Individuals usually operate with a limited resource budget and investment in sexual traits often reduces investment in non-sexual traits. Currently, we have little understanding of how tradeoffs between sexual and non-sexual traits are resolved. This study manipulated resource stress (food quality via manipulation of phosphorus) and predation stress (presence of dragonfly predator cues) to assess how these stressors interact to shape the development of sexual and non-sexual traits in a freshwater amphipod (Hyalella sp.). Specifically, we predicted predation stress would cause increased resource allocation to growth at the expense of sexual traits because dragonflies selectively prey on small individuals. Furthermore, we expected this tradeoff would be magnified in low P treatments, as fewer individuals would be able to meet the demand for increased growth and be able to invest in P-rich sexual traits.
The amphipods responded to both treatments in an interactive fashion. Without predator cues, the low P treatments caused no change in the non-sexual traits whereas the two sexual traits (antennae and posterior gnathopod size) were 4% and 8% smaller. With predator cues, however, the sexual traits no longer became smaller in the low P environment. These results, although contrary to our predictions about the effect of predator cues, might be explained by predators excreting nutrients (including P) into the water, thereby dampening differences in P availability between food quality treatments and negating the possible stress imposed by predators. Although predator cues did not cause a tradeoff between growth and sexual trait development, they did eliminate the effect of P limitation in the low P treatment. Our results indicate that different magnitudes of nutrient and predation stress will alter the amount of phenotypic variation available for selection. This work is particularly important given that many freshwater systems are currently experiencing increases in nutrient levels due to agricultural run-off. Thus, it is important to understand how individuals and populations will respond to changes in nutrient supply and consequently how these changes impact long-term evolutionary trends.