Although ubiquitous in food webs, parasites are even less considered in food-web studies than free-living invertebrates, and few food webs report parasites. Kelp forests are famous for strong trophic interactions, and are physically and biologically distinct from ecosystems where parasites have been thoroughly studied. Our objective was to build a high-resolution topological kelp forest food web that includes parasites. We defined the study system as rocky reef that supports giant kelp (Macrocystis pyrifera) in the Santa Barbara Channel, California, USA. Three trophic sub webs (predator-prey, parasite-host, and predator-parasite) were constructed using information from sources including: primary and grey literature, direct sampling (dissections), expert opinion, and logical inference. Individual life stages were included when present in the system and when ontogenetic shifts occurred in the life cycle (for both free living and parasitic). Very rare species were only included if species were cryptic and/or unlikely to be detected by monitoring method, or high trophic level and transient, so naturally would be rare in the system.
Results/Conclusions
Resolving free-living species more than doubled the size of the network relative to previously published kelp forest webs, and adding parasites doubled it again. Inclusion of parasites increased the number of links, but proportionally less than the increase in nodes. Inclusion of predator-parasite links (concomitant mortality) added as many links as between free-living species. Key network metrics (connectance, link density, and degree distribution) were affected by the addition of parasites and concomitant links. Maximum trophic level and food chain lengths increased via both improved resolution of free-living species at mid-trophic levels and the addition of parasites feeding on top predators. Larval stages of shark and bird parasites were prevalent and abundant in fishes, highlighting the importance of transient predators. The open and dynamic nature of kelp forest ecosystems is highlighted by the presence of many free-living and parasitic species with life cycle stages occurring outside the ecosystem, while also serving as source of parasitic larval stages infecting transient predatory hosts. Adding parasites to the kelp forest food web makes it the most specious food web to date.