Resource competition and coexistence in heterogeneous metacommunities: many-species coexistence is unlikely to be facilitated by spatial variation in resources.

Donald R Schoolmaster

doi:10.7717/peerj.136

Abstract

There is little debate about the potential of environmental heterogeneity to facilitate species diversity. However, attempts to show the relationship between spatial heterogeneity and diversity empirically have given mixed results. One reason for this may be the failure to consider how species respond to the factor in the environment that varies. Most models of the heterogeneity-diversity relationship assume heterogeneity in non-resource environmental factors. These models show the potential for spatial heterogeneity to promote many-species coexistence via mainly the spatial storage effect. Here, I present a model of species competition under spatial heterogeneity and resource factors. This model allows for the stable coexistence of only two species. Partitioning the model to quantify the contributions of variation-dependent coexistence mechanisms shows contributions from only one mechanism, growth-density covariance. More notably, it shows the lack of potential for any contribution from the spatial storage effect, the only mechanism that can facilitate stable many-species coexistence. This happens because the spatial storage effect measures the contribution of different species to specializing on different parts of the gradient of the heterogeneous factor. Under simple models of resource competition, in which all species grow best at high resource levels, such specialization is impossible. This analysis suggests that, in the absence of additional mechanisms, spatial heterogeneity in a single resource is unlikely to facilitate many-species coexistence and, more generally, that when evaluating the relationship between heterogeneity and diversity, a distinction should be made between resource and non-resource factors.

Highlights

The potential of spatial heterogeneity to promote plant species coexistence is well documented theoretically, but empirical support documenting the power and scope of its ability to support diverse natural communitiesHow to cite this article Schoolmaster Jr (2013), Resource competition and coexistence in heterogeneous metacommunities: manyspecies coexistence is unlikely to be facilitated by spatial variation in resources
One potential reason for the uncertainty observed in the relationship between plant species diversity and spatial environmental heterogeneity is that the strength of the effect depends on what aspect of the environment is varying; whether it is resource or non-resource factors that vary over space
I derive approximate analytical relationships for regional species coexistence from which metacommunity-scale population growth rates may be partitioned into the variation-dependent and variation-independent coexistence mechanisms. These mechanisms are used to argue why spatial variation for resources is less effective than non-resource spatial variation in facilitating coexistence of many species. The goal of this model is to answer the questions “how many species can coexist via spatial heterogeneity in resource supply and by what means?” To answer these questions, I define a simple model of plants growing in a spatially heterogeneous environment, and use the framework developed by Chesson (1994) to partition the regional growth rates implied by the model into contributions from variation-independent and variation-dependent mechanisms

Summary

Introduction

The potential of spatial heterogeneity to promote plant species coexistence is well documented theoretically (reviewed in Amarasekare, 2003), but empirical support documenting the power and scope of its ability to support diverse natural communitiesHow to cite this article Schoolmaster Jr (2013), Resource competition and coexistence in heterogeneous metacommunities: manyspecies coexistence is unlikely to be facilitated by spatial variation in resources. To understand why resource heterogeneity may less effectively facilitate species diversity requires insight into how these factors affect variation dependent species coexistence mechanisms (Chesson, 2000a).

Results

Conclusion