Effects of spatial subsidies and habitat structure on the ecology of two locally dominant geckos

Background/Question/Methods

While it is well established that ecosystem subsidies can affect consumer abundance, there is less information available on how subsidy levels may affect consumer diet, body condition, trophic position, and resource partitioning between consumer species.  There is also little information on whether changes in vegetation structure commonly associated with spatial variation in subsidies play an important role in driving consumer responses to subsidies.  To address these knowledge gaps, we studied changes in abundance, diet, trophic position, and morphometrics of two locally abundant, congeneric gecko species (Lepidodactylus spp.) that coexist across the Central Pacific. Using islets on Palmyra Atoll, which vary strongly  in quantity of subsidy input and habitat complexity (associated with changes in seabird density and forest type, respectively), we were able to test the direct, bottom-up effects of subsidies on consumers, and the indirect subsidy effects mediated by structural changes of vegetation communities.

Results/Conclusions

Contrary to other studies, we found that subsidy level had no impact on the abundance of either gecko species; it also did not affect resource partitioning between species. However, geckos in highly subsidized dicot forests were larger, had higher body condition, more diverse diets, and occupied a much higher trophic position than geckos found in low subsidy palm dominated forests.  Certain prey items also varied in diet importance across forest type.  Parsing out the separate effects of subsidy input and habitat structure on gecko responses showed that gecko trophic position was positively correlated with subsidy level, while changes in body size and body condition were associated with habitat structure.  Overall these results suggest that variation in subsidy levels may drive important physical and behavioral responses in predators, even when their numerical response is limited.  Strong changes in trophic position of consumers also suggest that subsidies may drive systematic changes in food web structure.