Parasites in kelp‐forest food webs increase food‐chain length, complexity, and specialization, but reduce connectance

Abstract

We explored whether parasites are important in kelp forests by examining their effects on a high-quality, high-resolution kelp-forest food web. After controlling for generic effects of network size, parasites affected kelp-forest food web structure in some ways consistent with other systems. Parasites increased the trophic span of the web, increasing top predator vulnerability and the longest chain length. Unique links associated with parasites, such as concomitant predation (consumption of parasites along with their hosts by predators) increased the frequency of network motifs involving mutual consumption and decreased niche contiguity of free-living species. However, parasites also affected kelp-forest food web structure in ways not seen in other systems.Kelp-forest parasites are richer and more specialized than other systems. As a result, parasites reduced diet generality and decreased connectance in the kelp forest. Although mutual consumption motifs increased in frequency, this motif type was still a small fraction of all possible motifs, so their increase in frequency was not enough to compensate for the decrease in connectance caused by adding many specialist parasite species.

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

“…Parasites are very common consumers in most systems and yet parasitism is often over-looked in food webs (Marcogliese and Cone 1997 ; Lafferty et al 2008 ). There are numerous reasons to include parasitic interactions in a food web (Marcogliese and Cone 1997 ), and a growing field of work reflects this (Lafferty et al 2006b ; Hernandez and Sukhdeo 2008 ; Amundsen et al 2009 ; Preston et al 2014 ; McLaughlin 2018 ; Morton and Lafferty 2022 ). Parasites can have lasting impacts on their host organisms, both at the individual and population levels, by exploiting the host’s energy for its own development (Lafferty and Shaw 2013 ), altering the infected host’s behavior or morphology to increase its vulnerability to predation from free-living predators (Poulin and Thomas 1999 ; Miura et al 2006 ), or driving trophic niche specialization and competitive release (or perhaps relaxation) (Hatcher et al 2006 ; Britton and Andreou 2016 ; Rovenolt and Tate 2022 ).…”

“…The inclusion of parasites has been found to alter food-web structure and common food-web metrics, such as the number of links and trophic levels, food chain length, connectedness, and nestedness—factors that are all considered important for food-web complexity and stability (Lafferty et al 2006b , 2008 ; Hernandez and Sukhdeo 2008 ; Amundsen et al 2009 ; Thieltges et al 2013 ; Morton and Lafferty 2022 ). As increasing numbers of studies have included parasites in community ecology studies, it is apparent that parasites’ wide diversity and unique functional roles alter our understanding of an ecosystem’s structure and functioning (e.g., Lafferty et al 2008 ; Frainer et al 2018 ; Morton and Lafferty 2022 ). Accordingly, excluding parasites and their links from food-web analyses may impede further developments of comprehensive food-web theory and understanding and may lead to erroneous conclusions concerning how ecosystems are structured and function.…”

“…We predict adjusted connectance will also follow this trend as this metric more accurately measures the percentage of possible links that are observed (Lafferty et al 2006a ). Food-web generality and vulnerability are also predicted to increase along with free-living taxa generality and vulnerability as parasites represent additional consumers that rely on energy obtained from free-living taxa to continue their life cycles and introduce infective agents to top predators (Lafferty et al 2008 ; Amundsen et al 2009 ; Morton and Lafferty 2022 ). However, because each compartment has distinct parasitic communities due to their dissimilar free-living communities, we hypothesize that the generality and vulnerability of each compartment may behave differently from the other.…”

“…We expect a larger increase in generality and vulnerability in the benthic compartment as we anticipate more general parasites with complex life cycles and broad host ranges, such as the speciose trematode community (Soldánová et al 2017 ), to be prevalent here. Benthic trematodes also have a free-living life cycle stage during which they are extremely vulnerable to predation by free-living predators (Johnson et al 2010 ; Born-Torrijos et al 2020 , 2021 ), and these trophic links also serve to increase benthic parasite vulnerability (Dunne et al 2013 ; Morton and Lafferty 2022 ). The parasite community of the pelagic food web is in contrast dominated by cestodes (Amundsen et al 2009 ), which expectedly are less speciose and not as general and vulnerable as the trematodes.…”

A meta‐analysis of how parasites affect host consumption rates

“…Specifically, parasites affect the efficiency of consumers in obtaining prey and the susceptibility of prey to predation (Poulin 1999, Hatcher et al 2012). Hence, these inconspicuous members of biological communities mediate species interactions at all trophic levels and form important nodes in natural food webs (Lafferty et al 2008, Hatcher et al 2012, Morton and Lafferty 2022).…”