Population density and infection with parasites often are important factors affecting the growth and development of individuals. How these factors co-occur and interact in nature should have important consequences for individual fitness and higher-order phenomena, such as population dynamics of hosts and their interactions with other species. However, few studies have examined the joint effects of density and parasitism on host growth and development. We examined the co-influences of rearing density and parasitism, by the trematode Echinostoma trivolvis, on the growth and development of larval frogs, Rana (=Lithobates) pipiens. We also examined the potential role of parasite-mediated intraspecific competition by observing how unparasitized individuals performed when housed with other unparasitized tadpoles, versus housing with a combination of unparasitized and parasitized hosts. Mean mass and mean developmental stage were reduced under high rearing densities. The presence of parasitized conspecifics had no significant effect, but there was a significant interaction of density and parasitism presence on host mass, due to the fact that parasitized conspecifics grew poorly at high densities. Unparasitized individuals reared with parasitized and unparasitized conspecifics fared no better than unparasitized 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 different processes, and parasitism, if it only affects the latter, might not have a discernible impact on competitive interactions.
Collection and maintenance of tadpoles
Use of tadpoles adhered to guidelines from the Canadian Council on Animal Care. Portions of three masses of R. pipiens eggs were collected in May 2005. Eggs were returned to the lab and hatched in aquaria filled with aer- ated, dechlorinated water and kept at 24 °C on a 14:10 LD cycle. Tadpoles were fed ad libitum with NutraFin® vegeta- ble flake fish food. The experiment began when the tad- poles were eight weeks old (stage 27-28; Gosner 1960).
Tadpole infection with cercariae
Helisoma trivolvis snails were collected in late June 2005, maintained in lab aquaria on a 14:10 LD cycle, and fed vegetable flake fish food. To stimulate release of cercariae, snails were put into small plastic dishes of aerated pond water placed approximately 30 cm away from 100 W incandescent lamps from 07.00 to 10.00 h on the day of tadpole infection, 5 July 2005. E. trivolvis cercariae, identified by the anterior collar of spines (Schell 1984) and distinct movement compared to the other two trematode species released, were pipetted and pooled together into a single dish of pond water to account for variation in age, genotype, and condition. Although we collected snails and cercariae from a different location than Qgg masses of frogs, we note that E. trivolvis is a generalist parasite and such generalists often show local maladaptation, meaning they can act as costly parasites when introduced to nonlocal populations (Lajeunesse and Forbes 2002).
A total of 674 cercariae were collected and pipetted into one of four plastic tubs containing 3.95 l of dechlorinated water. At 11.00 h, 720 tadpoles were randomly chosen from four rearing aquaria, such that 180 from each were selected. Tadpoles were pooled together in a plastic container and then haphazardly assigned to one of the four plastic tubs described above (180 tadpoles/tub, one tub con- taining cercariae). Tadpoles were left for 1 h to allow for cercarial penetration, previously demonstrated to result in substantial infection of R. pipiens tadpoles with E. trivolvis cercariae of this age (Fried et al. 1997), with approximately 3.7 cercariae/tadpole available in the single tub containing 674 cercariae. Immediately after exposure to cercariae, tadpoles were randomly assigned to density treatments and moved to the experimental tanks described below.
Experimental design and procedure
Plastic cattle watering tanks (capacity of 378.5 l) located inside a fenced and roofed outdoor enclosure on the University of Toronto at Mississauga campus were filled with approximately 284 l of tap water in May 2005. Tanks were unused for five weeks to allow chlorine to dissipate and were covered with 60% shade cloth to prevent colonization by other organisms. Little evaporation occurred so no top-up of water levels was required prior to or during the experiment. We used four treatments (low versus high tadpole density crossed with exposed, and likely infected, tadpoles versus only unexposed individuals), with six replicates of each. In order to account for possible effects of tank location within the enclosure, treatments were randomly assigned to the previously set up tanks. The low-density treatment was 20 tadpoles per tank (0.05 tadpoles/l); the high-density treatment was 40 tadpoles (0.11 tadpoles/l). Densities were based on previous studies of tadpole competition (Relyea 2004), and are consistent with natural ranges reported for R. pipiens (Smith et al. 2003). Each tank with infected tadpoles contained tadpoles both exposed and unexposed to cercariae (50:50); this allowed for competition between infected and uninfected individuals.