Asymmetric foraging in aquatic animals

This blog post is provided by Junjie Zheng and tells the #StoryBehindThePaper for the paper “Asymmetric foraging lowers the trophic level and omnivory in natural food webs“, which was recently published in Journal of Animal Ecology. The authors found a prevalence of asymmetric foraging in natural aquatic food webs. Featured image by Tingnan Zhou

Food webs depict the network of “who eats whom” in nature. Generally, species forage on more than one prey, and have different trophic positions (quantified as trophic level) due to their diverse energy resources. The exact foraging relationships and energy fluxes between species can govern the functioning and stability of ecosystems. Accordingly, understanding the foraging pattern as well as its drivers and implications is critical to exploring the rules of nature.

In a recent paper in the Journal of Animal Ecology, based on two structural metrics, trophic level (species position in a food web) and omnivory (degree of concentrated or discrete feeding), we investigate two main questions. First question is species feeding asymmetric and biased towards prey species with lower or higher trophic level? Based on a dataset with 157 food webs (Figure 1), we found that predators prefer prey at higher trophic levels, which usually have a higher nutrient concentration (i.e. food quality) inside the body. However, in most aquatic ecosystems, species biomass is shaped like a pyramid (Figure 2, a diagram), which means more abundant biomass stored in lower trophic levels. Thus, although they prefer prey with higher trophic positions, species have to obtain their energy mostly from lower trophic levels with larger abundance (i.e. food quantity; Figure 3D, Figure 4). This suggests that in nature, food quantity outperforms food quality and becomes the dominant driver for the bottom-up mechanism.

Figure 1 Map of the food webs

Second question what will happen if the asymmetric foraging is ignored (e.g. food webs lack energy flux information)? We found that for predators, lower food web resolution fails to identify the asymmetric foraging pattern. They thus distribute even foraging strength to each prey (Figure 3C). In this case, the trophic position and the degree of omnivory for the predator will be overestimated, compared to the scenario with high resolution. Such over-estimations will be larger for predators at lower trophic positions. For the whole food web, over-estimations on these structural properties are more pronounced for ecosystems with higher species diversity. Therefore, proper resolutions on energy flux information are crucial to accurately understand the foraging behavior of predators as well as the structural complexity of natural food webs.

Figure 2 A diagram of biomass pyramid

The more we know about the foraging characteristics of animals, the clearer it is to understand the relationships among food web structure, functioning and stability. For instance, previous studies raised contrasting relationships between omnivory and food web stability, but the higher resolution data can contribute to unify these results by clarifying that weak omnivorous interactions stabilize population dynamics, whereas strong interactions destabilize them. Moreover, scientists have found an exponential relationship between productivity and maximum trophic level. The higher resolution data can help calculate a more accurate trophic level and thus a more reliable productivity for an ecosystem.

Figure 3 Illustration of different foraging patterns for a four-species food web
Figure 4 Relationship between energy uptake (a) or feeding preference (b) with prey trophic level

This study highlights the importance of energy flux information in understanding foraging characteristics and food web structure, and discusses relating mechanisms and potential effects. Benefiting from the advance of both empirical and theoretical tools, the availability of high-resolution food web data has been increasing. This provides new opportunities to reconcile food web structure, functioning and stability, and may contribute to fishery policy-making and ecological management.

BIO: Junjie Zheng is a PhD student (2018- ) at the College of Urban and Environmental Sciences, Peking University. Her research interests currently focus on food web structure and functioning.

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