Sexes and species as rival units of niche saturation during community assembly

Abstract

AbstractAimCommunity assembly is traditionally assumed to result from speciation and colonization mediated by available niche space. This paradigm is expanded by the theory that niche space can also be saturated by intersexual adaptive divergence (ecological sexual dimorphism) when interspecific competition is relaxed. This theory (here termed ‘niche‐packing equivalence’) predicts that the evolution of ecological sexual dimorphism constrains the ecological opportunity that would otherwise lead to ecological speciation or colonization, and that saturation of niches by different species constrains divergent selection for divergence between the sexes. Therefore, sexes and species are equivalent, yet antagonistic units of niche occupation. We present the most comprehensive test of the niche‐packing equivalence theory at ecological time‐scales (assemblage level) to date.LocationSouth AmericaMajor taxa studiedLiolaemuslizards.MethodsWe identified 23Liolaemusassemblages varying in species richness and sexual size dimorphism (SSD), distributed across a wide environmental range. We used mixed effects models, permutation tests and Markov Chain Monte Carlo (MCMC) regressions to quantify the relationship between SSD and species richness. We then partitioned the body size niche dimension between the sexes and amongst species, and tested for non‐overlapping body size distributions. We regressed SSD and species richness of each assemblage against environmental predictors, using multi‐model inference and structural equation modelling.ResultsSexual dimorphism declines with increasing species richness, and a strong signal of tension between the two remains following phylogenetic control. This pattern is accompanied by evidence of constraints on body‐size partitioning amongst species and between the sexes: the two units of niche saturation tend not to overlap. However, across assemblages, species richness and SSD correlate with different environmental variables, suggesting that their tension is context‐specific.Main conclusionsOur evidence supports the prediction that sexual dimorphism and species richness are alternative outcomes of adaptive radiation. However, this antagonism is mediated by a suite of environmental predictors that influence dimorphism and species richness differentially.