Chance and necessity in the assembly of plant communities: Stochasticity increases with size, isolation and diversity of temporary ponds

Biodiversity emerges from niche mechanisms, in which the combination of traits determines species performance, and populations drift because of the inherent stochasticity of community assembly processes. Population biology dictates that small and isolated communities are more prone to show stochastic assemblages. However, a reduced mass effect in isolated communities may promote trait selection. In addition, large and connected communities have a larger species pool, higher functional redundancy, lower population sizes and more random recruitment, which also fosters stochasticity in community assembly. These contradictory expectations demand empirical analyses.Plant metacommunities in temporary ponds are assembled by the action of strong environmental filters and cover wide ranges of local community sizes and connectivity, representing ideal systems for identifying determinants of trait-selection processes. Using a deviance partition method introduced by the theory of community assembly by trait selection, we evaluated the role of plant traits in local community assemblies along 60 communities from a 14-year plant survey of temporary ponds.Variation in pond size, hydroperiod, connectitivity and heterogeneity determined a selection gradien in traits related to drought resistance, life history and disperal strategies; and also in the strength of trait-mediated community assembly. The taxonomic and functional diversity of a pond and its physical heterogeneity fostered stochasticity in the assembly of the community, which also presented a hump-shaped association with connectivity. The pond area increased taxonomic richness but decreased functional diversity, determining negative and positive indirect effects on stochasticity.Synthesis. Diversity provides the raw material for trait selection putatively reducing stochasticity, but here diversity was positively related to stochasticity. Having enough functional diversity, larger redundancy and lower population sizes in diverse communities is probably fostering stochastic assemblages. The hump-shaped association between stochasticity and connectivity supports a larger role of trait selection in isolated systems due to a weak mass effect, but also on connected communities in which a set of more optimal traits for the selection scenario could be available. In the ongoing state of ecosystem fragmentation, these empirical trends contribute to the mechanistic understanding of the connection between landscape structure and biodiversity assembly.