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Effects of nonlethal predation and competition on life-history characteristics and behavior of larval Oregon spotted frog (Rana pretiosa) and larval red-legged frog (Rana aurora) Peterson, Heidy Kristann

Abstract

Oregon spotted frogs (Rana pretiosa) and red-legged frogs (Rana aurora) co-occur in the northern portions of their range and use similar larval rearing habitat in southwestern British Columbia and Washington State. I conducted a field mesocosm study to test hypotheses about the effects of inter- and intraspecific competition and nonlethal predation on metamorphic characteristics. Tadpoles were exposed to treatments in the presence and absence of one another, two density levels and to the presence or absence of a predacious odonate larva (Aeshna palmata) isolated in an enclosure. I examined the metamorphic responses of both species by evaluating weight at metamorphosis, time to metamorphosis and survival to metamorphosis. Additionally, a laboratory study evaluated behavioral mechanisms potentially responsible for any changes observed in metamorphic characteristics. Results from the four-week laboratory test indicated that both species reduced activity and moved away from the predator in the presence of an enclosed dragonfly larva, thus I expected to see effects on characteristics associated with growth as previous studies have shown. In the field mesocosm study, red-legged frogs exhibited lengthened larval periods and were 12% larger at metamorphosis when exposed to Aeshna. In the presence of Oregon spotted frogs, they decreased time to metamorphose by a week and a half, and were 12% larger than those reared alone at metamorphosis. Individuals from high density treatments were 28% smaller that those metamorphosing from low density treatments, suggesting that interspecific competition influences metamorphic characteristics of red-legged frogs. The proportion of tadpoles surviving to metamorphose was very high at over 0.9 in all treatments for red-legged frogs. Although red-legged frogs and Oregon spotted frogs rear under similar conditions, their responses to experimental manipulations were different. Oregon spotted frogs in treatments with red-legged frogs were an average 14% larger at metamorphosis when a predator was present. However, in treatments where Oregon spotted frogs were alone with a predator, results indicate tadpoles weight at metamorphosis was 26% smaller than those in the absence of Aeshna. In addition, Oregon spotted frogs had the lowest survival rate observed in the experiment when alone with the predator. In low density treatments survival was approximately 0.8 while in treatments with Oregon spotted frogs alone in the presence of Aeshna was only 0.2. High density treatments caused tadpoles to metamorphose only 11% smaller than from low density tanks. The field mesocosm study demonstrated that the presence of a predacious invertebrate alters metamorphic characteristics of both species likely by changing their foraging behavior. Furthermore, results suggest that Oregon spotted frogs benefit through a facilitative interaction with red-legged frogs in the presence of a predator, however it is not known if similar behavioral adjustments are prevalent in natural populations. Ecological relationships like those discovered in these experiments should be considered when planning long-term conservation strategies for both species.

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