Experimental evidence for a strong influence of stickleback predation on the population dynamics and sex ratio of an aquatic moth

Abstract

The larvae of the water moth Acentria ephemerella are important herbivores in freshwater macrophyte beds. Due to the considerable damage that Acentria can inflict on the plants, the herbivore might influence the important role of macrophytes within littoral ecosystems. It is thus important to know which factors can control Acentria populations and consequently its interannual variability. Here we use a mesocosm experiment to test whether fish predation can control Acentria population development. We followed Acentria population development and size distribution using two different stickleback, Gasterosteus aculeatus, densities and one control treatment. Sticklebacks were able to reduce population growth of Acentria compared to the control treatment without fishes. Predation of sticklebacks on Acentria was confirmed by gut content analyses of sticklebacks caught at the end of the experiment. In addition to a reduction of population growth, stickleback predation resulted in malebiased sex ratios of Acentria, which are typically observed in situ in Lake Constance. A comparison of Acentria size distributions in the experiment and in stickleback guts suggests that the male bias is not due to size-selective predation of sticklebacks on presumably larger females. In summary, stickleback predation influenced Acentria population dynamics and dampened population growth (1) directly by consuming Acentria larvae and (2) indirectly by shifting the sex ratio via sex-specific mortality towards a stronger male bias. Top-down control by fishes of Acentria, a key herbivore in macrophyte beds, suggests the existence of a previously overlooked trophic cascade of major importance in aquatic ecosystems.