Abstract
Some consumers depend on the contemporaneous growth of the resources they feed on. For example, Tanytarsus gracilentus midges feed on algae, and because midge generation time is much longer than that of algae, midges benefit not just from the standing stock but also from the growth of algae. This implies that an intermediate consumption rate maximizes midge somatic growth: low consumption rates constrain midge growth, whereas high consumption rates reduce algae and future food abundance. In an experiment, we manipulated midge presence and initial algal abundance. We found that midges could suppress algal growth, as measured by changes in algal primary production (GPP). We also found a positive relationship between GPP and midge growth. We then fit a consumer-resource model to the experimental data, which showed the expected hump-shaped relationship between midge consumption rates and their somatic growth. In the model, projected midge growth rates were only positively associated with GPP when midge consumption was below the value that optimized midge growth. Therefore, the observed relationship between instantaneous primary production and midge growth seen in the experiment suggests that midges did not overexploit algae. This work highlights the challenges faced by consumers that depend on contemporaneously growing resources.