur ( Enquist et al ., 1998 ).

Competition–density effect

The early pioneering studies on plant monocultures stimulated considerable interest in the mathematical description of the relationship between plant performance and density. The decline in plant performance with density as a result of competition was found to be best described by a reciprocal equation ( Shinozaki and Kira, 1956 ; Bleasdale and Nelder, 1960 ; Watkinson, 1980 ) of the form

w = w m (1 + aN ) – b (1)

where w is mean plant weight, N is plant density, w m is the mean dry weight of an isolated plant at a given time, and a and b are parameters ( Watkinson, 1980 ). Parameter a is related to the density at which intraspecific competition has an impact on yield and parameter b determines whether the yield‐density relationship is over‐turning ( b > 1), asymptotic ( b = 1) or monotonically increasing ( b < 1) with density. The parameters w m and a are typically positively correlated ( Watkinson, 1984 ; Li et al ., 1996 ) as parameter a can be considered as the area of resources required to produce a yield of w m in an isolated plant. This model or similar has been successfully used to describe yield–density relationships in a wide range of plant monocultures and lies at the heart of describing density‐dependent processes that result from competition in plant populations.

Alteration in the size structure of the population

Quantifying the size structure of a population is clearly an important pre‐requisite for determining the role of plant competition; the most often used measurements are the shape of histograms (skewness), the coefficient of variation and the Gini coefficient ( Weiner and Solbrig, 1984 ). Koyama and Kira (1956 ) made the first studies of changes in the frequency distribution of biomass in plant monocultures with time and density. They presented evidence that frequency distributions of herbaceous and tree species were symmetrical during the early stages of growth, with the distribution shifting progressively towards a positive skew with time. They pointed out that this could be explained as the inevitable consequence of exponential growth and a symmetrical distribution of relative growth rates. Hence, the development of log‐normal weight frequency distributions is not proof of competitive interactions between plants.

There is, however, evidence that competition plays a major role in generating the plant‐to‐plant variability in relative growth rates that affect frequency distributions of weight ( Weiner and Thomas, 1986 ) and that the symmetry of competition also affects the development of frequency distributions. Asymmetrical competition occurs when a small number of large individuals utilize a disproportionately large share of the available resources to the detriment of the growth of smaller neighbours. In symmetrical competition the growth of each plant is in proportion to its size. In general, asymmetrical competition leads to greater inequality of biomass within a population. There are, however, complex interactions between the spatial arrangement of plants, the nature of the resource, the spatial heterogeneity of the resource, the episodic availability of the resource and the plant’s physiological and morphology response to levels of resource supply ( Schwinning and Weiner, 1998 ). Because of these complex interactions, it is not possible to conclude that the development of inequality of size is proof of asymmetrical competition.

The development of a size hierarchy has been described by numerous population models (e.g. Westoby, 1982 ; Firbank and Watkinson, 1985 a ; Benjamin, 1988 ; Pacala and Weiner, 1991 ), and many factors, such as the number of neighbours and relative emergence time, have been considered as important in determining the position of an individual within a size hierarchy ( Benjamin and Hardwick, 1986 ; Wyszomirski et al ., 1999 ). However, despite the obvious commercial importance of variability in plant size, especially in vegetable crops that need to meet strict size limits to be marketable, models of the development of size hierarchies have not been exploited to this end. Models to predict the effect of agronomic practices on changes in size structure of populations have instead relied upon estimating changes in the parameter values of frequency distribution curves from empirically derived relationships ( Benjamin et al ., 1999 ).

Agronomic studies aimed at quantifying competition between two species most commonly consider a weed and crop species and, to a considerably lesser extent, two crops grown in an intercrop. A variety of experimental designs and statistical analyses have been used to study competition in mixtures ( Gibson et al ., 1999 ; Freckleton and Watkinson, 2000 ). Here the focus is on the replacement series, and additive and neighbourhood designs.

Replacement series

The replacement series approach involves growing two species in varying proportions, including monoculture, whilst maintaining a constant overall stand density ( de Wit, 1960 ). Considered as pioneering in the analysis of competition within mixtures, it has also attracted much criticism, particularly regarding the dependence of the model coefficients on total stand density ( Inouye and Schaffer, 1981 ; Connolly, 1986 ) and the inability of the model to dissociate the separate effects of intra‐ and interspecific competition ( Firbank and Watkinson, 1985 b ; Snaydon, 1991 ) especially under changing conditions ( Watkinson and Freckleton, 1997 ). Notwithstanding these shortcomings, the replacement series still has adherents, albeit conditional. We would, however, argue that it is an inappropriate design for the analysis of competition in agronomic environments where understanding of how the effects of competition vary with density is of key concern (see below).

Additive design

Additive designs refer to those experiments where both the density and proportion of species are varied in mixtures. In its simplest and most typically applied form in an agricultural context, the so‐called ‘partial additive design’ allows the density of one species to be held constant whilst the second species is varied over a range of densities. Consequently, this design is particularly favoured for the study of crop–weed competition, although the data generated by this design can provide only a limited picture of the interaction between species, because it provides no information on the effect of the crop on the weed. One formulation of the hyperbolic model that has been used commonly for describing the damage to crop yield caused by competition from weeds ( Cousens, 1985 ) is

Y L = iN w (1 + ia –1 N w ) –1 (2)

where Y L is the proportion of yield lost, N w is weed density, i is the percentage yield loss per weed plant at low weed densities and a is the upper limit to yield loss at high weed densities. This equation may be further modified to take account of differences in competition between the weed and crop that result from differences in emergence time ( Cousens et al ., 1987 )

Y L = bN w ( e cT + ba –1 N w ) –1 (3)

where T is the time in days between emergence of the crop and the weed and a , b and c are parameters. Kropff and Spitters (1992 ) took an alternative approach for accounting for differences in emergence times by weighting densities with the average leaf area index.

The popularity of the hyperbolic model is in part due to its ability to satisfy what Cousens (1985 ) reasons to be the four fundamental biological truths of crop–weed competition: (1) there will be no yield loss in the absence of weeds; (2) the effects of increasing weed numbers will be additive at low weed densities; (3) crop yield loss can never exceed 100%; and (4) there is a non‐linear response of crop yield to weed density at high weed densities. The model is sometimes criticized because it offers little explanation of the underlying processes determining the outcome of competition and because there are difficulties in extrapolating to a broad range of species or locations. The model is, however, readily parameterized and with data taken from a range of sites at different times, it is possible to generalize about the factors playing the greatest role in determining yield loss.

While the partial additive design typically involves growing one species at a constant density while varying that of the other, an extreme version involves growing a species with and without interspecific competition. This has led to the development of a range of indices to quantify competition by essentially comparing the performance of a plant in monoculture with that in a mixture. Despite some endorsement of the use of such simple composite indices in conjunction with this experimental design ( Cousens, 1991 ; Snaydon, 1991 ), they allow only a crude picture of the competitive process to emerge. Once again, the indices are sensitive to density and may erroneously attribute the effects of a change in environmental conditions on relative performance in mixtures to changes in interspecific competition ( Freckleton and Watkinson, 1997 ).

At the opposite end of the extreme, the additive series involves replication of the full complement of density combinations for two species over a wide range of densities. It allows the quantification of both intra‐ and interspecific competition when analysed using a two‐species regression model ( Pantone and Baker, 1991 ; Park et al ., 2002 ). More generally for this form of analysis, the single species model (eqn 1) can be extended to two or more species using the relationship

w i = w m ,i (1 + Σα ij N j ) – b (4)

where w is a measure of plant performance, w m is the performance of an isolated plant and α represents the per capita effects of intra‐ (α ii ) and interspecific (α ij ) competition ( Watkinson, 1985 ).

Neighbourhood design

The neighbourhood approach to analysing plant competition was pioneered by Mack and Harper (1977 ) and involves relating the performance of an individual target species to the density of a neighbouring species within a given proximity. This design is based on the assumption that the performance of a target plant is related to the number, biomass, cover, aggregation or distance of the neighbouring species. However, the data requirements for neighbourhood models may be particularly resource‐intensive and can yield similar results to less spatially explicit mean density models ( Pacala and Silander, 1990 ). Nevertheless they are of particular value where competition needs to be quantified under different spatial arrangements of plants, although they have been used little within this context in agricultural studies.

Experimental design and analysis

Whilst the debate regarding the most appropriate experimental design and method of quantifying competitive intensity in mixed species stands continues to attract a lively discussion ( Freckleton and Watkinson, 1999 ; Jolliffe, 2000 ; Connolly et al ., 2001 ), there is an increasing consensus that the range of inferences that may validly be drawn from a study are principally determined by the experimental design used ( Gibson et al ., 1999 ; Freckleton and Watkinson, 2000 ). There is, however, no optimum design for competition experiments since the aims, objectives and practicalities vary from study to study and species to species. However, the fact that an overwhelming number of studies have shown that the effects on performance of competition in plant mixtures may be described by simple hyperbolic models, indicates that the problem of measuring plant competition is one of regression ( Freckleton and Watkinson, 2000 ). Moreover, theory has shown that, of the available methods, the regression approach is generally the most robust for analysing competition under field conditions ( Freckleton and Watkinson, 2001 a ). This argues for comparable approaches to the study of plant competition under both controlled environment and field conditions if we are to understand better how changing conditions and resources affect the process of competition in the agricultural environment and the consequent impacts of competition on crop yield and weed performance.

Models of plant competition are predominantly categorized as being either phenomenological, providing only a description of the outcome of competition, or mechanistic in structure, offering a representation of the physiological processes underlying plant growth. Competition studies that consider only final yield are inevitably limited as to the inferences that may be drawn about the process of competition. Quantitative measures of growth taken during the course of a growing period are necessary to understand the changing dynamics of species interactions and elucidate the competitive mechanisms determining the growth of individuals over time. Further, dynamic growth analysis allows the dissociation of ontogeny, the phenotypic developmental trajectory of an individual, from environmental effects on growth ( Evans, 1972 ).

Mechanistic models and, in particular, eco‐physiological models based on the response of physiological processes in plants to their environment, generally contain many parameters. As parameter estimates may be difficult to derive and consequently contain substantial error, the use of mechanistic models as a tool for decision‐making is regularly found to be impractical. However, the development of more parsimonious mechanistic plant growth models has resulted in a general increase in their use in recent years ( Graf and Hill, 1992 ; Kropff and Spitters, 1992 ; Aikman and Scaife, 1993 ; Deen et al ., 2003 ).

The capture of resources, particularly the interception of solar radiation, is an important factor in determining the competitive ability of species and this is reflected in light interception being the most developed aspect of many eco‐physiological models. For example, Spitters and Aerts (1983 ) proposed a model that was further developed by Kropff and colleagues ( Kropff, 1988 ; Lotz et al ., 1990 ; Kropff and Spitters, 1992 ), which took account of the spatial position of leaves and roots by dividing the canopy and root zone into a number of horizontal strata. Simulated growth was then partitioned between two species according to their relative proportional contribution to total leaf area. A less detailed approach to simulating growth within a multispecies canopy was taken by Ryel et al . (1990 ), who estimated the photosynthetic potential of foliage positioned in sunlit and shaded areas of the canopy. Rimmington (1984 ) provided a simpler model in which competition for light was simulated by dividing the canopy into only a small number of strata.

The above models all differ in the amount of detail expended on quantifying the local availability and interception of light. Interestingly, Deutschman et al . (1999 ) demonstrated that the amount of detail used to describe the local availability of light using the mechanistic, spatially explicit, stochastic forest simulation model, SORTIE ( Pacala et al ., 1996 ), had surprisingly little effect on the accuracy of its predictions at the forest development level. Nonetheless, a less detailed quantification of light does alter the predicted growth and mortality rates at the level of the individual tree.

Eco‐physiological models have been developed to include resources, such as nitrogen ( Graf et al ., 1990 ; Wilkerson et al ., 1990 ), in addition to light while the Conductance Model ( Aikman and Scaife, 1993 ) offers a simple mechanistic approach to simulating the growth of similar and different height species in monoculture and mixtures as a function of multiple resources ( Park et al ., 2001 ). Cellular automata models have also been used to describe the development of an individual in response to a heterogeneous resource supply ( Colasanti and Hunt, 1997 ).

Dynamic models of plant growth and competition, however, have had little impact to date on the design of weed management programmes. Despite the obvious potential application of such models to crop–weed competition through, for example, the manipulation of the canopy of the crop to suppress weeds ( Weiner et al ., 2001 ) and the delay in herbicide spraying to allow weed growth during the early stages of crop development to promote biodiversity ( Dewar et al ., 2003 ). The problem lies in the intensive studies required for successful parameterization.

Population dynamics

To understand the population dynamics of a species through time requires understanding of the various density‐independent and density‐dependent processes that affect the numbers of births and deaths in a population. Population models are based on censuses of plants at either flowering or germination and predict population size of species i ( N i ) at time t + 1 as a function of the population sizes at time t using a hyperbolic equation of the form

N i ( t + 1) = λ i N i ( t ) (1 + Σα ij N j ) – b (5)

The parameters of eqn (5) are the finite rate of increase, λ, defined as the maximal mean rate of population increase from low densities and competition coefficients, α, that model the per capita effects of intraspecific (α ii ) and interspecific (α ij ) competitors. This formulation can readily be extended to include a seed bank ( Freckleton et al ., 2000 ; Watkinson et al ., 2000 a ) and allows the dynamics of the species to be modelled using parameters that can be estimated directly from data on counts of numbers of plants.

Population models of the sort described by eqn (5) have been applied to a range of weed species including Alopecurus myosuroides ( Doyle et al ., 1986 ), Anisantha sterilis ( Firbank et al ., 1985 ; Smith et al ., 1999 ), Avena fatua ( Pandey and Medd, 1991 ; Jones and Medd, 1997 ), Chenopodium album ( Freckleton and Watkinson, 1998 ) and Vulpia bromoides ( Freckleton et al ., 2000 ). A critical component of all of the above models is quantification of the strength of both intra‐ and interspecific competition. In all cases, this was carried out through the manipulation of densities and the use of regression analysis to estimate the competition parameters in eqn (5), using either population growth rate or some measure of plant performance (e.g. seed production) as the dependent variable. Sensitivity analysis allows the understanding of key parameters of the life cycle that determine population numbers and highlights areas of the life cycle at which controls may be effective. In the case of Chenopodium album , this allowed Freckleton and Watkinson (1998 ) to conclude that predicting the effects of changing management on long‐term abundance of the weed will benefit more from improved systems for understanding germination behaviour than through management of the competitive ability of the crop, which will be effective only if very large changes in competitive effect can be achieved.

If the intrinsic weed‐suppressing ability of a crop is to be exploited ( Weiner et al ., 2001 ), it is necessary to identify the ecological and life‐history traits that confer competitive ability. Numerous life‐history traits have been credited as determining the competitive ability of an individual plant or species. Many of these traits are morphological (e.g. biomass partitioning) and display considerable phenotypic plasticity that can be exploited by a plant in a competitive environment. Several other traits have also been identified as potential determinants of competitive ability; these include seed size ( Rees, 1995 ), seedling size ( Schwinning and Fox, 1995 ), emergence time ( Kropff and Spitters, 1991 ) and plant size ( Goldberg and Landa, 1991 ). All of these parameters, in one way or another, either influence or reflect the ability of an individual plant to capture resources.

Whilst life history traits per se offer some explanation for the competitive ability of an individual or a species, the relative difference between two competing individuals or species is increasingly being recognized as an important determinant of the outcome of competition ( Freckleton and Watkinson, 2001 b ). The most common life history traits considered in terms of the relative difference between individual species are the relative time of emergence ( Elberse and de Kruyf, 1979 ; Cousens et al ., 1987 ), relative leaf area ( Kropff, 1988 ) and relative biomass ( Goldberg and Landa, 1991 ; Freckleton and Watkinson, 2001 b ). In an analysis of the competitive relationships between seven species, Freckleton and Watkinson (2001 b ) found that competition coefficients relate strongly to differences between the maximum sizes, root allocation, emergence time and seed size of species. The best predictor, however, was the difference in the maximum size of plants grown in isolation; correlations of the other traits with the competition coefficients occur through effects on the maximum size. The analysis also revealed coefficient reciprocity (inverse relationships between the interspecific coefficients for species pairs) and transitivity (numerically predictable hierarchies of competition between species). The theoretical basis for expecting coefficients to follow these patterns relates to short‐term competition for limiting resources.

It should be noted at this point that the strength of competition measured from experiments that consider plant weight as the dependent variable is not necessarily the same as the strength of competition in a population dynamic sense ( Chesson and Huntly, 1997 ). Caution should therefore be exercised in making inferences about the outcome of competition from studies on plant performance alone. The outcome of competition in a population dynamics sense depends not only on the magnitude of the competition coefficients but also on the finite rate of population increase ( Watkinson et al ., 2000 a ).

Crop–weed competition models have been used extensively for determining the yield loss of crops that result from varying densities of weeds. In one of the simplest extensions of this approach, knowledge of crop–weed competition has been combined with herbicide–weed resource curves to simulate the effects of herbicide use on crop yield and provide a rudimentary economic evaluation of herbicide treatments ( Streibig, 1989 ). A more refined economic analysis has been achieved through the use of bioeconomic models. These consist of several sub‐models, typically describing the life cycle of the crop and weed, crop–weed competition and economic system ( Dunan et al ., 1993 ; Jones and Medd, 1997 ).

In an alternative approach, crop–weed models have been applied to the task of identifying the minimum, or ‘threshold’, density of weeds justifying weed control. The threshold is calculated as the weed density at which the cost of chemical control is equal to the net benefit on crop yield gained through a reduction in weed competition. Such a modelling approach is not without its critics, particularly on the grounds of insufficient data, uncertainty and the non‐random distribution of weeds ( Auld and Tisdell, 1987 ; Dent et al ., 1989 ). In a recent review, Wilkerson et al . (2002 ) argued that weed management decision models should be evaluated from the perspectives of biological accuracy, quality of recommendations and ease of use. They further argued that future use depends upon finding cost‐effective methods to assess weed populations, demonstrating that the use of models makes more better decisions and that there is stable long‐term funding for the maintenance and support for the models.

Whilst the primary motivation behind many threshold models has been an improvement in the cost‐effectiveness of using herbicides, integrated weed management models, which simulate a combination of different chemical, mechanical, cultural, generic and biological weed control methods, provide a more sustainable approach to weed control. By using a selection of sub‐models to describe biological processes, including crop–weed competition, decision aids such as WEEDSIM ( Swinton and King, 1994 ) and WEEDCAM ( Lybecker et al ., 1991 ) simulate the long‐term outcome of a mix of different management options on the environment, given an initial estimate of the weed seed bank or seedling population. Despite offering a potentially valuable tool for assessing the environmental, as well as the economic costs of weed management strategies, the use of mathematical models in integrated crop protection has, to date, been markedly under‐utilized.

The quantification of competition together with the finite rate of population increase is at the heart of these models. Examples of their range of application include explanation of the decline in a previously common weed ( Firbank and Watkinson, 1986 ), predicting strategies that provide economic control of weeds ( Cousens et al ., 1986 ; Doyle et al ., 1986 ; Watkinson et al ., 2000 a ), contrasting the impacts of broad‐scale changes in farm management for the dynamics of weeds ( Smith et al ., 1999 ) and predicting the potential impacts of new technology on the species that feed on weed seeds ( Watkinson et al ., 2000 b ). Weed population models are thus being used to address a range of questions that would be impossible to tackle without quantification of plant competition. It is for this reason that the experimental designs and analyses that are used to quantify competition are of such importance.

Crop–weed models incorporating competition have had considerable success in describing how the process of competition affects crop yield and how strategic weed management decisions impact on weed numbers for a limited range of economically important species. There is, however, a need to increase our understanding of the spatial and temporal variability in model parameters if they are to be used more in a predictive context and to pull together data for a wide range of weeds and crops.

In contrast, mechanistic models have to date had limited success in describing crop–weed competition and limited utility within the weed management process. The problem with such models is that they require very intensive studies to be successfully parameterized and are constrained by their inherent need for detailed information relating to key physiological processes. For this reason, the development of more parsimonious models would be an advantage, requiring a more general approach to the study of competitive and physiological processes, enabling insight beyond that of the individual species.

Exploration of integrated weed management requires that we understand how weed management decisions within the crop growing‐season affect: ( a ) the yield of the crop through competition for resources, and ( b ) the biodiversity and numbers of weeds in the current and future crops. Both mechanistic and phenomenological models have a role to play here. The former include sufficient detail of the relationships between plant traits and the environment to allow exploration of within‐season management decisions on crop yield, while the latter, although not including such intricate detail, allow exploration of strategic management decisions on the abundance of weeds through the crop rotation.

S.E.P. received a Biotechnology and Biological Sciences Research Council (BBSRC) CASE studentship. Rothamsted Research receives grant‐aided support from the BBSRC.

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Author notes

1School of Biological Sciences, University of East Anglia, Norwich NR4 7TJ, UK, 2Rothamsted Research, Harpenden, Hertfordshire, UK, 3Schools of Biological and Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK and 4Tyndall Centre for Climate Change Research, University of East Anglia, Norwich NR4 7TJ, UK

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Intraspecific Competition and Evolution

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Adaptation through natural selection is one of the key elements of the Darwinian theory of evolution. This process has been the focus of much attention, partly because of its importance in the understanding of biological evolution, and partly because it appears as a nice directed deterministic process. The annoying aspect of Darwin’s theory of natural selection is, however, that the raw material of the process, the pheno- typic variation, originates as random errors in the hereditary transmission.

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Natural Selection and Evolution

Evolution as a Largely Autonomous Process

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Christiansen, F.B. (1988). Intraspecific Competition and Evolution. In: Wolff, W., Soeder, CJ., Drepper, F.R. (eds) Ecodynamics. Research Reports in Physics. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-73953-8_3

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On the intraspecific competition of yellowfin sole limanda aspera (pallas, [1814]) (pleuronectidae) in the eastern part of the sea of okhotsk.

Based on the longterm observations for 1963–2019, the article provides an assessment of the yellowfin sole generation mortality dynamics depending initial abundance and population dynamics in view of abundance and biomass. Individual growth and matiration rates were evaluated in generations with different initial abundance and in different states of population dynamics. Results indicated about intraspecific competition in yellowfin sole in the eastern part of the Sea of Okhotsk, expressed in specifics of the dynamics of stock abundance, growth and maturation. Forming generation stock abundance in early and later ages has different character. The more exceeding number of parental eggs spawned, the more generation abundance of yearlings getting exactly compensated by mortality (complete compensation). Older generations demonstrate the phenomen of “overcompensation”, when mortality of generations appeared in the years of higher egg production exceeds fertility. In the period of population growth and stabilization at a high level the period of the Yellowfin sole abundance fluctuation cycle gets shorter and the amplitude – smaller. Effects of intraspecific competition on the growth of individuals are revealed. Negative effects of the competition authentically revealed in elder age groups were not observed in younger age groups. An increase of the Yellowfin sole abundance brings negative effects on maturation rate of males with almost no such effects on females. To the greatest extent such effects can be seen in young age groups, at the beginning and middle stage of maturation.

The transcriptomic signature of physiological trade-offs caused by larval overcrowding in Drosophila melanogaster

Intraspecific competition at the larval stage is an important ecological factor affecting life-history, adaptation and evolutionary trajectory in holometabolous insects. However, the molecular pathways and physiological trade-offs underpinning these ecological processes are poorly characterised. We reared Drosophila melanogaster at three egg densities (5, 60 and 300 eggs/ml) and sequenced the transcriptomes of pooled third-instar larvae. We also examined emergence time, egg-to-adult viability, adult mass and adult sex-ratio at each density. Medium crowding had minor detrimental effects on adult phenotypes compared to low density and yielded 24 differentially expressed genes (DEGs) including several chitinase enzymes. In contrast, high crowding had substantial detrimental effects on adult phenotypes and yielded 2107 DEGs. Among these, upregulated gene sets were enriched in sugar, steroid and amino acid metabolism as well as DNA replication pathways, whereas downregulated gene sets were enriched in ABC transporters, Taurine, Toll/Imd signalling and P450 xenobiotics metabolism pathways. Overall, our findings show that larval overcrowding has a large consistent effect on several molecular pathways (i.e., core responses) with few pathways displaying density-specific regulation (i.e., idiosyncratic responses). This provides important insights into how holometabolous insects respond to intraspecific competition during development.

Interactive Effects of Intraspecific Competition and Drought on Stomatal Conductance and Hormone Concentrations in Different Tomato Genotypes

Plant physiological responses to various stresses are characterized by interaction and coupling, while the intrinsic mechanism remains unclear. The effects of intraspecific competition on plant growth, stomatal opening, and hormone concentrations were investigated with three tomato genotypes (WT-wild type, Ailsa Craig; FL-a abscisic acid (ABA) deficient mutant, flacca; NR-a partially ethylene-insensitive genotype) under two water regimes (full irrigation, irrigation amount = daily transpiration; deficit irrigation, 60% of irrigation amount in full irrigation) in this study. Three kinds of competitions were designed, i.e., root and canopy competition, non-root competition, and non-canopy competition, respectively. Intraspecific competition reduced plant leaf area and stomatal conductance (gs) of wild-type tomato, accompanied by ABA accumulation and ethylene evolution. Intraspecific competition-induced decrease in gs was absent in FL and NR, indicating ABA and ethylene involved in plant response to intraspecific competition. As soil water becomes dry, the competition decreased gs by elevating ABA and ethylene accumulations. Under severe drought, the competition-induced decline in gs was covered by the severe drought-induced decrease in gs, as hydraulic signals most probably dominate. The absence of canopy competition insignificantly influenced plant stomatal opening of well-watered tomato, as canopy separation minimized the plant neighbor sensing by ethylene and other signals. Whereas under water deficit condition, the absence of canopy competition significantly reduced ABA accumulation in roots and then stomatal conductance, indicating the belowground neighbor detection signals maybe enhanced by soil drought. The absence of root competition increased ethylene evolution, confirming the importance of ethylene in neighbor detection and plant response to environmental stress.

Mycorrhizal symbiosis changes host nitrogen source use

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Host-mediated, cross-generational intraspecific competition in a herbivore species

Intraspecific competition and persistence in acanthoscelides macrophthalmus (coleoptera: chrysomelidae): an experimental analysis in a stage‐structured population, am fungi endow greater plant biomass and soil nutrients under interspecific competition rather than nutrient releases for litter.

Plant competition affects belowground ecological processes, such as litter decomposition and nutrient release. Arbuscular mycorrhizal (AM) fungi play an essential role in plant growth and litter decomposition potentially. However, how plant competition affects the nutrient release of litter through AM fungi remains unclear especially for juvenile plants. In this study, a competitive potting experiment was conducted using juvenile seedlings of Broussonetia papyrifera and Carpinus pubescens from a karst habitat, including the intraspecific and interspecific competition treatments. The seedlings were inoculated by AM fungus or not inoculated, and the litter mixtures of B. papyrifera and C. pubescens were added into the soil or not added. The results were as follows: Litter addition significantly increased the root mycorrhizal colonization of two species in intraspecific competition. AM fungus significantly increased the biomass of B. papyrifera seedings and nitrogen release and decreased nitrogen concentration and N/P ratio of litter and further improved the total nitrogen and N/P ratio of soil under litter. The interspecific competition interacting with AM fungus was beneficial to the biomass accumulation of B. papyrifera and improvement of soil nutrients under litter. However, intraspecific competition significantly promoted nutrient releases via AM fungus. In conclusion, we suggest that AM fungi endow greater plant biomass and soil nutrients through interspecific competition, while intraspecific competition prefers to release the nutrients of litter.

Behavioural Indicators of Intra- and Inter-Specific Competition: Sheep Co-Grazing with Guanaco in the Patagonian Steppe

In extensive livestock production, high densities may inhibit regulation processes, maintaining high levels of intraspecific competition over time. During competition, individuals typically modify their behaviours, particularly feeding and bite rates, which can therefore be used as indicators of competition. Over eight consecutive seasons, we investigated if variation in herd density, food availability, and the presence of a potential competitor, the guanaco (Lama guanicoe), was related with behavioural changes in domestic sheep in Chilean Patagonia. Focal sampling, instantaneous scan sampling, measures of bite and movement rates were used to quantify behavioural changes in domestic sheep. We found that food availability increased time spent feeding, while herd density was associated with an increase in vigilant behaviour and a decrease in bite rate, but only when food availability was low. Guanaco presence appeared to have no impact on sheep behaviour. Our results suggest that the observed behavioural changes in domestic sheep are more likely due to intraspecific competition rather than interspecific competition. Consideration of intraspecific competition where guanaco and sheep co-graze on pastures could allow management strategies to focus on herd density, according to rangeland carrying capacity.

Group size mediates effects of intraspecific competition and forest structure on productivity in a recovering social woodpecker population

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Published: 01 August 2019

Interspecific competition affects the expression of personality-traits in natural populations

Lucas A. Wauters 1 , 2 na1 ,
Maria Vittoria Mazzamuto 1 na1 ,
Francesca Santicchia 1 ,
Stefan Van Dongen 2 ,
Damiano G. Preatoni 1 &
Adriano Martinoli 1

Scientific Reports volume 9 , Article number: 11189 ( 2019 ) Cite this article

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Animal behaviour
Behavioural ecology

Competition between animal species can cause niche partitioning and shape an individual’s phenotype, including its behaviour. However, little is known about effects of interspecific competition on personality, the among-individual variation in behaviour that is consistent across different spatial and temporal contexts. We investigated whether alien grey squirrels ( Sciurus carolinensis ) influenced the expression of personality traits in native red squirrels ( Sciurus vulgaris ). In Italy, alien grey squirrels replaced native reds through competition for food resources and space, reducing breeding and recruitment in the native species. We compared personality of red squirrels in red-only (no interspecific competition) and red-grey (with interspecific competition) sites, using arena-tests. The trait activity was measured by Open Field Test while sociability and avoidance were quantified by Mirror Image Stimulation test. Red squirrels co-occurring with the alien species had higher sociability scores and higher between-individual variation in sociability than in red-only sites. Differences in activity and avoidance were not significant. Personality – fitness relationships were not affected by presence or absence of grey squirrels, suggesting that the expression of sociability in red squirrels was not due to short-term selection, but was likely the result of context-related advantages when co-occurring with the competing species.

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Introduction.

Intraspecific competition among individuals in a population can be an important driver of natural selection 1 , 2 . Those individuals that are best adapted to local conditions, through their genotype, morphology, physiology and/or behaviour, will achieve a higher fitness. Not only intrinsic factors affect the outcome of competition, but spatio-temporal variation in extrinsic environmental conditions can produce extra selective pressures that differ among populations and with time. One of these extrinsic factors is the intensity of interspecific competition with a species that occupies an overlapping ecological niche 3 , 4 , 5 . Interspecific competition for limited resources (food, nest sites) can exert selective pressures on all aspects of an animal’s phenotype, including its behaviour 4 , 6 , 7 .

Since many behaviours have both a heritable and a flexible component, and their costs and benefits in terms of fitness will vary with environmental changes, the maintenance of behavioural variation can be explained by an evolutionary stable strategy or by a conditional strategy (e.g. 8 , 9 ). In a conditional strategy, the behavioural tactic an individual will adopt depends on some aspect of its environmental or physiological state 8 . Hence the absence or occurrence of competitors belonging to another species, presenting a change in the environmental state, might induce changes in the behaviour of the target species and behaviours less adapted in a single-species situation might become more adaptive when co-occurring with one or more competing species 4 , 7 , 10 , 11 .

Together with a suite of flexible behaviours, animals also display behaviours that differ consistently between individuals across different spatial and temporal contexts, referred to as personality traits 12 , 13 . Where it has been shown that spatio-temporal variation in the intensity of intraspecific competition (e.g. differences in population density, food availability, habitat use) can affect the relationship between an animal’s personality and its fitness 14 , 15 , 16 , only few studies that considered also a possible relationships between interspecific competition and the expression of personality traits 3 , 11 , 17 , 18 .

A special case of interspecific competition can occur as the result of human-induced biological invasions 7 . Understanding how animals respond to the occurrence of alien (invasive) species, is a critical ecological and evolutionary issue: behavioural responses can play an important role in the interactions between the native and the alien species and certain personality types might be better adapted than others to cope with the new challenge 7 .

In this study we explore whether the occurrence of a competing alien species results in changes in personality traits in comparison to a “single species” situation. We use the well-known study system of interspecific competition between the introduced invasive Eastern grey squirrel ( Sciurus carolinensis ) and the native Eurasian red squirrel ( Sciurus vulgaris ) 19 , 20 , 21 . Although, eventually, competition between the two species results in the replacement of red by alien grey squirrels (e.g. 20 , 21 ; but see 22 ), the earlier phases of colonization by the alien species and the years of co-occurrence of both species allow us to test the predictions of the hypothesis that interspecific competition influences the adaptiveness and hence the relative occurrence of different personality traits in the target species. Grey squirrels compete with native reds for limited food resources (tree seeds) and for space to establish home ranges, reducing recruitment of individuals of the native species 20 , 23 , 24 . Under the scenario of interspecific competition, more sociable red squirrels should be better adapted to persist the increasing pressure from co-occurring grey squirrels than individuals that tend to avoid close proximity of other squirrels, in particular when dispersal could be personality-trait (avoidance) dependent 7 . Moreover, active, exploring individuals of the native species should be more likely to acquire sufficient food resources, despite the interspecific home range overlap 19 , 25 , 26 , 27 . Therefore, we expect more explorative and/or active red squirrels and more individuals with a high score for sociability in areas where the native species has to compete with the invader, than in areas with only red squirrels. To test these predictions, we measured personality traits in six populations of the Eurasian red squirrel, three in areas where only the native species occurs and three in areas with both red and grey squirrels. Details of study sites and arena tests are given in the methods and in the Supplementary Material (Table S1 and Section S2 ).

Expert-based personality traits

During OFT, red squirrels spent most time in behaviours related to activity and shyness and little time in exploration. Sociability and avoidance were the most commonly expressed personality traits during MIS (Tables 1 and 2 ). We did not record any event of attack towards the mirror. Activity and shyness had high repeatability, but repeatability of exploration was very low and so was time spent exploring the arena (Tables 1 and 2 ). During MIS, the personality traits sociability, avoidance and other had moderate repeatability, while alert had not (Table 2 ).

Interspecific competition and personality

Since exploration had low repeatability and its average score did not differ between red-only and red-grey sites (Table 2 ), we will only report the traits activity and shyness from OFT. There was no effect of the presence of grey squirrels on the expression of activity (estimate β = −0.08, 95% CI = −0.45 to 0.26, pMCMC = 0.59) or on the expression of shyness (β = 0.06, 95% CI = −0.29 to 0.39, pMCMC = 0.71) (Fig. 1 ). Red squirrels tended to be more active during OFT in 2017 than in 2016 (β = 0.32, 95% CI = 0.08 to 0.56, pMCMC = 0.009) and scores for activity were highest in the first test, while those for shyness were lowest in the first test (for estimates see Supporting Information, Table S3 ). Hence, when red squirrels were in the arena for the first they performed more activity-related behaviours than in subsequent tests.

Box and Whisker plots of the personality trait scores (squareroot transformed proportion of time spent in behaviours that are part of the given trait) for the four main traits. Comparison between red-only (dark grey) and red-grey (light grey) area-type. Diamonds = mean. Data shown using the first arena test for each individual (n = 184; 95 from red-only area type and 89 from red-grey area type).

Patterns of sociability measured by MIS differed between red-only and red-grey sites (Fig. 1 ). Behaviours related to sociability were more expressed in red-grey than in red-only situation (β = 0.44, 95% CI = 0.06 to 0.83, pMCMC = 0.034). Moreover, red squirrels tended to express slightly more avoidance in red-only than in red-grey situation (β = 0.17, 95% CI = −0.16 to 0.49, pMCMC = 0.29), but this difference was not significant. Individual variation in personality was not affected by sex or body mass (Supporting Information, Table S3 ), except for a sex-effect on the trait avoidance: females expressed less avoidance than males (β = −0.29, 95% CI = −0.55 to −0.02, pMCMC = 0.033). The correlations between personality traits are reported in Supplemental Information (Table S4 ).

Relationships of personality with local survival and reproduction

We estimated local survival on 224 observations (112 red-only sites; 112 red-grey sites) of 180 different red squirrels. Overall, in 71% of cases red squirrels survived the year (73% in red-only sites, 70% in red-grey sites). There was no effect of area-type (red-grey vs red-only) on probability to survive (β = −14.8, 95% CI = −53.2 to 19.1, pMCMC = 0.31). Personality traits were not related with the probability of survival (Table 3 ) and neither was body mass (β = 4.89, CI = −9.46 to 19.4, pMCMC = 0.38).

We estimated the probability to reproduce using 77 observations (32 red-only sites, 45 red-grey sites). In 62% of cases female red squirrels produced a litter (59% in red-only sites, 64% in red-grey). The probability to produce at least one litter per year did not differ between red-grey and red-only sites (β = −27.9, 95% CI = −83.2 to 17.8, pMCMC = 0.16). Reproductive output was lower in 2017 than in 2016 (β = −48.1, 95% CI = −91.3 to −20.3, pMCMC < 0.001) and increased with a female’s body mass (β = 43.5, 95% CI = 15.1 to 81.9, pMCMC < 0.0001). The probability to reproduce was not related with any of the personality traits (Table 3 ).

Next, we run two sub-models, one for each area-type, and compared their posterior slopes for the various correlations between personality traits and fitness components to explore whether high levels of activity and/or sociability had a fitness advantage in red-grey sites, but not in red-only. Correlations with survival or reproductive rate were weak for all traits reported and in both red-only and red-grey sites, and there was no significance difference in the posterior slopes for any of the traits (Table 3 ; Supporting Information, Table S5 ). Hence, there was no evidence for selection favouring active or social red squirrels in populations co-occurring with the alien competitor. However, the between-individual variance in sociability and avoidance of red squirrels was higher when the competitor was present than in red-only sites (Table 4 ).

In small mammals personality is studied using capture-mark-recapture data and/or by arena tests 15 , 28 , 29 . Using arena test (Open Field Test followed by a Mirror Image Stimulation test) we found that red squirrels co-occurring with grey squirrels expressed more the personality trait sociability than in areas without the invasive competitor. The tendency to show less avoidance of the mirror image in red-grey than in red-only areas was weak and non significant, and in contrast with our predictions, we found no difference in the expression of activity between red-grey and red-only areas. The between-individual variation in the traits sociability and avoidance among red squirrels was higher in sites with the alien competitor than in red-only sites. This indicates more variation among individual red squirrels in the expression of these traits in the situation with interspecific competition (see also Fig. 1 ).

There are two, non mutually exclusive explanations for the pattern of sociability expression. The first implies interspecific competition as a driver for natural selection favouring certain phenotypes (personality traits) over others (character displacement, e.g. 3 , 4 , 6 ); the second that the observed differences are the results of context related (with vs without competitor) plasticity in the behaviour of red squirrel.

In the first case, interspecific competition between grey and red squirrels favours those individuals of the native species that have a higher sociability, measured by the reactions to their mirror image. In other words, red squirrels that behave in a sociable, non-aggressive way to conspecifics are more common in red-grey than in red-only areas because they have a selective advantage. This advantage could be related to a general social personality trait, implying non-aggressive behaviour and tolerance to close proximity not only to a conspecific, measured with MIS, but also to the competing invasive species 26 . If these personality trait differences are the result of selection, we expect that red squirrels with a higher sociability score have a survival and/or reproduction benefit in the woodlands with grey squirrels, but not in woodlands without the competitor. We expected positive correlations of sociability with one or both fitness components only in red-grey sites, but no significant associations were found. The same was true for the trait avoidance in red-only sites. Hence, our results suggested that the observed differences in the expression of personality traits by red squirrels in red-only than in red-grey study sites were not due to selection on these traits (sociability and avoidance). It must be noted that differences in personality among animal can also affect their dispersal behaviour 7 , 30 . Since we were unable to follow juvenile cohorts throughout the dispersal and settlement process 31 , we could not rule out that selection for higher sociability occurred in red squirrels when co-existing with grey squirrels during this phase. Studying the personality of juvenile and subadult red squirrels and its relationship with dispersal and settlement success can reveal possible selection mechanisms for certain traits that might differ between areas with and without grey squirrels.

One important limitation of our dataset is that the proxy of fitness did not account for variation in number of young weaned/female as a component affecting variation in reproductive success 32 . Moreover, our study may have been too short (2 years) to measure any selective advantage of a given personality trait (see also 18 ). Despite these potential problems, we argue that it is unlikely that competition with grey squirrels asserts a selective pressure on co-occurring native red squirrels in our study system. Grey squirrels colonized our study sites very recently, between 2 and 8 years before the arena test experiments. Hence, selection on personality traits should have occurred in only two-three generations, which seems unlikely (but see 33 ). Also, keeping grey squirrel densities low by removal could decrease the intensity of interspecific competition resulting in reduced selective pressure on local adaptations of personality. Finally, other phenotypic characteristics may have more pronounced effects on fitness than personality traits; in fact we found strong positive effects of a squirrel’s body mass on reproduction, in agreement with earlier studies 16 , 32 , 34 .

The second explanation of having more red squirrels with high sociability in sites with grey squirrels is that sociability has a marked flexible component itself, or is related with other behaviours that have context-related plasticity and facilitate red squirrels to share the woodland with the invasive competitor. In woodlands occupied by both species, the interspecific overlap of the foraging-niche, daily activity pattern and home ranges (core-areas) are high 19 , 26 and more sociable red squirrels are likely to sustain such pressure, that increase with grey squirrel density, better than individuals with a tendency to avoid conspecifics. Also, higher sociability could be related with a lower susceptibility to physiological stress induced by the invader 35 . Conversely, dispersal as a conditional strategy 36 , could result in red squirrels with a strong avoidance personality being the first to emigrate from woodlands invaded by grey squirrels, as supported also by low local recruitment rates of juvenile red squirrels in areas of co-occurrence 20 . Finally, Sih et al . 7 reported that personality traits can influence the intensity of interspecific interactions and/or increase intraspecific variation of certain traits, which might result in higher functional diversity for one or both of the competing species. Our data supported this hypothesis, since we found higher between-individual variance in sociability among red squirrels co-occurring with grey squirrels than among red squirrels in red-only sites.

As far as the relationship between interspecific competition and activity was concerned, we predicted more active squirrels in areas where the native species has to compete with the invader, since higher activity was thought to be related with greater food resource acquisition. However, we did not find any differences in the activity trait comparing the two situations (red-only vs red-grey). We believe this was due to our grey squirrel control to keep their densities low. At such low densities, interspecific competition for food is reduced and might be insufficient to create a marked advantage for more active red squirrels.

The study was carried out in six study sites that were not identical in tree species composition or red squirrel density (see also study design below). This was addressed statistically by modelling study site nested within situation as a random effect in the MCMCglmm, thereby correcting for any potential between site variation in the test for the situation effect (red-only vs red-grey situation). Since we did find a significant effect of situation on sociability expression, this effect was much larger than any potential between study site variation. In other words, any variation in the expression of personality traits, potentially due to differences between study sites in the proportions of conifers and deciduous tree species, or other ecological variables, was much smaller than the effect of the presence of grey squirrels on the expression of sociability.

Few studies investigated individual differences in personality in relation to outcomes of interspecific competition. Experiments with two ecologically similar fish species, the threespine and ninespine sticklebacks ( Gasterosteus aculeatus and Pungitius pungitius ) showed that more active individuals of both species spend more time in open waters than in vegetation, and bolder fish had a higher prey-consumption rate than more shy individuals, irrespective of species 17 . Authors suggested that individual variation in personality traits can facilitate interspecific niche overlap, which might affect prevailing selection pressures in areas where interspecific competition is more important compared to single-species situations (see also 3 , 11 ). In birds, territorial aggression can be very important in the context of interspecific competition for limited high-quality nesting sites 18 . Eastern bluebirds ( Sialia sialis ) showed a strong tendency toward assortative mating in areas of both high and low interspecific competition with tree swallows ( Tachycineta bicolor ), but pairs that behaved the most similarly and displayed either extremely aggressive or extremely non-aggressive phenotypes experienced higher reproductive success only in areas of high interspecific competition 18 . However, since the study was over a single breeding season, they could not measure ongoing selection of bluebird personality traits driven by interspecific competition. These authors suggested that interspecific competition may select for certain personality traits and that animal personality may be an important factor influencing the outcome of interactions between native and invasive species 18 .

In conclusion, our data showed that, of the different personality traits investigated, only the sociability of red squirrels changed in sites invaded by grey squirrels. Red squirrels competing with the invasive species had higher sociability scores and higher between-individual variance in sociability than in sites without grey squirrels. Although it was recently shown that natural selection of personality traits and emergence of behavioural syndromes can be rapid 33 , we found no evidence that the observed differences in personality traits were the consequence of character displacement driven by interspecific competition. However, differences in dispersal tendency of individual red squirrels that are either social or avoiders could explain the higher average scores of sociability in woods shared with grey squirrels than in woods without the invasive competitor. Further studies over a longer time-period should investigate whether the flexible component of the activity, sociability and avoidance personality traits vary over time with the increasing experience of the individual squirrel. Moreover, allowing grey squirrel density to increase in some study sites might reveal whether interspecific competition can drive selection for personality phenotypes that allow red squirrels to cope with the alien invasive species. More research on naturally co-occurring species in a guild and how both intra- and interspecific interactions contribute to the selection of personality traits is mandatory to increase our insight in the role of interspecific competition in shaping individual variation in personality.

Study design and trapping squirrels

The six red squirrel populations (study sites) we monitored are independent replicates in the same geographic area (North Italy): three with only red squirrels and three with both red and grey squirrels. Since we used a natural setting, the six study sites were not identical in forest composition or red squirrel density. However, the range of densities was comparable between red-only and red-grey sites (Table S1 ) and social organisation, mating behaviour, foraging behaviour and activity patterns are similar and consistent over a wide range of habitat types 23 , 25 , 26 , 37 . Therefore, there should be no confounding ecological variables associated with the different study sites that could influence the main effect of absence/presence of the alien competitor. Moreover, site heterogeneity was addressed statistically by adding study site nested in area-type as random effect in the MCMCglmm model (see statistical analyses). The red-grey sites are mature mixed broadleaf-conifer woods dominated by oaks ( Quercus robur , Q. petraea ) and hornbeam ( Carpinus betulus ) with different proportion of conifers. The red-only sites are mixed conifer forests and data on forest structure and composition are reported elsewhere 37 , 38 .

Trapping was carried out in two to four periods per year between January 2016 and December 2017 (Supporting Information, Table S1 ). A trapping session involved the use of Tomahawk “squirrel” traps (models 201 and 202, Tomahawk Live Trap, WI, USA) placed on the ground or at breast height against tree trunks. Traps were more or less homogeneously distributed over the study area, with average trap densities varying among sites, in relation to expected squirrel density (Supporting Information, Table S1 ).

Traps were pre-baited with sunflower seeds and hazelnuts 4 to 6 times over a 30 day period, then baited and set for 4–5 days. Traps were checked two times per day. Each trapped red squirrel was flushed into a light cotton handling bag with a zipper or a wire-mesh “handling cone” to minimize stress during handling, and individually marked using numbered metal ear-tags (type 1003 S, National Band and Tag, Newport, KY, USA). It was weighed to the nearest 5 g using a spring-balance (Pesola AG, Baar, Switzerland). Sex, age class and reproductive condition were determined on the basis of external genitalia, condition of the nipples (females) and body mass, with juvenile red squirrels weighing less than 250 g 32 .

We used capture-mark-recapture (CMR) data to define local annual survival (binary variable: 1 = survived, trapped from first to last trapping session in a given year; 0 = not survived, no longer trapped in the last trapping session of the given year, nor in subsequent sessions). Capture probabilities in red squirrel populations are high, and both bold and shy animals are trapped at least once per year; moreover, radio-tracking data confirm survival estimates based on CMR 16 . For females we also determined a measure of reproductive output: each individual female was scored 1 (binary variable) when it produced a litter (trapped pregnant and/or lactating in at least one session), it was scored 0 when no litter was produced (anoestrus and non lactating in all trapping sessions in a given year).

In the experimental sites, captured grey squirrels were removed as part of a red squirrel conservation project: animals were euthanized by CO 2 inhalation, following the EC and AVMA guidelines 39 . Doing so, grey squirrel densities were kept low, making any result of the relationships between interspecific competition and red squirrel personality conservative. Trapping and handling squirrels complied with current laws on animal research and welfare in Italy.

Ethical approval for fieldwork with animals

Trapping, marking and handling of red squirrels and arena-test experiments were carried out in accordance with the Guidelines for the Use of Animals in Research (Animal Behaviour, 2018, 135, I-X). Grey squirrel control was carried out in accordance to the indications in Leary S. et al . 2013 AVMA Guidelines for the Euthanasia of Animals: 2013 Edition. Approval and legal requirements according to the Italian Wildlife Protection and Hunting Law L.N. 157 from 1992 and authorizations N.294–34626 of 12/09/2014 (2014–2016) from the Provincia di Torino and N62-3025 (2017–2019) from the Città Metropolitana di Torino, and Decreto N. 11190 (29/11/2013) and decrees n°9523 of 15/10/2014 and n° 198 (13/01/2017) from Direzione Generale Agricoltura, Regione Lombardia; and the permission Protocol n° 414 of 28/02/2014 of the Stelvio National Park.

Measuring personality

Details of arena tests in Supplementary material 2 (and see 40 ). To quantify individual personality, we performed two different experiments inside the arena: Open Field Test (OFT) to estimate activity and exploration levels in a novel environment and Mirror Image Stimulation (MIS) to test aggressiveness, sociability or avoidance towards conspecifics 28 , 40 , 41 , 42 . The two tests were performed in the same testing session, with the OFT also serving as habituation time before the MIS. We performed arena tests for each individual only once per capture-session to reduce stress and habituation in animals (minimum time between tests for the same individual: 77 days). In addition, to check the assumptions of repeatability of personality traits we repeated both experiments (OFT and MIS) in different capture-sessions to have at least two arena tests for most individuals.

In total we performed 323 arena tests (156 in red-only sites, 167 in red-grey sites) on 184 different red squirrels (95 in red-only sites, 89 in red-grey sites; Table S1 ). We analysed digital videos of OFT and MIS with CowLog 3.0.2 software 43 and used the ethogram from Mazzamuto et al . 40 (Table 1 ); for each experiment, the software calculates the time that an individual spent in each behaviour.

Statistical analysis

We first transformed the time calculated by CowLog 3.0.2 in proportion of time spent by each squirrel in a given behavioural state. To reduce the number of behaviours observed into few personality-linked variables we used the expert-based method described previously 40 . With the expert-based approach the researcher defines groups of behaviours, with each group related to a specific personality trait, summing the values of the single behaviours to obtain scores for each personality trait 40 . The method was validated by comparing its performance of grouping behaviours into personality traits with the outcomes of statistical grouping based on PCA or Factor Analysis 40 . Aggressiveness was considered as the number of attacks towards the mirror during MIS.

All analyses and interpretations were based on a multivariate mixed model fitted in a Bayesian framework using the package MCMCglmm in R 44 . Personality-trait scores were squareroot transformed before analysis. All expert-based personality traits, survival and reproduction were treated as dependent variables after standardisation. For all expert-based personality traits, a Gaussian residual error distribution was used, while survival and reproduction were treated as binomial. Assumption of multivariate normality of the personality traits was supported by the QQ-plot of the Mahalanobis distances of the model residuals (r-squared value = 0.92). As repeated observations were present, individual was added as a random effect. Because 91 individuals (60 males, 31 females) were caught in at least two trapping sessions (a total sample of 230 tests), we were able to estimate the repeatability of the expert-based personality traits as the between-individual variation divided by the sum of the between-individual and residual variation. For both the residual and between-individual variation, an unstructured variance-covariance matrix was modelled, allowing the estimation of correlations among the response variables (covariance divided by the square root of the product of the variances). Area-type, red-only vs red-grey, was treated as fixed effect, and area nested within area-type was added as random effect (as a heterogeneous identity matrix) to avoid pseudoreplication problems during the parameter estimation process. In addition, sex, body mass, year and arena test order (first to fourth test of the same animal) were added as fixed effects. We did not include body mass measures of pregnant females to avoid a bias due to extra weight of developing embryos. The effect of sex was set to zero for the dependent variable reproduction and the effect of arena test order was set to zero for both reproduction and survival. Posterior distributions were based on 10000000 iterations with a burnin of 50000 iterations and thinning of 100, such that 100000 iterations were used to obtain point estimates and 95% credibility intervals (model with 1000000 iterations, 50000 burnin and 40 thinning produced the same results). For all fixed effects, the prior distribution was Gaussian with zero mean and variance equal to 1. For the random effects and residual variation and inverse Wishard prior was set with diagonal elements equal to 0.5, 0.5 and 0.1 for the residual, between-individual and nested area effect respectively. The believe parameter was set to 0.01. Full model outputs are provided in Supporting Information, Table S3 .

To explore whether high levels of activity and/or sociability had a fitness advantage in red-grey sites but not in red-only sites, we ran sub-models, one for each area-type. These models were constructed as the full model except for the fixed effect of area-type (full outputs in Supporting Information, Table S5 ). We then tested explicitly for the interactions with area-type by comparing the slopes of the posterior distributions from the two separate models, for the various correlations between personality traits and fitness components (survival and reproduction).

Data Availability

MCMCglmm outputs are available in the Supplemental Information. The MCMCglmm and other data analyses R-scripts and the datafile are available at https://doi.org/10.5281/zenodo.1451460 .

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Acknowledgements

Authors thank Regione Lombardia, Provincia di Torino, Stelvio National Park, Oasi WWF Vanzago and the owners of private estates for permits and access to the woodlands. We thank Jeff Dolphin for checking the English. This is paper number 30 of Alpine Squirrel Population Ecology Research (ASPER).

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Lucas A. Wauters and Maria Vittoria Mazzamuto contributed equally.

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Environment Analysis and Management Unit, Guido Tosi Research Group, Department of Theoretical and Applied Sciences, University of Insubria, Varese, Italy

Lucas A. Wauters, Maria Vittoria Mazzamuto, Francesca Santicchia, Damiano G. Preatoni & Adriano Martinoli

Department of Biology, University of Antwerp, Antwerp, Belgium

Lucas A. Wauters & Stefan Van Dongen

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L.A.W. and M.V.M. conceived and designed the study and drafted the manuscript; F.S. and L.A.W. collected the data; M.V.M., S.V.D. and D.P. performed modelling work and analysed output data; A.M. and D.P. coordinated the study. All authors critically commented on the ms and gave final approval for submission.

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Correspondence to Damiano G. Preatoni .

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Wauters, L.A., Mazzamuto, M.V., Santicchia, F. et al. Interspecific competition affects the expression of personality-traits in natural populations. Sci Rep 9 , 11189 (2019). https://doi.org/10.1038/s41598-019-47694-4

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The exploration of mechanisms that enable species coexistence under competition for a sole limiting resource is widespread across ecology. One classical example is the coexistence of herbaceous and woody species in self-organised dryland vegetation patterns. Previous theoretical investigations have explained this phenomenon by making strong assumptions on the differences between grasses and trees (e.g. contrasting dispersal behaviours or different functional responses to soil moisture). Motivated by classical theory on Lotka-Volterra competition models, I argue that the interplay between interspecific and intraspecific competition can explain species coexistence without relying on such assumptions. I use an ecohydrological reaction-advection-diffusion system that captures the interactions of two plant species with an explicitly modelled resource to show that coexistence is facilitated by strong intraspecific competition of the species superior in its colonisation abilities, if its competitor’s local average fitness is higher. The inclusion of a spatial self-organisation principle yields significant differences from the nonspatial case in which strong intraspecific competition of the locally superior species enables coexistence. Results presented in this paper also capture the empirically observed spatial species distribution in single bands of vegetation and propose differences in plants’ dispersal behaviour as its cause.

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Interspecific competition affects the expression of personality-traits in natural populations

Lucas a. wauters.

1 Environment Analysis and Management Unit, Guido Tosi Research Group, Department of Theoretical and Applied Sciences, University of Insubria, Varese, Italy

2 Department of Biology, University of Antwerp, Antwerp, Belgium

Maria Vittoria Mazzamuto

Francesca santicchia, stefan van dongen, damiano g. preatoni, adriano martinoli, associated data.

MCMCglmm outputs are available in the Supplemental Information. The MCMCglmm and other data analyses R-scripts and the datafile are available at 10.5281/zenodo.1451460.

Introduction

Intraspecific competition among individuals in a population can be an important driver of natural selection 1 , 2 . Those individuals that are best adapted to local conditions, through their genotype, morphology, physiology and/or behaviour, will achieve a higher fitness. Not only intrinsic factors affect the outcome of competition, but spatio-temporal variation in extrinsic environmental conditions can produce extra selective pressures that differ among populations and with time. One of these extrinsic factors is the intensity of interspecific competition with a species that occupies an overlapping ecological niche 3 – 5 . Interspecific competition for limited resources (food, nest sites) can exert selective pressures on all aspects of an animal’s phenotype, including its behaviour 4 , 6 , 7 .

Together with a suite of flexible behaviours, animals also display behaviours that differ consistently between individuals across different spatial and temporal contexts, referred to as personality traits 12 , 13 . Where it has been shown that spatio-temporal variation in the intensity of intraspecific competition (e.g. differences in population density, food availability, habitat use) can affect the relationship between an animal’s personality and its fitness 14 – 16 , only few studies that considered also a possible relationships between interspecific competition and the expression of personality traits 3 , 11 , 17 , 18 .

In this study we explore whether the occurrence of a competing alien species results in changes in personality traits in comparison to a “single species” situation. We use the well-known study system of interspecific competition between the introduced invasive Eastern grey squirrel ( Sciurus carolinensis ) and the native Eurasian red squirrel ( Sciurus vulgaris ) 19 – 21 . Although, eventually, competition between the two species results in the replacement of red by alien grey squirrels (e.g. 20 , 21 ; but see 22 ), the earlier phases of colonization by the alien species and the years of co-occurrence of both species allow us to test the predictions of the hypothesis that interspecific competition influences the adaptiveness and hence the relative occurrence of different personality traits in the target species. Grey squirrels compete with native reds for limited food resources (tree seeds) and for space to establish home ranges, reducing recruitment of individuals of the native species 20 , 23 , 24 . Under the scenario of interspecific competition, more sociable red squirrels should be better adapted to persist the increasing pressure from co-occurring grey squirrels than individuals that tend to avoid close proximity of other squirrels, in particular when dispersal could be personality-trait (avoidance) dependent 7 . Moreover, active, exploring individuals of the native species should be more likely to acquire sufficient food resources, despite the interspecific home range overlap 19 , 25 – 27 . Therefore, we expect more explorative and/or active red squirrels and more individuals with a high score for sociability in areas where the native species has to compete with the invader, than in areas with only red squirrels. To test these predictions, we measured personality traits in six populations of the Eurasian red squirrel, three in areas where only the native species occurs and three in areas with both red and grey squirrels. Details of study sites and arena tests are given in the methods and in the Supplementary Material (Table S1 and Section S2 ).

Expert-based personality traits

During OFT, red squirrels spent most time in behaviours related to activity and shyness and little time in exploration. Sociability and avoidance were the most commonly expressed personality traits during MIS (Tables 1 and and2). 2 ). We did not record any event of attack towards the mirror. Activity and shyness had high repeatability, but repeatability of exploration was very low and so was time spent exploring the arena (Tables 1 and and2). 2 ). During MIS, the personality traits sociability, avoidance and other had moderate repeatability, while alert had not (Table 2 ).

Ethogram for Open Field and Mirror-Image Stimulation tests.

Open Field Test			Mirror Image sStimulation Test
Behaviour	Behaviour description	Personality trait	Behaviour	Behaviour description	Personality trait
Locomotion	Jump, walk		Locomotion	Jump, walk
Rise	Rise up on hind legs		Rise	Rise up on hind legs
Scan	Head moving		Scan	Head moving
Scratch	Scratch or chew floors/walls		Scratch	Scratch or chew floors/walls
Sniff	Sniff the corner of arena		Sniff	Sniff the corner of arena
Head dip	Put head in holes in the floor		Head dip	Put head in holes in the floor
Hang	Hang on walls		Hang	Hang on walls
Immobile	No movement		Back	Immobile in back half of arena furthest from mirror
			Slow	Slow approach towards mirror, with hind legs stretched out behind
			No-aggressive	Non aggressive contact with the mirror
			Front	Immobile in front half of arena closest to mirror
			Watch	Immobile, watching directly to mirror
			Attack	Strike the mirror with front legs or head

Description of the single behaviours and indication of the expert-based grouping into categories that represent personality traits 40 .

The average proportion of time (raw data) red squirrels were engaged in behaviours related to the different personality traits defined by the expert-based approach during OFT and MIS.

		red-only (n = 156)		red-grey (n = 167)		Repeatability (n = 230)
Personality trait		Mean	SD	Mean	SD	R	95% CI	Posterior mode
OFT	Activity	0.36	0.19	0.33	0.18	0.50	0.36–0.63	0.50
	Shyness	0.55	0.22	0.57	0.21	0.52	0.39–0.65	0.53
	Exploration	0.06	0.05	0.06	0.07	0.09	0.02–0.18	0.09
MIS	Sociability	0.12	0.26	0.26	0.33	0.19	0.05–0.33	0.18
	Avoidance	0.57	0.31	0.47	0.35	0.20	0.07–0.33	0.18
	Alert	0.12	0.10	0.12	0.13	0.09	0.01–0.17	0.07
	Other	0.17	0.17	0.13	0.13	0.34	0.20–0.49	0.35
	Aggressiveness	0 attacks to the mirror

Data grouped by situation (study sites with only red squirrels = red-only; study sites with both red and grey squirrels = red-grey). Repeatability estimated with the MCMCglmm model (see methods).

Interspecific competition and personality

An external file that holds a picture, illustration, etc.
Object name is 41598_2019_47694_Fig1_HTML.jpg

Relationships of personality with local survival and reproduction

Correlation coefficients and 95% credibility intervals derived from the MCMCglmm models between the two fitness components (probability of local survival and probability to produce a litter) and four personality traits of red squirrels.

Study area type	Fitness variables	Activity	Shyness	Sociability	Avoidance
All	Local survival	−0.03 (−0.28–0.23)	0.03 (−0.21–0.28)	−0.25 (−0.79–0.27)	0.13 (−0.35–0.64)
All	Female reproduction	0.29 (−0.11–0.77)	−0.34 (−0.73–0.02)	0.24 (−0.63–0.92)	−0.32 (−0.91–0.37)
Red-only	Local survival	−0.09 (−0.47–0.29)	0.18 (−0.20–0.55)	−0.38 (−0.96–0.37)	0.19 (−0.52–0.89)
Red-only	Female reproduction	0.46 (−0.13–0.98)	−0.53 (−0.99–0.02)	0.30 (−0.45–0.92)	−0.30 (−0.93–0.36)
Red-grey	Local survival	−0.12 (−0.49–0.24)	0.05 (−0.28–0.39)	−0.06 (−0.48–0.36)	0.04 (−0.37–0.43)
Red-grey	Female reproduction	−0.09 (−0.62–0.41)	0.07 (−0.40––0.57)	−0.30 (−0.77–0.23)	0.26 (−0.28–0.73)
Difference slopes	Local survival	−0.03 (−0.56–0.52)	−0.13 (−0.64–0.38)	0.31 (−0.62–1.03)	−0.16 (−0.94–0.72)
Difference slopes	Female reproduction	−0.55 (−1.41–0.45)	0.60 (−0.35–1.45)	−0.60 (−1.38–0.31)	0.57 (−0.37–1.37)

All correlations include 0 in the 95% CI. Differences between posterior slopes of the correlation estimates for red-only and red-grey area-type.

Mean (95% CI) between-individual variances and differences (Diff) in between-individual variances of the personality traits in the Red-grey and Red-only areas (Differences Red-grey – Red-Ony) based on the two MCMCglmm models (iterations 1000000, burnin 50000, thinning interval 40, sample size per chain 25000).

Personality trait	Red-Grey areas	Red-Only areas	Diff. Mean ± SD	Diff. 95% CI
Activity	0.33 (0.13–0.56)	0.49 (0.21–0.80)	−0.16 ± 0.19	−0.54 to 0.20
Exploration	0.11 (0.01–0.23)	0.24 (0.05–0.45)	−0.13 ± 0.12	−0.40 to 0.09
Shyness	0.45 (0.21–0.71)	0.53 (0.21–0.88)	−0.08 ± 0.22	−0.52 to 0.34
Sociability	0.42 (0.11–0.73)	0.06 (0.01–0.14)	0.35 ± 0.17	0.06 to 0.72
Avoidance	0.43 (0.16–0.74)	0.10 (0.01–0.21)	0.33 ± 0.16	0.05 to 0.68
Alert	0.10 (0.01–0.23)	0.11 (0.01–0.25)	−0.01 ± 0.09	−0.20 to 0.18
Other	0.30 (0.10–0.52)	0.45 (0.15–0.77)	−0.15 ± 0.19	−0.55 to 0.22

Study design and trapping squirrels

Ethical approval for fieldwork with animals

Measuring personality

Details of arena tests in Supplementary material 2 (and see 40 ). To quantify individual personality, we performed two different experiments inside the arena: Open Field Test (OFT) to estimate activity and exploration levels in a novel environment and Mirror Image Stimulation (MIS) to test aggressiveness, sociability or avoidance towards conspecifics 28 , 40 – 42 . The two tests were performed in the same testing session, with the OFT also serving as habituation time before the MIS. We performed arena tests for each individual only once per capture-session to reduce stress and habituation in animals (minimum time between tests for the same individual: 77 days). In addition, to check the assumptions of repeatability of personality traits we repeated both experiments (OFT and MIS) in different capture-sessions to have at least two arena tests for most individuals.

Statistical analysis

Supplementary information

Acknowledgements.

Author Contributions

Data Availability

Competing interests.

The authors declare no competing interests.

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Lucas A. Wauters and Maria Vittoria Mazzamuto contributed equally.

Supplementary information accompanies this paper at 10.1038/s41598-019-47694-4.

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Adaptation through natural selection is one of the key elements of the Darwinian theory of evolution. This process has been the focus of much attention, partly because of its importance in the understanding of biological evolution, and partly because it appears as a nice directed deterministic process. The annoying aspect of Darwin's theory ...
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Investigation of this rainfall threshold shows that increases in local intraspecific competition shift the Turing-Hopf bifurcation to lower rainfall levels (Fig. 2.1).The stabilisation of the spatially uniform vegetated state is caused by a reduction in plant equilibrium density under strong local intraspecific competition which reduces the water requirements of the spatially uniform state.
intraspecific competition Latest Research Papers
Specific Regulation. Intraspecific competition at the larval stage is an important ecological factor affecting life-history, adaptation and evolutionary trajectory in holometabolous insects. However, the molecular pathways and physiological trade-offs underpinning these ecological processes are poorly characterised.
Interspecific competition affects the expression of ...
Intraspecific competition among individuals in a population can be an important driver of natural selection 1,2.Those individuals that are best adapted to local conditions, through their genotype ...
Intraspecific competition can generate species coexistence in ...
The exploration of mechanisms that enable species coexistence under competition for a sole limiting resource is widespread across ecology. One classical example is the coexistence of herbaceous and woody species in self-organised dryland vegetation patterns. Previous theoretical investigations have explained this phenomenon by making strong assumptions on the differences between grasses and ...
Interspecific and intraspecific competition as causes of direct and
Finally, by separating interspecific and intraspecific effects, existing hypotheses for the cycles can be challenged to explain interspecific as well as intraspecific density depen-dence. In particular, we may ask whether vole cycles are single-species or community-level phenomena (29, 30). In this paper, we focus on the strongly cyclic ...
Interspecific competition affects the expression of personality-traits
Intraspecific competition among individuals in a population can be an ... More research on naturally co-occurring species in a guild and how both intra- and interspecific interactions contribute to the selection of personality traits is mandatory to increase our insight in the role of interspecific competition in shaping individual variation in ...
On the Prevalence and Relative Importance of Interspecific Competition
In a strictly defined sample of competition studies using controlled field experiments, covering 215 species and 527 experiments, competition was found in most of the studies, in somewhat more than half of the species, and in about two-fifths of the experiments. In most of these experiments interspecific competition was not distinguished from intraspecific competition. In the few studies in ...

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Competition and coexistence in plant communities: intraspecific competition is stronger than interspecific competition

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Competition–density effect

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Interspecific competition affects the expression of personality-traits in natural populations

Maria Vittoria Mazzamuto

Introduction

Expert-based personality traits

Interspecific competition and personality

Relationships of personality with local survival and reproduction

Study design and trapping squirrels

Ethical approval for fieldwork with animals

Measuring personality

Statistical analysis