Competition and habitat filtering shape Neotropical canopy butterfly communities across a rainfall seasonality gradient

The stability hypothesis posits that stable environments provide species with predictable resources, where mechanisms that limit similarity, such as competition, are able to drive niche specialization and shape community composition. By contrast, variable and unpredictable environments are expected to allow only those species with appropriate adaptive traits to persist, the process of habitat filtering. To date, only a limited number of studies have tested these hypotheses, especially for tropical insects, which dominate terrestrial biodiversity, and whose spatial patterns of diversity, and the mechanisms driving them, remain poorly explored. Here, we used a combined approach of phylogenetic- and functional trait-based analyses to test these hypotheses across a rainfall seasonality gradient, examining whether tropical butterfly communities are structured more by competition in less seasonal rain forests, and conversely more by habitat filtering in seasonal dry forests. A 3-year survey was carried out at three sites across a climatic gradient in western Ecuador. Canopy butterflies were sampled using traps baited with rotting banana and shrimp every 2 months from November 2010 to September 2013, and forewing length and aspect ratio were measured from adults as potential functional traits. A total of 4075 specimens representing 126 species of the families Nymphalidae and Hesperiidae were included in the analyses. We found strong evidence that both assembly mechanisms, namely habitat filtering and competition, shape butterfly communities in both seasonal and aseasonal forests. In general, significant levels of habitat filtering were inferred from phylogeny-based analyses. Trait-based analyses, however, showed that competition shapes the distribution of traits within communities. Our interpretation is that while canopy butterfly lineages are broadly adapted to differing rainfall seasonality regimes, leading to phylogenetic clustering, within each environment, there can be competition among relatively closely related species for available resources, leading to an over-dispersion of traits.