Islands on the central coast of British Columbia tend to be nutrient poor compared to the ocean water surrounding them. They also vary greatly in terms of their physical and spatial characteristics, such as area, isolation, shoreline slope, shoreline substrate, perimeter-to-area ratio, and topography. These features, among several others, are fundamental in determining each island’s ability to intercept and retain marine-derived nutrients, which mainly cross the land-sea interface through beach-cast seaweeds and river otter activity. Terrestrial birds are upper-level consumers in simple island food webs. As such, they integrate key trophic processes that are driven by both terrestrial and marine-derived nutrients, making them ideal test subjects to explore theories about how nutrients from the sea impact community ecology. We will investigate some of the complex links between marine and terrestrial ecosystems on the central coast of British Columbia by testing how classical island biogeography predictions of birds on islands are mediated by inputs of marine subsidies. We will determine how islands’ physical and spatial features predict patterns in terrestrial breeding bird ecology on 100 representative islands.
We have analyzed results for 5173 birds of 55 different species across 72 islands. The final 28 islands will be surveyed in summer 2017. Rarefied species richness estimates range from 1 species on our smallest island (124m2) to 23 species on several of our larger islands, supporting the classical island biogeography prediction that species diversity generally increases with island size (Adj R2 = 0.6). We find no significant trends regarding species richness and isolation, suggesting that isolation is less relevant to shaping bird communities on these islands than predicted by classical theory. This, in addition to strong positive correlations between bird diversity and certain estimators of marine influence, such as sea wrack biomass, shoreline width and slope, and island shape, suggests that marine inputs mediate patterns in bird ecology on these islands.
Overall, our results indicate strong correlations between measures of bird ecology and marine nutrient input, suggesting that the understudied pathway of nutrients from sea to land is one with substantial implications for community ecology on islands.