EFFECTS OF PREDATION RISK ON POPULATION VARIATION IN ADULT SIZE IN A STREAM-DWELLING ISOPOD

I used a combination of laboratory experiments and field surveys to ex amine the role that population-specific predation risk may play in sha ping the life history strategy of a stream-dwelling isopod Lirceus fon tinalis. Two focal populations were identified that were exposed to di fferent predator types. The first population was exposed to larvae of the streamside salamander (Ambystoma barbouri) and the second to bande d sculpin (Cottus carolinae). A laboratory experiment, in which differ ent size classes of prey were offered simultaneously to individual pre dators, revealed that L. fontinalis suffered greatest mortality risk a t small sizes with A. barbouri. Alternatively, with C. carolinae the r isk of mortality was independent of size. Life history theory predicts that L, frontinalis from populations exposed to the gaps-limited sala mander larvae should be larger at maturity relative to individuals fro m populations exposed to C. carolinae. Field surveys on the two focal populations both within 1 year and across 4 years supported this predi ction. Four other populations, two exposed to streamside salamander la rvas and two to fish, provided additional support for the prediction. I concluded that L. fontinalis exhibited an adaptive response in size at maturity in response to population-specific predation risk. I then used gut content assays of the major predators to assess whether the p opulation-specific life history strategies adopted by L. fontinalis we re successful in avoiding predation.