The silent force in the food web

Addition of parasites (red spheres) visibly increases connectivity of species in this representation of an Arctic food web.

Studies of food webs fascinate community ecologists. There seems to be a never-ending supply of interactions to observe, analyze and use in predictions. From the largest apex predators, feeding once a week, to the smallest alga, constantly converting sunlight to energy, there’s a kind of wonder in the idea that all living things are connected.

In truth, however, ecologists are just beginning to realize that in this picturesque painting of a community in harmony, some less cuddly players are conspicuously absent: parasites.

In the May issue of the Journal of Animal Ecology, Per-Arne Amundsen of the Norwegian College of Fishery Science and his colleagues wanted to know whether including parasites in a food web would significantly alter the connectivity of the web itself. The connectivity is the proportion of possible interactions among species that are actually realized in the food web.  These numbers are usually low, since the possible number of connections equals the square of the number of species – if there are 20 species in a community, there are 400 possible connections.

The authors examined a lake community where the interactions among species are especially well known.  They then produced a different food web that included parasites living within organisms in the original food web, and compared the two.  As expected, the number of connections increased. Predators often acquire parasites by preying on infected organisms; the authors found that each of the parasite types they studied was ingested by one-third of the free-living organisms.

It’s well-known that parasites are ubiquitous within the food web, but, as pointed out in a commentary in the same issue, ecologists are at the point where they’re still talking about including parasites in food webs, but most are not actually doing it. The commentary, by Andrew Beckerman and Owen Petchey of the University of Sheffield, UK, also notes that another emerging line of research will be the study parasites’ effects not only on their hapless hosts, but also on each other.

Amundsen, P., Lafferty, K., Knudsen, R., Primicerio, R., Klemetsen, A., & Kuris, A. (2009). Food web topology and parasites in the pelagic zone of a subarctic lake
Journal of Animal Ecology, 78 (3), 563-572 DOI: 10.1111/j.1365-2656.2008.01518.x

Beckerman, A., & Petchey, O. (2009). Infectious food webs Journal of Animal Ecology, 78 (3), 493-496 DOI: 10.1111/j.1365-2656.2009.01538.x