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.
Results/Conclusions
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.
