Oecologia (2008) 155:641–649 DOI 10.1007/s00442-007-0937-2 123 COMMUNITY ECOLOGY - ORIGINAL PAPER Larval amphibian growth and development under varying density: are parasitized individuals poor competitors? J. Koprivnikar · M. R. Forbes · R. L. Baker Received: 9 July 2007 / Accepted: 3 December 2007 / Published online: 20 December 2007  Springer-Verlag 2007 Abstract Population density and infection with parasites often are important factors aVecting the growth and devel- opment of individuals. How these factors co-occur and interact in nature should have important consequences for individual Wtness and higher-order phenomena, such as population dynamics of hosts and their interactions with other species. However, few studies have examined the joint eVects of density and parasitism on host growth and development. We examined the co-inXuences of rearing density and parasitism, by the trematode Echinostoma tri- volvis, on the growth and development of larval frogs, Rana (=Lithobates) pipiens. We also examined the potential role of parasite-mediated intraspeciWc competition by observing how unparasitized individuals performed when housed with other unparasitized tadpoles, versus housing with a combi- nation of unparasitized and parasitized hosts. Mean mass and mean developmental stage were reduced under high rearing densities. The presence of parasitized conspeciWcs had no signiWcant eVect, but there was a signiWcant interac- tion of density and parasitism presence on host mass, due to the fact that parasitized conspeciWcs grew poorly at high densities. Unparasitized individuals reared with parasitized and unparasitized conspeciWcs fared no better than unpara- sitized individuals reared only with one another. This result indicates that infected hosts compete as much as uninfected hosts for resources, even though infected individuals have reduced mass under high-density conditions. Resource acquisition and resource allocation are diVerent processes, and parasitism, if it only aVects the latter, might not have a discernible impact on competitive interactions. Keywords Density · IntraspeciWc competition · Parasitism · Tadpole · Trematode Introduction Ecologists are interested in the role that parasites play in the population dynamics of their hosts, and the interactions between those hosts and other species (Poulin 1999; Parris and Beaudoin 2004). Mechanisms by which parasites are thought to aVect population dynamics are varied and include direct changes in host mortality and reproductive output, as well as indirect changes in behavior that may aVect foraging, mating, and avoidance of predators (Minchella and Scott 1991; Poulin 1999; Worden et al. 2000; Moore 2002). Such direct and indirect changes in reproductive out- put and behavior are also expected to result from other Communicated by Craig Osenberg. J. Koprivnikar Department of Biology, University of Toronto at Mississauga, 3395 Mississauga Road North, L5L 1C6 Mississauga, ON, Canada M. R. Forbes Department of Biology, Carleton University, 209 Nesbitt Building, K1S 5B6 Ottawa, ON, Canada e-mail: mforbes@connect.carleton.ca R. L. Baker Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks Street, M5S 3B2 Toronto, ON, Canada e-mail: rbaker@eeb.utoronto.ca J. Koprivnikar (&) Department of Biological Sciences, University of the PaciWc, Stockton, CA 95211, USA e-mail: jkoprivnikar@paciWc.edu