Closing the loop: Canada geese on islands exert bottom-up effects on intertidal communities and trigger top-down effects by Pisaster ochraceus
The impacts of cross-ecosystem nutrient flow from marine systems to island and coastal communities are well documented. However, allochthonous nutrients may occasionally run off to the near-shore environment where they could also stimulate population and community level responses. Seabird nutrient impacts on intertidal algae and emergent insects are documented in a few studies, and grazing waterfowl impacts on freshwater systems are widely understood. In the San Juan Islands of Washington, U.S.A., Canada geese (Branta canadensis) graze on larger islands and on marine algae and return to small, satellite islands to nest and roost. Geese deposit large quantities of feces on these islands, which run off into intertidal zones during rains. We investigated the potential impacts of those nutrient inputs and particulate matter on intertidal communities. We predicted that intertidal zones below high-density goose areas would support higher abundance and richness of invertebrates. We also predicted that the presence of Pisaster ochraceus as a high trophic level predator would alter the abundance or richness patterns. In 2012-14 we sampled three to eight 0.25 m2 quadrats around 13 small (<3 ha) islands, noting uphill presence or absence of goose activity and if P. ochraceuswas present or absent.
Quadrats in intertidal zones below high goose density areas had 54% higher abundance of invertebrates when including barnacles in the counts, and 33% lower abundance when excluding barnacles compared to quadrats below areas without goose activity. Order-level richness did not vary between zones below areas of high goose activity and areas without goose activity. Where P. ochraceus was present, and they were only present below some areas of high goose activity but not all, total abundance of all invertebrate species including barnacles was 284% higher than when they were absent. However, when excluding barnacles from the count, abundance of all other invertebrates was 45% lower when P. orchraceus was present than when it was absent. Richness did not differ between zones with and without P. orchraceus. In summary, Canada geese feeding on larger islands or in the ocean deposit guano on small islands that then runs off back into the intertidal zone, exerting a bottom-up effect on population growth of barnacles and attracting P. ochraceus. However, those bottom up effects are not evident for other intertidal invertebrates, which may be explained by much stronger top down effects by P. orchaceus.