# Habitat Complexity Reduces the Feeding Strength of Freshwater Predators

## TL;DR

This study shows that complex habitats reduce how much freshwater predators eat, which could help stabilize ecosystems.

## Contribution

The study reveals distinct effects of habitat complexity on two predator types, showing reduced prey consumption in complex environments.

## Key findings

- Ambush predators (Ischnura elegans) had feeding rates mainly influenced by presence or absence of habitat structures.
- Pursuit predators (Notonecta glauca) showed unique maximum feeding rates and decreasing attack rates with increasing habitat complexity.
- Prey consumption by both predators was significantly reduced in complex habitats, with some cases showing more than a 50% decrease.

## Abstract

The physical structure of an environment potentially influences feeding interactions among organisms, for instance, by providing refuge 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 ran 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 the functional response pattern. The experimental design and analysis allowed us to test for effects of structure presence, amount, and complexity level on functional response in one pass, without confounding predictors. Across all complexity levels, the feeding for both predators 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 explained by the absence versus presence of structure. Yet, in the case of
N. glauca
, feeding rate was strongly dependent on habitat complexity with the predator showing a unique maximum feeding rate (i.e., the inverse of the handling time) for each complexity level and a decreasing attack rate with increasing amount of habitat. On average, prey consumption by both predators was reduced when complex structures were present, compared to the ‘no habitat structure’ environment (e.g., consumption more than halved for some treatments). Our findings demonstrate that habitat complexity dampens feeding rates and therefore plays a key role in the stability of freshwater ecosystems.

We investigated how habitat complexity influences the feeding responses of an ambush predator (Ischnura elegans) and a pursuit predator (
Notonecta glauca
) preying on
Asellus aquaticus
. Feeding rates of
I. elegans
were primarily affected by the presence or absence of structure, whereas
N. glauca
exhibited a unique maximum feeding rate and a declining attack rate with increasing habitat complexity. Overall, both predators consumed less prey in complex habitats, with consumption reduced by more than half in some treatments.