Research Article Body Size Mediated Coexistence in Swans Katharina A. M. Engelhardt, 1 Mark E. Ritchie, 2 and James A. Powell 3 1 Appalachian Laboratory, University of Maryland Center for Environmental Science, Frostburg, MD 21532-2307, USA 2 Department of Biology, Syracuse University, Syracuse, NY 13244-1270, USA 3 Department of Mathematics and Statistics, Utah State University, Logan, UT 84322-3900, USA Correspondence should be addressed to Katharina A. M. Engelhardt; kengelhardt@umces.edu Received 28 August 2013; Accepted 24 October 2013; Published 4 February 2014 Academic Editors: R. Julliard and S. Rossi Copyright © 2014 Katharina A. M. Engelhardt et al. his is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Diferences in body sizes may create a trade-of between foraging eiciency (foraging gains/costs) and access to resources. Such a trade-of provides a potential mechanism for ecologically similar species to coexist on one resource. We explored this hypothesis for tundra (Cygnus columbianus) and trumpeter swans (Cygnus buccinator), a federally protected species, feeding solely on sago pondweed (Stuckenia pectinata) tubers during fall staging and wintering in northern Utah. Foraging eiciency was higher for tundra swans because this species experienced lower foraging and metabolic costs relative to foraging gains; however, trumpeter swans (a) had longer necks and therefore had access to exclusive resources buried deep in wetland sediments and (b) were more aggressive and could therefore displace tundra swans from lucrative foraging locations. We conclude that body size diferentiation is an important feature of coexistence among ecologically similar species feeding on one resource. In situations where resources are limiting and competition for resources is strong, conservation managers will need to consider the trade-of between foraging eiciency and access to resources to ensure ecologically similar species can coexist on a shared resource. 1. Introduction Ecological theory predicts that the number of coexisting species within a community should be inite and that species difer in their morphological traits more than what would be expected by chance [1–4]. Patterns of size diferentiation have been observed among various guilds and communities [5–7] and across ecosystems [3, 8], leading to the hypothesis that size diferences drive functional diferences among species [9] and, therefore, resource partitioning in space [5, 10, 11]. Resource partitioning occurs by eating diferent types of foods or diferent size classes of the same food [2, 12–15], where coexistence is possible when species are limited by the resources they can exploit best [16–18]. Much less is known about how body size diferentiation may lead to coexistence in the absence of resource partitioning [14]. We illustrate this idea by examining a system featuring ecologically similar swan species feeding on the same resource in space and time. One species, the tundra swan (Cygnus columbianus), is common whereas the other species, the trumpeter swan (Cygnus buccinator), is rare and federally protected in the United States. Understanding whether and how the two species can coexist on a shared resource is important to the management and conservation of this group of species when food resources are limited. Two species can coexist on a shared resource when a body size mediated trade-of between foraging eiciency and access to resources exists. (1) Foraging eiciency is deined here as the ratio between foraging gains and foraging costs [19]. Larger-bodied animals incur greater foraging costs when they require more energy for metabolism and locomotion than their smaller-bodied counterparts [3, 20, 21]. However, larger animals gain more energy per unit foraging time because they are more efective in searching, handling, and processing their prey [20, 22–24]. hus, how foraging ei- ciency is related to body size depends on body size mediated foraging gains relative to foraging costs. (2) Similarly, access to resources can be mediated by body size through physical or behavioral mechanisms. Longer necks and bodies allow larger-bodied animals physical access to exclusive resources, such that smaller species are included entirely within the niche of the larger species [3, 14, 25]. Likewise, larger Hindawi Publishing Corporation e Scientific World Journal Volume 2014, Article ID 643694, 12 pages http://dx.doi.org/10.1155/2014/643694