Land-bridge islands created by dam constructions can be viewed as ideal 'natural laboratories' to explore patterns of species diversity and community composition in shaping community assembly. However, the classic island biogeography theory (IBT) proposed by MacArthur & Wilson considers all species being equally different from each other and thus disregards species’ different ecological functions. Thus, exploring taxonomic and functional aspects of biodiversity simultaneously can better understand the processes of community assembly.
Here, we tested the predictions of the IBT and its extended hypotheses about target and rescue effects, and analyzed taxonomic and functional alpha and beta diversity to examine community assembly using long-term bird data collected on land-bridge islands in the Thousand Island Lake, a large man-made reservoir in China.
In our study, breeding birds were surveyed on 37 islands annually since 2007. We estimated the effects of area, isolation on the frequencies of colonization and extinction events using multivariate logistic regression. We then decomposed beta diversity into spatial turnover and nestedness-resultant components, and related taxonomic and functional dissimilarities to island variables using multiple regression models on distance matrices. Randomization tests were used to assess the strength of the correlations between taxonomic and functional diversity.
Results/Conclusions
Our results revealed high species temporal turnover of bird communities on study islands, and found taxonomic and functional alpha diversity of birds have clear relationships with island area, but not isolation. The taxonomic nestedness-resultant and turnover components increased and decreased with difference in area, respectively, but functional counterparts did not. By partitioning beta diversity, we found low levels of overall taxonomic and functional beta diversity among islands (i.e. spatial turnover). The functional nestedness-resultant component dominates overall functional beta diversity, whereas taxonomic turnover is the dominant component for taxonomic beta diversity. The simulation showed that functional alpha and beta diversity were significantly correlated with taxonomic diversity, and the observed values of correlations were significantly different from null expectations of random extinction.
Our assessment of island bird assemblages validated the predictions of area and target effects, but not distance and rescue effects due to the relatively small separations between islands (1.47 ± 0.86 km) and the strong dispersal ability of most birds. The simulation suggested that selective extinction, the deterministic process of environmental filtering, drives taxonomic and functional diversity. The contrasting turnover and nestedness-resultant components of taxonomic and functional beta diversity demonstrate the importance of considering the multifaceted nature of biodiversity when examining community assembly.