Does water velocity influence optimal escape behaviors in stream insects?

Optimal escape theory can successfully explain variation in the distance to an approaching predator at which prey initiate flight (the flight initiation distance, FID). However, for animals without access to refuges, optimal escape theory may also explain variation in the distance that prey flee (the retreat distance, RD). In benthic stream habitats, both the risk of predation and the costs of escape may be mediated by water velocity; optimal escape theory then predicts that FID and RD of slow-current insects should vary little with increasing current velocity, whereas the FID and RD of fast-current insects should decrease. To test this prediction, a simulated predator (SP) was used to initiate escape responses in 3 mayflies found in different habitats--Ameletus (slow pools), Baetis (fast riffles), and Epeorus (very fast cascades)--across a range of water velocities. Unexpectedly, the FID of all 3 prey did not vary with water velocity. In contrast, the RD of Epeorus decreased with velocity (RD at the lowest velocity about 4.5 greater than the highest velocity), whereas the RD of Ameletus did not vary significantly with velocity. Escape behaviors of Baetis did not vary strongly with velocity. Variation in the proportion of larvae that escaped by drifting or swimming rather than crawling (Ameletus > Baetis, Epeorus) suggests that for fast-current prey, the costs associated with leaving the streambed exceed the risks of benthic predation. Water velocity may thus influence ecological processes such as predator--prey interactions and emigration from patches in substantial, but previously unexplored, ways. Copyright 2010, Oxford University Press.