Concomitant predation links have unique structural roles within food webs

Background/Question/Methods

Contrary to the idea that they have little ecological effect, parasites can have large impacts on host populations and are involved in a wide variety of interactions within their communities. These facts have provoked a debate about how parasites shape food-webs and how this is influenced by different interaction types. Since parasitism, unlike predation, is generally not fatal to hosts, it seems likely that parasitic interactions would be distributed differently throughout a food-web than interactions between free-living species. Conversely, concomitant predation on parasites and their hosts represents an unusual trophic interaction that depends on predation between free-living species. We therefore expect that concomitant predation links and links between free-living species would be distributed in similar ways.

To rigorously test for differences in how various feeding links appear in food webs, we quantified the structural roles of each interaction in seven highly-resolved empirical food-webs made up of 77-138 free-living species and 18-77 parasites. Here we define a link's structural role within its food web based on its participation in three-species motifs. By enumerating the frequencies with which an empirical interaction appears across the 24 unique link positions that make up these motifs, we obtain a holistic representation of that link's "role".

Results/Conclusions

Here, we compared the roles of links between free-living species with those of parasitism links, concomitant predation links, and links between parasites. We found that different types of feeding links occupy distinct roles. Surprisingly, parasitism had similar roles to predation among free-living species while concomitant predation links had very different roles. Further, the roles of concomitant predation links were more variable than those of other types of links. This suggests that the structural effects of concomitant predation may not be direct consequences of the links between free-living species that facilitate them and may be difficult to predict.

Further, the unique structural roles of concomitant predation links have important consequences for the food-web roles of the parasites themselves. Without taking these links into account, parasites have roles between those of free-living intermediate consumers and top predators. When concomitant predation is included, however, parasites' roles approach those of basal resources. This highlights the importance of concomitant predation as a source of mortality for parasites but also as an infection route for free-living species. If infected free-living species are nutritionally different from their uninfected conspecifics, or are easier to catch and process, there may also be key energetic consequences for the free-living consumer.