Why do mayflies thrive in trout streams?

There are many examples of aquatic insects that avoid ovipositing in risky habitats, and whose populations are decimated by vertebrate predation if they make mistakes. Stream-dwelling mayflies are not among those examples. In fact, some mayfly species may preferentially oviposit and achieve higher densities in streams with the highest risk of vertebrate predation. My students, colleagues and I have spent many years trying to understand that perplexing pattern. Competing selection pressures independent of predation risk (hydro-geomorphology maximizing hatching success) may explain why Baetis mayflies selectively oviposit in trout streams. Adaptive behavior and life history strategies may explain the high relative abundance of Baetis populations in streams with trout compared to those in fishless streams. Consequently, non-consumptive effects of predators may have a greater influence on the rate of Baetis population growth than does consumption by trout. Models of predator-prey interactions in streams need to incorporate such effects.