Comparing predator-induced changes in stream insects with varying vulnerabilities to risk of predation

Abstract

Mayflies in high-elevation streams in Colorado are more vulnerable to risk of predation based on their relative drift propensisties. Baetis bicaudatis, a mayfly that has high drift propensity, has been shown to respond to predation risk by accelerating its metamorphosis into an adult, which results in smaller body size and a reduced fecundity. Other species of mayflies with lower drift propensities (either as a result of a fixed behavior, or a response to predation risk) , as well as other aquatic insects, such as, caddisflies that do not drift may have a different response to predators. We conducted field surveys of five fish and five fishless streams to determine the effect of predation risk on aquatic insect response (i.e., body size and fecundity). Three species of mayfly were collected: Rhithrogena robusta, Epeorus deceptivus, and Cinygmula spp. and one species of caddisfly was collected: A. grandis. We also conducted a stream micrcosm experiment to determine if the patterns seen in nature for E. deceptivus and A. grandis could be induced with just fish chemical cues. A general trend was seen in field surveys that showed an increase in R. robusta ( power = 0.8264) body size in fish streams. A. grandis did not show a body size response to stream treatment in field surveys, but there was a higher number of eggs produced in fish streams (power = 0.0542). These results indicate that insects with high vulnerabilities but low risk to predation may not be affected by predator-induced changes in body size.

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