Parasites alter food-web topology of a subarctic lake food web and its pelagic and benthic compartments

Abstract

We compared three sets of highly resolved food webs with and without parasites for a subarctic lake system corresponding to its pelagic and benthic compartments and the whole-lake food web. Key topological food-web metrics were calculated for each set of compartments to explore the role parasites play in food-web topology in these highly contrasting webs. After controlling for effects from differences in web size, we observed similar responses to the addition of parasites in both the pelagic and benthic compartments demonstrated by increases in trophic levels, linkage density, connectance, generality, and vulnerability despite the contrasting composition of free-living and parasitic species between the two compartments. Similar effects on food-web topology can be expected with the inclusion of parasites, regardless of the physical characteristics and taxonomic community compositions of contrasting environments. Additionally, similar increases in key topological metrics were found in the whole-lake food web that combines the pelagic and benthic webs, effects that are comparable to parasite food-web analyses from other systems. These changes in topological metrics are a result of the unique properties of parasites as infectious agents and the links they participate in. Trematodes were key contributors to these results, as these parasites have distinct characteristics in aquatic systems that introduce new link types and increase the food web's generality and vulnerability disproportionate to other parasites. Our analysis highlights the importance of incorporating parasites, especially trophically transmitted parasites, into food webs as they significantly alter key topological metrics and are thus essential for understanding an ecosystem's structure and functioning.

Keywords: Connectance; Food webs; Parasite ecology; Trematoda; Trophic transmission.

Fig. 1

Food web of Takvatn’s (a) total pelagic web, (b) total benthic web, and (c) total whole-lake web and their respective links. The nodes representing detritus and other non-living taxa are depicted in brown, the producers are depicted in green, and all other free-living taxa are depicted in blue. The links between these free-living nodes are blue. All parasites are depicted in red and their respective parasitizing links are also red

Fig. 2

The number of links in the (a) total pelagic compartment, (b) total benthic compartment, and (c) total whole-lake web. Each of the four link categories (predator–prey in pink, predator-parasite in blue, parasite-host in green, and parasite-parasite in yellow) is subdivided into its corresponding link types (twelve total). The number of links observed in each link type is presented in parentheses next to the link type name. The pelagic compartment did not contain any parasite-parasite links as only trematodes exhibited this link category, and all trematodes were allocated to the benthic compartment. The number of links in the free-living webs of each web version are represented by the number of predator–prey links present in each total web

Fig. 3

Niche model errors (ME) for four network structure properties for four web versions of the three Takvatn compartments. MEs >|1| indicate a significant difference between the empirical value and niche model prediction (1000 simulated networks). Negative MEs indicate the empirical value was less than model predicted value; positive MEs indicate the empirical value was greater than model predicted value

Fig. 4

Density plots depicting generality and vulnerability distributions of (a) the free-living pelagic web and (b) the total pelagic web, (c) the free-living benthic web and (d) the total benthic web, and (e) the free-living whole-lake web and (f) the total whole-lake web. In each panel, generality and vulnerability for each web are further sub-divided into free-living taxa generality and vulnerability densities and parasite taxa generality and vulnerability densities. Free-living taxa values are represented in blue and parasite taxa values are represented in red