The focus of metacommunity models has largely been from a species sorting or patch-dynamic perspective, where species differences – like spatial or environmental niches – facilitate coexistence. In terms of community patterns, however, the possibility for species differences is considered alongside the possibility for species equivalence, usually invoked as two competing hypotheses: the Heterogeneity-Diversity Relationship (HDR) hypothesis posits that if species have niche differences, diversity will be related to the heterogeneity, and not the mean, of abiotic conditions. The alternative – the Available Energy (AE) hypothesis – posits that if species share a similar niche, that diversity will be related to the mean, not the heterogeneity, of abiotic conditions. Perhaps surprising for many theoretical ecologists is the abundance of empirical support for the AE hypothesis in plant communities. Motivated by these studies, we used a spatially explicit simulation model to investigate plant community patterns when species showed niche similarity along one environmental gradient, and niche differentiation along another. Within landscapes, the role of niche similarity vs. differentiation was assessed by allowing communities to vary along a continuum of complete equivalence to complete differentiation. The role of spatial scale was assessed by varying dispersal rates, and the spatial autocorrelation of each abiotic factor independently.
Results/Conclusions
When plant communities responded differently to more than one abiotic factor – e.g. showing niche differentiation along one axis and niche similarity along another – the correlation between the abiotic mean and abiotic heterogeneity predicted simulated community patterns better than when either measure was used alone. These results were sensitive to changes in plant responses along each gradient, and limiting dispersal rate obscured trends as species lost their ability to track abiotic conditions via universal dispersal.
Based on previous observational studies and other empirical work, it seems likely that species will show niche similarity along some abiotic gradients, while differentiating along others, but it is unrealistic to assume that environmental factors are varying at the same spatial scales. For this reason we are hopeful that these results can be used to advance other areas of research, particularly since previous studies show that extinction debts and thresholds will be sensitive to abiotic conditions.