Eutrophication and predation risk interact to affect sexual trait expression and mating success

Sexual traits are especially sensitive to low food resources. Other environmental parameters (e.g., predation) should also affect sexual trait expression by favoring investment in viability traits rather than sexual traits. We know surprisingly little about how predators alter investment in sexual traits, or how predator and resource environments interact to affect sexual trait investment. We explored how increasing phosphorous (P) availability, at a level mimicking cultural eutrophication, affects the development of sexual, non-sexual, and viability traits of amphipods in the presence and absence of predators. Sexual traits and growth were hyper-sensitive to low P compared to non-sexual traits. However, a key sexual trait responded to low P only when predator cues were absent. Furthermore, investment tradeoffs between sexual traits and growth only occurred when P was low. The phenotypic changes caused by predator cues and increased P availability resulted in higher male mating success....