Competition In Natural Communities Research Articles

Examining the link between competition and negative co‐occurrence patterns

Negative species co‐occurrence patterns have long intrigued ecologists because of their potential link to competition. Although manipulative field experiments have consistently revealed evidence of competition in natural communities, there is little evidence that this competition produces negative co‐occurrence patterns. Evidence does suggest that abiotic variation, dispersal limitation and herbivory can contribute to patterns of negative co‐occurrence among species; it is possible these influences have obscured a link with competition. Here, we test for a connection between negative co‐occurrence and competition by examining a small‐scale, relatively homogeneous old‐field plant community where the influence of abiotic variation was likely to be minimal and we accounted for the impact of herbivory with an herbivore exclosure treatment. Using three years of data (two biennial periods), we tested whether negatively co‐occurring pairs of species, when occasionally found together, experienced asymmetric abundance decline more frequently than positively co‐occurring pairs, for which there is no such expectation. We found no evidence that negatively co‐occurring pairs consistently suffered asymmetric abundance decline more frequently than positively co‐occurring pairs, providing no evidence that competition is a primary driver of negative co‐occurrence patterns in this community. Our results were consistent across control and herbivore exclosure treatments, suggesting that herbivores are not driving patterns of negative species co‐occurrence in this community. Any influence of competition or herbivory on co‐occurrence patterns is small enough that it is obscured by other factors such as substrate heterogeneity, dispersal and differential species responses to climatic variation through time. We interpret our results as providing evidence that competition is not responsible for producing negative co‐occurrence patterns in our study community and suggest that this may be the case more broadly.

Intransitive competition is common across five major taxonomic groups and is driven by productivity, competitive rank and functional traits

Abstract Competition can be fully hierarchical or intransitive, and this degree of hierarchy is driven by multiple factors, including environmental conditions, the functional traits of the species involved or the topology of competition networks. Studies simultaneously analysing these drivers of competition hierarchy are rare. Additionally, organisms compete either directly or via interference competition for resources or space, within a local neighbourhood or across the habitat. Therefore, the drivers of competition could change accordingly and depend on the taxa studied. We performed the first multi‐taxon study on pairwise competition across major taxonomic groups, including experiments with vascular plants, mosses, saprobic fungi, aquatic protists and soil bacteria. We evaluated how general is competition intransitivity from the pairwise competition matrix including all species and also for each possible three‐species combination (triplets). We then examined which species were likely to engage in competitive loops and the effects of environmental conditions, competitive rank and functional traits on intransitive competition. We found some degree of competition intransitivity in all taxa studied, with 38% to 5% of triplets being intransitive. Variance in competitive rank between species and more fertile conditions strongly reduced intransitivity, with triplets composed of species differing widely in their competitive ranks much less likely to be intransitive. Including functional traits of the species involved more than doubled the variation explained compared to models including competitive rank only. Both trait means and variance within triplets affected the odds of them being intransitive. However, the traits responsible and the direction of trait effects varied widely between taxa, suggesting that traits can have a wide variety of effects on competition. Synthesis. We evaluated the drivers of competition across multiple taxa and showed that productivity and competitive rank are fundamental drivers of intransitivity. We also showed that not only the functional traits of each species, but also those of the accompanying species, determine competition intransitivity. Intransitive competition is common across multiple taxa but can dampen under fertile conditions or for those species with large variance in their competitive abilities. This provides a first step towards predicting the prevalence of intransitive competition in natural communities.

The ghost of hybridization past: niche pre-emption is not the only explanation of apparent monophyly in island endemics.

Summary 1 Published molecular phylogenies show that many plant groups in the Canary Islands are monophyletic despite the fact that the short distance between the islands and Africa should have led to many independent colonization events. 2 Low establishment rates of later migrants owing to niche pre-emption by earlier, already established, colonists could explain these patterns. The apparent monophyly is, however, also compatible with multiple colonizations, with later colonizers making only limited contributions to the total gene pool (and therefore being undetected in the molecular phylogeny) or being wiped out by stochastic processes. 3 Experimental evidence for niche pre-emption and species-specific interactions in plants is weak, with survival and establishment of a newly immigrant species depending on the overall composition of the community, rather than on the presence of a particular ‘ecologically similar’ species. 4 Although niche pre-emption might have contributed to the observed patterns of monophyly, we do not think that phylogeographical data from Macaronesia can be taken as evidence for its action in the geological past.

Is the assembly of ant communities mediated by parasitoids?

Studies of species interactions in ant communities have been a major source of evidence for the importance of interspecific competition in natural communities. One consequence of the overwhelming evidence for competition in ant communities is that the role of such “top‐down” processes as predation and parasitism has been ignored. Recent evidence, however, suggests that the composition and dynamics of ant communities are influenced by highly specialized parasitoids that mediate the outcome of competition among ant species. Here I review this evidence and develop a general framework for integrating the roles of competition and parasitism in the assembly of ant communities. I then use invasions by the red imported fire ant (Solenopsis wagneri) and the Argentine ant (Linepithema humile) to show how this new framework can be used to develop testable hypotheses regarding the ecological success (or failure) of invasive ant species.

Assessing the Impact of Competition on Community Assembly: A Case Study using Small Mammals

Competition has been shown to be an important factor in natural systems. Yet, its effect on community structure remains poorly understood and is often disputed. Null models have been employed by researchers in recent years, but these generally suffer from a lack of power estimation and unclear distinction between statistical significance and biological importance. Models that employ an appropriate alternative hypothesis allow for powers estimates and for quantitative evaluation of the effects of competition. We have developed methodologies to (1) allow for testing the significance of competition in natural communities, (2) determine the power of these estimates, and (3) produce maximum likelihood estimates of interaction parameters or these communities. Using these methologies we show that small mammal communities at the interface between Valdivian temperate rain forest and Patagonian steppe in southern South America appear significantly and importantly structured by competition, although we cannot distinguish between resource competition, interference competition, and apparent competition. Secondarily, we show that the geographic origin of the component species (Valdivian vs. Patagonian) is relatively more important than their habitat affinities in structuring these communities.

Simulation of responses of community structure to species interactions driven by phenotypic change

We developed a community model which considers the effect of phenotype on species interaction in order to elucidate general relationships between phenotype, species interactions and community structure. The model consists of community matrices with density-dependent species interactions and a limit on the total population of each species. A general natural pattern forms the basis of the model: organisms which are most similar compete, those which are less similar may enter into a predator-prey relationship, and those which are most different may be mutualistic. In this model, contrary to some previously published studies, increasing the number of species increased stability of community matrices. The rate of change in phenotype was varied to assess the response of community characteristics. At the highest rate of random phenotypic change there were the most species; the minimum number of species was found at intermediate rates of change. The models were run in the absence of predation, competition, or mutualism. The absence of mutualism led to more extinction. The absence of competition caused communities to quickly reach a population ceiling. Results suggest that it is extremely important to understand the relative occurrence of mutualism and competition in natural communities. Furthermore, the rate of phenotypic change could be an important but subtle determinant of community structure.

Patterns and Consequences of Interspecific Competition in Natural Communities: A Review of Field Experiments with Plants

We present a quantitative literature review to assess the extent to which field experi- ments with plants have addressed questions about patterns of competition over time and space, consequences of competition for community structure, and comparisons of competitive ability among species. We outline the necessary treatment comparisons and statistical analyses to answer each question and then describe the number of experiments that meet these criteria and their results. Although we found a total of 101 experiments in 89 studies, 63% of these experi- ments only addressed whether competition significantly affected some component of individual fitness of a single species at a single time and site. Despite the limited data base to address more complex questions about competitive interactions, we did find consistent results for a few of the questions we reviewed. Where tested, competition always had significant effects on distribution patterns (five experiments), on relative abundances (two experiments), and on diversity (four experiments), consistent with the notion that competition has strong effects on community structure. On the other hand, intraspecific competition was not usually stronger than interspe- cific competition for either competitive effect (four experiments) or response (three experi- ments), which suggests that resource partitioning may not be an important mechanism of coexis- tence in plants.

A comparative approach to predicting competitive ability from plant traits

Decades of study of interspecific competition in community ecology has yielded an overwhelming body of special cases but few general principles1–3. This is largely because of the phenomenological, non-predictive approach used4. Further progress requires a predictive approach5 that will enable general principles to be deduced that apply beyond the species and conditions of a particular study or site. General principles are best sought using a comparative approach, that is, the systematic screening of a large number of species under standardized experimental conditions6,7 We used 44 wetland plant species to test whether competitive ability could be predicted from plant traits. Multiple linear regression showed that there was a strong relationship between plant traits and competitive ability (r2 = 0.74). Plant biomass explained 63% of the variation in competitive ability and plant height, canopy diameter, canopy area and leaf shape explained most of the residual variation. This study represents a major step in escaping the current phenomenological approach to competition in community ecology, and provides a general predictive tool for studying competition in natural communities.

Seasonally fluctuating resources and temporal variability of interspecific competition.

The influence of seasonal availability of two critical resources (food and substrates from which food was harvested) on interspecific competition between striped surfperch (Embiotoca lateralis) and black surfperch (Embiotoca jacksoni) was examined. There was a strong depth-related gradient in density of prey and in cover of foliose algae; both declined with increasing bottom depth. Density of prey was reduced 5-10 fold during the winter season, but cover of substrates remained constant throughout the year. Although both fishes co-occurred throughout the same depth range, striped surfperch were more common in shallow habitats and black surfperch were more abundant deeper. Local abundance and distribution patterns of both surfperch species did not change seasonally. Stepwise regression analyses suggested that availability of favored substrates was a proximate influence on local patterns of surfperch distribution and abundance, and that interspecific competition depressed abundance of the two species to the same degree. Removal experiments conducted during the cold-water season revealed that interspecific competition influenced depth distribution of black surfperch but not striped surfperch. Seasonal change in density of prey was accompanied by marked changes in overlap in use of foraging substrates by the surfperches. The pattern of change in interspecific overlap suggested that surfperch competed for food only when prey were seasonally scarce. There was no difference in the agonistic tendencies of the two fishes, and the absolute and relative frequency of interspecific chases was independent of food level. These results have important implications regarding the impact of temporal variability of interspecific competition in natural communities. In the surfperch system, competition was characterized by constant and time-varying elements that had symmetrical and asymmetrical effects and involved both interference and exploitation mechanisms.