ETIB-T: an Equilibrium Theory of Island Biogeography for plant traits

Aim: The Equilibrium Theory of Island Biogeography (ETIB) posits that species richness on islands represents a dynamic equilibrium between immigration and extinction. ETIB makes predictions about numbers of species and biogeographical rates, but not about species identities or functional traits. However, functional traits provide additional information in understanding the assembly of island biotas. Here, we build on ETIB's principle of community equilibria and investigate how these processes affect plant functional traits over time. 

Location: Fifteen islands from 164 m² to 19 km² in Western Australia. 

Taxon: Angiosperms. 

Methods: We assembled an island-trait dataset linking seed mass, plant height and leaf area of 156 species to their occurrences on 15 islands sampled four times within four decades. We estimated community trait means and functional diversity for each island and sampling period and tested whether both remained at equilibrium over time. Using linear models, we tested whether temporal species turnover is linked to specific traits. We used generalised linear mixed-effect models to test for the effect of environmental characteristics on species and trait turnover. 

Results: Species richness on the islands was at equilibrium as predicted by ETIB despite high species temporal turnover. Functional diversity and community trait means also were stable over time. Species most susceptible to turnover were on average smaller and had lower seed mass than persisting species. Environmental island characteristics had no strong effect in explaining species and trait turnover. 

Main conclusions: We provide evidence that ETIB can be extended to functional traits, which we suggest to term Equilibrium Theory of Island Biogeography for Traits. Trait equilibria on islands suggest that locally extinct species are replaced by new ones sharing similar traits. Being small with light seeds increases both immigration probability and extinction risk.