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PLoS BIOLOGY Food Web Models Predict Species Abundances in Response to Habitat Change Nicholas J Gotelli1 Aaron M Ellison2 1 Department of Biology University of Vermont Burlington Burlington Vermont United States of America 2 Harvard Forest Harvard University Petersham Massachusetts United States of America Plant and animal population sizes inevitably change following habitat loss but the mechanisms underlying these changes are poorly understood We experimentally altered habitat volume and eliminated top trophic levels of the food web of invertebrates that inhabit rain filled leaves of the carnivorous pitcher plant Sarracenia purpurea Path models that incorporated food web structure better predicted population sizes of food web constituents than did simple keystone species models models that included only autecological responses to habitat volume or models including both food web structure and habitat volume These results provide the first experimental confirmation that trophic structure can determine species abundances in the face of habitat loss Citation Gotelli NJ Ellison AM 2006 Food web models predict species abundances in response to habitat change PLoS Biol 4 10 e324 DOI 10 1371 journal pbio 0040324 Unfortunately published studies of the effects of habitat contraction have relied on conventional analyses that do not explicitly compare these alternative frameworks 17 18 Although analysis of variance and other statistical protocols can quantify community change they cannot be used to distinguish between simple responses of species to habitat contraction single factor volume model and more complex responses to changes in the abundance of other species foodweb and keystone species models A third possibility is that trophic responses dominate the responses even in the face of habitat alterations In this study we used realistic eld manipulations of habitat volume and removal of top trophic levels of entire aquatic communities These manipulations induced major alterations in habitat size and community structure that have been studied previously in nonexperimental settings 1 For the rst time we have experimentally assessed the relative importance of autecological responses keystone species effects and trophic interactions in accounting for changes in species abundance Introduction The loss of natural habitat area often is accompanied by the disappearance of large bodied top predators and the upper trophic levels of food webs 1 3 However several pieces of evidence suggest that habitat area alone may be insuf cient to predict changes in population size Predictions of ecological models 4 5 patterns of food web structure in small versus large habitat fragments 6 and recent observations of collapsing island communities 1 7 all suggest that trophic interactions must be considered in order to predict how abundances of populations will change in the face of habitat loss and alteration However existing tests of the role of trophic interactions in determining species abundances as habitats contract are correlative only In this study we provide the rst evidence from a controlled eld experiment for the importance of trophic structure in controlling abundances of multiple species in an aquatic food web Moreover we demonstrate that models of trophic structure account for the results better than do simpler models that focus only on responses of individual species to changes in habitat size or structure or models that include both foodweb structure and habitat volume For multitrophic assemblages two broad classes of community models predict the potential responses of populations to habitat change 1 Single factor models emphasize the unique responses of individual species to variation in habitat area This framework includes island biogeographic models 8 as well as single species demographic analyses 9 and assessments of extinction risk Single factor models also include keystone species effects which emphasize responses of populations to changes in the abundance of a single keystone species such as a top predator 10 11 This framework includes much current research on habitat alterations by foundation species 12 and ecosystem engineers 13 2 Food web models emphasize the shifts in abundance that result from multiple trophic interactions and the transfer of energy and biomass through a food web This framework includes top down and bottom up processes 14 trophic cascades 15 and more complex interactions across multiple trophic levels 16 PLoS Biology www plosbiology org Results The Aquatic Food Web of Sarracenia The macroinvertebrate community associated with the northern pitcher plant Sarracenia purpurea Figure 1 is a model system for testing mechanisms controlling abundance in the face of habitat change 19 S purpurea is a long lived perennial plant that grows in peat bogs and seepage swamps throughout southern Canada and the eastern United States 20 The plant grows as a rosette and produces a set of six to 12 new tubular leaves each year During the growing season Academic Editor Robert Holt University of Florida United States of America Received January 22 2006 Accepted August 1 2006 Published September 26 2006 DOI 10 1371 journal pbio 0040324 Copyright 2006 Gotelli and Ellison This is an open access article distributed under the terms of the Creative Commons Attribution License which permits unrestricted use distribution and reproduction in any medium provided the original author and source are credited To whom correspondence should be addressed E mail ngotelli uvm edu 0001 October 2006 Volume 4 Issue 10 e324 Food Web Models Predict Abundance which is shredded and partially consumed by midge Metriocnemus knabi and sarcophagid y Fletcherimyia etcheri larvae 23 Shredded prey are then processed by a subweb of bacteria and protozoa 24 which respectively are prey to lter feeding rotifers Habrotrocha rosi and mites Sarraceniopus gibsonii Larvae of the pitcher plant mosquito Wyeomyia smithii feed on bacteria protozoa and rotifers 25 Large third instar larvae of F etcheri feed on rotifers and small rst and second instar larvae of W smithii 26 Thus the Sarracenia food web exhibits the same complex linkages across multiple trophic levels that characterize other aquatic and terrestrial food webs 16 Furthermore the same assemblage of macroinvertebrate species can be found associated with S purpurea throughout its broad geographic range from the Florida panhandle to Labrador and west to the Canadian Rocky

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