The effect of intraspecific variation on predator-prey interactions
Whenever underlying phenotypic traits have non-linear relationships with ecological processes, variation in those traits is mathematically expected to have, per se, an effect on those ecological processes. In a previous theoretical study we showed how intraspecific variation on an underlying trait simultaneously controlling attack rate, handling time and interference competition could have important yet largely overlooked effects upon the dynamics of a predator-prey system through its effect on foraging rates. We also predicted that foraging rates should generally decrease with variation, leading to more stable dynamics and increased persistence. However, the effect of variation was found to change whenever prey densities were high or predator densities low. Because incorporating variation to predator-prey models adds complexity and loss of analytical tractability, it should only be undertaken if its effects are important and can be measured empirically. Here, we address this important point by testing some of our theoretical predictions on a simple copepod-protist predator-prey system. We ran foraging experiments were both prey and predator levels were varied, then estimated predator foraging rates and measured every individual predator so that both mean body-size and standing variance within each trial could be estimated.
Our copepod predators showed high levels of interference, which conformed to the assumptions of our model. Intraspecific variation in body size explained most of the residual variation in foraging rates once the functional response had been taken into account, and this effect was more pronounced than that of mean body-size, a priori expected to be important as copepods are gape-limited. Lastly, our theoretical predictions held true, as individual variation decreases foraging rates unless prey levels are high or predator levels low. The implications of our results are threefold: first, the effect of intraspecific variation can be important and detectable empirically. Second, this effect can be teased apart from that of other important ecological factors such as prey availability and interference competition but can also interact with those, leading to predictable outcomes in predator-prey dynamics. Last, variation in traits a priori expected to be ecologically relevant, such as body size in our copepod-protist system, may be more informative of the ecological role of the trait than their mean value. Together, our results highlight the need to explore how the genetic makeup leading to trait distributions may be responsible for a large fraction of the poorly understood variation observed in ecological dynamics.