Multiple effects of hurricane-induced marine subsides on island food webs

Background/Question/Methods

We are investigating the effects of significant seasonal 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, which in turn consume terrestrial herbivores. Additionally, seaweed decomposes directly into the soil under plants. Our research focuses on how pulsed marine subsidies directly and indirectly affect various components of the terrestrial food web. We conducted a 12-month-long field experiment in which seaweed was manipulated in shoreline plots on large islands and a complementary survey of unmanipulated plots experiencing natural variation in seaweed deposition. We will also present preliminary results of an ongoing experiment designed to test how the amount and temporal frequency of pulsed resource subsidies affect food-web processes on small islands. This experiment will run for 5 years using a crossed two-factor design; the first factor is amount and frequency of seaweed deposited on islands and the second factor is presence/absence of lizards.

Results/Conclusions

In the large-island experiment, lizard density was 63% higher in plots with seaweed added than in control plots. Stable isotope analysis revealed a shift in lizard-diet composition towards more marine-based prey in subsidized plots. Leaf damage was 70% higher in subsidized plots after 8 months, but subsequent damage was about the same in the two treatments. Foliage growth rate was 70% higher and nitrogen content was 17% higher in subsidized plots after 12 months. We suggest three sequential effects caused by a pulse of marine subsidies on terrestrial plants: 1) "predator-diet-shift" in which lizards shift from eating local prey (including terrestrial herbivores) to eating mostly marine detritivores, 2) “predator numerical response” in which marine subsidies increase predator reproduction, and 3) "fertilization" in which seaweed adds nutrients to plants, increasing their growth rate, and susceptibility to herbivory. Path analysis on the survey data indicated that the bottom-up fertilization effect on herbivory was stronger than top-down effects of lizards. Preliminary results from the ongoing small-island experiment showed that herbivory increased markedly only when large amounts of seaweed were repeatedly added, and only on islands without lizards, suggesting that the amount of seaweed deposition and the top-down effect of lizards are both important.