Islands provide some of the most dramatic examples of ecological and evolutionary phenomena. At the same time, island ecosystems are among the most anthropogenically disturbed habitats on Earth. Island avifaunas exemplify this pattern. It is estimated that nearly 90% of bird extinctions have occurred on islands while less than 20% of the world’s birds reside on islands. Compounding this biodiversity loss, islands have experienced 70% of avian introductions. The reshuffling of island bird biodiversity has resulted in dramatically altered communities.
Here, we quantify the effect of invasions and extinctions on Phylogenetic Diversity (PD) for 152 oceanic islands from the historic time period (i.e., before European colonization) to the current time period. We developed a global island bird phylogeny based on species lists and then measured how species losses, invasions, and the combination of both altered PD at different spatial scales (island, archipelago, and ocean). We then related patterns of PD to island characteristics such as island size, isolation, topography, and the number of invasions and extinctions.
Results/Conclusions
We found that extinctions and invasions have resulted in island bird communities with lower PD than what would be expected at random. The pattern holds for invasions and extinction separately and in combination at the island, archipelago, and ocean scale. Islands are losing non-randomly long branch lengths and gaining non-randomly short branch lengths. Islands that have a longer colonization history and are less isolated tend to have lower PD than expected due to extinctions. Islands that are less isolated and have a greater number of exotic species tend to have lower PD than expected due to invasions. Overall, we show that non-random patterns of invasion and extinction have combined to reduce the PD of island bird communities.