Monday, August 4, 2008 - 3:40 PM

COS 6-7: Impact of marine subsidies on coastal food webs in the Bahamas

David A. Spiller1, Jonah Piovia-Scott1, Amber N. Wright1, Louie H. Yang2, Gaku Takimoto3, and Thomas W. Schoener1. (1) University of California, Davis, (2) University of California, Santa Barbara, (3) Toho University


The importance of materials or organisms transported across ecosystem boundaries has received much recent attention.  We are investigating the effects of significant seaweed-deposition events, caused by hurricanes and other storms, on species inhabiting subtropical islands.  The seaweed represents a pulsed resource subsidy that is consumed by amphipods and flies, which are eaten by lizards and predatory arthropods; those predators also consume terrestrial herbivores.  Additionally, seaweed decomposes directly into the soil.  We added seaweed to six shoreline plots and removed seaweed from another six plots; all plots were repeatedly monitored for 12 months after the initial manipulation.


Lizard density responded rapidly and became 57% higher in subsidized than in removal plots.  Web-spider density tended to be higher in subsidized than in removal plots after 8 months, but the overall difference between treatments was not statistically significant.  Leaf damage was 70% higher in subsidized than in removal plots after 8 months, but subsequent damage was about the same in the two treatments.  Foliage-growth rate was 70% higher in subsidized plots after 12 months.  Results of a complementary study on the relationship between natural variation in marine subsidies and island food-web components are qualitatively consistent with the experimental results.  We suggest two causal pathways for the effects of marine subsidies on terrestrial plants: 1) the "fertilization effect" in which seaweed adds nutrients to plants, increasing their growth and susceptibility to herbivores, and 2) the "predator-switching effect" in which lizards switch from eating local prey (including terrestrial herbivores) to eating mostly marine detritivores.