Tailoring species abundances distributions with trait-environment correlations
The pattern of few abundant and several rare species is considered a general law in ecology. Since both niche and neutral models can explain species abundances distributions (SADs), incorporating species differences in relation to its traits can help us understand to what extent functional divergence and equivalence can affect SADs. We used a trait-environment approach to understand which ecological strategies of species, if any, affect SADs in a fern metacommunity along three mountain ranges in a Brazilian Atlantic rainforest. We simultaneously tested hypothesis for purely neutral, purely niche and combination of both processes based on the emergent group (EG) approach. EG represents a combination of groups of species defined by functional divergence that within behave as functional equivalents. We built statistical models using generalized linear mixed models (glmm) to represent our hypothesis. Our framework is based on the idea that we can translate neutral and niche processes into random and fixed effects in glmm's. We modeled the effect of dispersal limitation and EG as random effects and used fixed effects to represent niche influences due to species ecological strategies and the altitudinal gradient. We used model selection to test 35 models representing niche, drift, EG and EG with drift processes.
Based on the best model we found that species abundances are defined by a hierarchical combination of niche and neutral processes. We identified that combination of different states of laminar thickness and life form defines an ecological strategy and the EG. Although some ecological strategies are related to high abundance values in a particular altitudinal level, we still found variance in abundance among fern species sharing the same ecological strategy. Variance was explained by random effects representing functional equivalence within EG and limited dispersal among altitudinal levels and mountain ranges. Given that there are tradeoffs of an EG with its position on the gradient, we attribute it to strategies for reducing water loss across the altitudinal gradient. Additionally, we found that abundance predicted by our model fitted perfectly with SADs on each altitudinal level. Finally, on the niche side, we defined EG in fern community based on synthetic and objective traits. On the neutral side, our modelling approach unpack neutrality in drift and limited dispersal mechanisms. By adding information of species traits we do not rejected the idea of functional equivalence of species, we refined the scope of the effect of neutral processes in terms of EG and limited dispersal.