Habitat complexity reduces feeding strength of freshwater predators

The physical structure of an environment potentially influences feeding interactions among organisms by providing refuges for prey. We examined how habitat complexity affects the functional feeding response of an ambush predator (damselfly larvae Ischnura elegans ) and a pursuit predator (backswimmer Notonecta glauca ) feeding on the isopod Asellus aquaticus . We run experiments in aquatic microcosms with an increasing number of structural elements (0, 2, or 3 rings of plastic plants in different spatial configurations), resulting in five habitat complexity levels. Across these levels, predators were presented with different prey densities to determine their functional response. The experimental design and statistical analysis allowed us to test for effects of structure presence, amount, and complexity level on functional response in one pass, without confounding predictors. The feeding for both predators across all complexity levels was best described by a Type II functional response model and habitat drove feeding strength. Regarding the latter, the predators showed different responses to the complexity treatments. The overall feeding rate of I. elegans was mainly driven by the absence vs. presence of structure. Yet, in the case of N. glauca feeding rate was strongly dependent on habitat complexity with the predator showing unique maximum feeding rates (i.e. the inverse of the handling time) for each complexity level and a decreasing attack rate with increasing amount of habitat. Overall, prey consumption was less than half for both predators when complex structures were present, compared to the no habitat structure environment. Our findings demonstrate that habitat complexity dampens feeding rates and therefore plays a key role in the stability of freshwater ecosystems.