Red: free-living animals
(A) Web with free-living species only.
(B) Web with parasite species but not concomitant predation links.
(C) Web with parasite species and concomitant links.
PLoS Biology, 11 (6) DOI: 10.1371/journal.pbio.1001579.Adding any large group of organisms into a food web is going to modify the whole network, and parasites can make up a quarter of the species in a food web. However, they have a couple of unique traits that alter the dynamics more than most species.
Dunne, J., Lafferty, K., Dobson, A., Hechinger, R., Kuris, A., Martinez, N., McLaughlin, J., Mouritsen, K., Poulin, R., Reise, K., Stouffer, D., Thieltges, D., Williams, R., & Zander, C. (2013). Parasites Affect Food Web Structure Primarily through Increased Diversity and Complexity PLoS Biology, 11 (6) DOI: 10.1371/journal.pbio.1001579.