UC San Diego

Island systems have long been valuable to ecological research as they provide natural experiments for the study of ecosystem processes. We examined Allen, Leaf, U and Flat Rock Reef Cays in the Bahamas to study the effects of seabird driven marine subsidies and invasive mice on island food webs on the body size and foraging ecology of the Allen Cays Rock Iguana (Cyclura cychlura inornata). Iguanas on an island with nesting seabirds (Allen Cay) had 6 times the body mass and 1.7 times the snout-vent length than those on non-seabird islands. We used stable carbon ([delta]¹³C) and nitrogen ([delta]¹⁵N) isotope analysis of representatives from all levels of the food webs on each cay to test the predictions that 1) the food web on Allen Cay exhibited the influence of nutrient subsidies from seabirds whereas those from the three non-seabird cays did not, and 2) size differences in iguanas between Allen and the other cays were due to either a) supplemental food availability from mice and/or seabird carcasses and/or b) access to more nutrient rich and denser vegetation as a result of regular fertilization of plants by seabird guano. Food web components from islands with marine subsidies had higher [delta]¹⁵N values by ̃5 to 9⁰/₀₀ than those on the other three cays. Plants on the seabird island had 1.6-1.9 times the nitrogen in foliar tissues, indicating seabird guano caused an increase in plant nutrient availability. There was no isotopic evidence for significant consumption of animal matter by iguanas on any island