Effects of Spatial Subsidies and Habitat Structure on the Foraging Ecology and Size of Geckos Amy A. Briggs 1. , Hillary S. Young 1 * a. ¤ , Douglas J. McCauley 1¤b , Stacie A. Hathaway 2 , Rodolfo Dirzo 1 , Robert N. Fisher 2 1 Department of Biology, Stanford University, Stanford, California, United States of America, 2 Western Ecological Research Center, U.S. Geological Survey, San Diego Field Station, San Diego, California, United States of America Abstract While it is well established that ecosystem subsidies—the addition of energy, nutrients, or materials across ecosystem boundaries—can affect consumer abundance, there is less information available on how subsidy levels may affect consumer diet, body condition, trophic position, and resource partitioning among consumer species. There is also little information on whether changes in vegetation structure commonly associated with spatial variation in subsidies may play an important role in driving consumer responses to subsidies. To address these knowledge gaps, we studied changes in abundance, diet, trophic position, size, and body condition of two congeneric gecko species (Lepidodactylus spp.) that coexist in palm dominated and native (hereafter dicot dominated) forests across the Central Pacific. These forests differ strongly both in the amount of marine subsidies that they receive from seabird guano and carcasses, and in the physical structure of the habitat. Contrary to other studies, we found that subsidy level had no impact on the abundance of either gecko species; it also did not have any apparent effects on resource partitioning between species. However, it did affect body size, dietary composition, and trophic position of both species. Geckos in subsidized, dicot forests were larger, had higher body condition and more diverse diets, and occupied a much higher trophic position than geckos found in palm dominated, low subsidy level forests. Both direct variation in subsidy levels and associated changes in habitat structure appear to play a role in driving these responses. These results suggest that variation in subsidy levels may drive important behavioral responses in predators, even when their numerical response is limited. Strong changes in trophic position of consumers also suggest that subsidies may drive increasingly complex food webs, with longer overall food chain length. Citation: Briggs AA, Young HS, McCauley DJ, Hathaway SA, Dirzo R, et al. (2012) Effects of Spatial Subsidies and Habitat Structure on the Foraging Ecology and Size of Geckos. PLoS ONE 7(8): e41364. doi:10.1371/journal.pone.0041364 Editor: Anna Dornhaus, University of Arizona, United States of America Received March 14, 2012; Accepted June 20, 2012; Published August 10, 2012 This is an open-access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication. Funding: This work was supported by funding from the National Science Foundation, the National Geographic Society, the Woods Institute for the Environment, and Stanford University. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: hyoung@fas.harvard.edu . These authors contributed equally to this work. ¤a Current address: Center for the Environment, Harvard University, Cambridge, Massachusetts, United States of America ¤b Current address: University of California, Berkeley, Department of Environmental Science, Policy, and Management, Berkeley, California, United States of America Introduction Spatial subsidies, or the movement of nutrients or energy between ecosystems, can have substantial impacts on the abundance and community composition of primary producers and consumers, ultimately leading to large scale alterations in food webs and changes in ecological processes [1–3]. These subsidies have been shown to affect consumer abundance across a variety of consumer and ecosystem types [4–7], ranging from large vertebrate predators in desert systems [8] to microbial communi- ties in temperate salt marsh systems [9]. The strength of response to variation in subsidy level in terms of consumer abundance has been shown to decline with trophic level of the consumer, with secondary consumers having less marked numerical response to subsidies than herbivores and detritivores [10]. This reduction in response for higher consumers is not surprising, considering their tendency to have longer generation times, lower fecundity, and more flexible diets [1]. Additionally, food and nutrient resources are often not the primary limiting factors of predator population sizes. Social limitations like territoriality and mate availability can regulate population densities at levels below the resource- determined carrying capacity [11,12]. Compensation from other consumers may also contribute to lower numerical responses of predators to allochthonous inputs (i.e. [3]). The aforementioned evidence has led to the idea that the importance of subsidies to consumers attenuates at higher levels of the food web [10,13,14]. However, a lack of observed numerical response by predators does not necessarily indicate that subsidy additions are not important to predator populations. There are multiple other avenues by which predators may respond to subsidy inputs. They may, for example, exhibit changes in behavior (i.e. shifts to subsidized foraging areas or subsidized diet items), instead of, or in addition to, changes in abundance, particularly in regions where subsidies are episodic [15]. Likewise, while slow reproduc- tion or territoriality may prohibit numerical responses, subsidized consumers may show increased body size, fecundity, or other fitness metric [7,16]. There may also be shifts of resource PLOS ONE | www.plosone.org 1 August 2012 | Volume 7 | Issue 8 | e41364