Functional niche occupation and species richness in herbaceous plant communities along experimental gradients of stress and disturbance.

Abstract

The patterns of niche occupation in functional trait space have been widely studied to understand the processes of community assembly, but are rarely linked to environmental conditions (here, stress and disturbance). In this study, we investigate (1) how the pattern of functional niche occupation, incorporating intraspecific trait variation and covariation, varies along experimental gradients of stress and disturbance, (2) whether habitat filtering and/or limiting similarity modify the pattern, and (3) whether their strength varies as a function of species richness or levels of stress and disturbance. We constructed an experimental system consisting of 24 herbaceous plant mesocosms under different levels of stress and disturbance, and measured ten traits on five individuals for each species in each mesocosm. We quantified the total functional niche volume occupied by an entire mesocosm, the functional niche overlap among species within a mesocosm and the average functional niche volume occupied per species, and investigated how these metrics varied from species-poor to species-rich mesocosms along gradients of stress and disturbance. Species richness and functional niche overlap correlated positively to disturbance at low and medium levels of stress, but peaked at the intermediate level of disturbance when stress was high. The total functional niche volume and average functional niche volume did not change significantly along these gradients. Compared to null models, each mesocosm occupied a smaller total functional niche volume (habitat filtering) and the species within each mesocosm overlapped less and were more functionally specialized (limiting similarity). Moreover, the standardized metrics (to the null expectations) did not change significantly under different levels of stress and disturbance. This experimental evidence shows that both habitat filtering and limiting similarity determine the patterns of functional niche occupation and species richness, but their strength does not change along environmental gradients of stress and disturbance.