The organization of biological communities is driven by deterministic and stochastic processes, and the relative importance of each varies with patch size. Smaller patches are more strongly affected by demographic stochasticity, leading to lower diversity within a patch and greater diversity between patches (alpha and beta diversity respectively). The effects of each process on traits are not well understood, however, in part because deterministic processes can have opposing effects on trait variation. For example, competitive processes and trait-environment fitness surfaces can either select for an optimal trait value with increased fitness, or greater trait variances with limited functional overlap. Trait-based metrics may also fail to detect stochasticity at the species level due to functional redundancies within communities.
We tested the effects of deterministic processes and ecological drift on diversity and functional variation within and among communities. To influence the importance of drift, we manipulated patch size by four to sixteen times across 168 experimental communities and measured ten functional traits from 72 plant species. Because species richness varies with community size and has a known statistical effect on trait variation, we used sampling controls (nested plots of identical sizes) and statistical corrections to distinguish the effects of each process.
Results/Conclusions
Patch size caused deterministic shifts in traits and species diversity within and among communities, as well as changes consistent with ecological drift. Variation in species abundances among patches (beta diversity) was highest in small communities, a pattern consistent with drift. This pattern was only partially observed for functional variation, however. Among-patch variation in trait means and variances tended to be higher in larger communities and better explained by abiotic variables, such that R2 values increased with patch size.
Within-patch (alpha) dynamics also showed a clear signature of deterministic effects on diversity and trait variation. Diversity was highest per unit area in small patches, a trend inconsistent with ecological drift alone. Trait means and variances also depended on patch size, with the largest variances found in large patches once confounding effects of species richness were accounted for. Although this result was consistent with ecological drift, its hypothesized cause (increased diversity) was not supported.
Overall, ecological drift had a greater impact on species than traits, with distinct consequences for trait means and variances. These differences between functional and compositional responses challenge the use of functional traits in lieu of species identity for community ecologists.