Concomitant predation on parasites is highly variable but constrains the ways in which parasites contribute to food web structure

Abstract

Summary Previous analyses of empirical food webs (the networks of who eats whom in a community) have revealed that parasites exert a strong influence over observed food web structure and alter many network properties such as connectance and degree distributions. It remains unclear, however, whether these community‐level effects are fully explained by differences in the ways that parasites and free‐living species interact within a food web.To rigorously quantify the interrelationship between food web structure, the types of species in a web and the distinct types of feeding links between them, we introduce a shared methodology to quantify the structural roles of both species and feeding links. Roles are quantified based on the frequencies with which a species (or link) appears in different food web motifs – the building blocks of networks.We hypothesized that different types of species (e.g. top predators, basal resources, parasites) and different types of links between species (e.g. classic predation, parasitism, concomitant predation on parasites along with their hosts) will show characteristic differences in their food web roles.We found that parasites do indeed have unique structural roles in food webs. Moreover, we demonstrate that different types of feeding links (e.g. parasitism, predation or concomitant predation) are distributed differently in a food web context. More than any other interaction type, concomitant predation appears to constrain the roles of parasites. In contrast, concomitant predation links themselves have more variable roles than any other type of interaction.Together, our results provide a novel perspective on how both species and feeding link composition shape the structure of an ecological community and vice versa.

Untangling the roles of parasites in food webs with generative network models

“…Previous work found that food webs including concomitant links deviated in motif frequencies [14,18]. However, motifs and niches describe the network at fundamentally different levels, and so there is no conflict between such observations and our results.…”

“…showed that parasites have unique structural roles compared to free-living species: when concomitant links were excluded, parasites have diverse roles similar to freeliving consumers that have both predators and prey (i.e., intermediate taxa), but when concomitant links were included, their roles were more constrained and different [18]. This study also found that concomitant links represent the most structurally diverse type of interaction.…”

Why infectious disease research needs community ecology

“…(39) used network approaches to show that co-infecting parasites of humans were organized into dense clusters around distinct locations in the body (e.g., organs) and tended to interact with each other via shared resources within the host, rather than via the immune system. Similar approaches have been applied across other scales of organization–for example, to define contact pathways for transmission among individual hosts (40, 41) and to identify the role of parasites in structuring ecological food webs (42). Although the focus of network approaches thus far has often been on the patterns of links among species, emerging tools allow for more explicit examination of interaction strengths, which will help to forecast dynamic changes in the system (43).…”

Host taxonomy constrains the properties of trophic transmission routes for parasites in lake food webs

“…, Dunne et al. , Cirtwill and Stouffer ). As well as affecting the free‐living community, however, many parasites rely on the free‐living food web to complete their life cycles.…”