Extinctions and the loss of ecological function in island bird communities

Abstract Aim Because of the negative impact that ongoing biodiversity loss may have on ecosystem properties that are critical for humans, understanding the relationship between extinction and functional diversity over time is of critical importance for conservation. However, empirical evidence concerning the sensitivity of vertebrate community function to species loss is very limited. Here we assess documented prehistoric and historic extinctions of birds on P acific islands in an effort to quantify the consequences of extinctions for functional structure and diversity in natural communities over broad spatial scales. Location Forty‐four islands from across the P acific. Methods We estimated functional aspects of island bird communities before and after Holocene extinctions based on body size, foraging niche, diet and activity period. We used four separate metrics to measure ecological function: functional diversity ( FD ), functional richness ( FRic ), functional evenness and functional divergence. We employed null models to separate the effects of observed extinctions from changes expected due to declining species richness. Results We find that Holocene bird extinctions led to substantial changes in community‐level functional diversity. Observed declines in FD and FRic were predictable from the pre‐extinction composition of communities, and did not differ from null model expectations. Across all islands, we observed non‐random changes in functional trait composition, with shifts away from ground‐level foraging, granivory and herbivory after extinctions. Extinctions have resulted in the loss of up to 80% of original functional diversity on some islands and caused a sharp decrease in the variety of ecological functions provided by birds. Main conclusions Our findings illustrate the significant losses of functional diversity that are already taking place on many islands and demonstrate its close connection with the loss of species. Accounting for the functional roles of species allows a more integrative understanding of ecological function and helps to bridge species and ecosystem perspectives in conservation science.