Focusing on individual species reveals the specific nature of assembly mechanisms in a tropical dry-forest

Abstract

We employed an individual-species approach based on the plant’s eye perspective to disentangle the effects of individual species on community assembly in a dry tropical forest of southern Ecuador. We completely mapped a forest plot of 9 ha, and measured several functional traits (leaf area, specific leaf area, wood density, seed mass and maximum height) for tree and shrub species. To account for stochastic and habitat filtering effects, we fitted spatial point processes for the 23 more abundant species in the plot, which confirmed that all species responded to plot scale habitat filtering and 14 were dispersal-limited. We tested the hypothesis that facilitative interactions would be prevalent in this dry forest. For this, we compared the distribution of taxonomic (TD), functional (FD) and phylogenetic (PD) diversity in the neighborhood of the studied species with the diversity expected under a null model combining habitat filtering and stochastic assembly. We found that in the fine spatial scales where species interactions are expected to occur (i.e., neighborhoods of 1–20 m) eight species did not show any significant pattern for TD, FD or PD. Eleven species showed evidences of facilitation (i.e., accumulated more TD than expected) but in some cases the facilitated neighborhoods had more FD or PD than expected, suggesting the joint effect of facilitation and competition based on niche differences. One species showed less TD than expected, accompanied by lower FD and higher PD, suggesting competition based on fitness differences.

Our study shows that in this dry tropical forest, where abiotic stress is prevalent, the assembly of diversity is controlled by environmental heterogeneity and both facilitative and competitive biotic processes, all of them acting simultaneously and at the same scale in the same neighborhoods.