PREDATOR MEDIATED PATCH USE BY TADPOLES (HYLA-REGILLA) - RISK BALANCING OR CONSEQUENCE OF MOTIONLESSNESS

To quantify the impact of garter snakes (Thamnophis hydrophilus, forme rly T. couchii) on tadpole prey (Hyla regilla) I experimentally examin ed whether: (1) garter snakes have greater impact on tadpole numbers o r on behavior; (2) tadpole patch choice follows the rule of minimizing the ratio of mortality risk, mu, to foraging gain, g: and (3) predato r-induced resource avoidance influences algal production. In replicate d enclosed pools constructed on gravel bars of a northern California r iver, garter snakes did not significantly reduce the number of tadpole s but did alter patch choice. Tadpoles spent less time in high food qu ality algae mats (Cladophora glomerata with epiphytic diatoms) and men u! time in low food quality patches (Zygnematales algae mats and sedim ents) when garter snakes were present than when they were absent in mo rtality risk experiments, however, there were no significant differenc es in the numbers of tadpoles consumed among patch types, and garter s nakes were not size selective. Therefore, patch choice did not follow the rule of minimizing mu/g. The change in patch choice was likely a c onsequence of the sublethal effect of garter snakes in which tadpole a ctivity is decreased. For negatively buoyant tadpoles, decreased activ ity results in sinking away from floating algal resources. This appare nt resource avoidance by tadpoles did not affect algal mass over the r elatively short duration of the experiments.