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Friday, July 5, 2013

Adding parasites to the food web

When I was a kid, we learned about ‘food chains’. These were links between organisms and the things that they eat. 'Plants photosynthesize, rabbits eat plants, coyotes eat rabbits, coyotes die and decompose, detritus fertilizes plants' is an example of a food chain. In recognition that real life is far more complicated (voles also eat plants, owls eat voles and rabbits, owls compete with coyotes, etc.), we now usually talk about ‘food webs’. These networks show the interconnectedness of all organisms within an ecosystem.

Despite this improvement, there’s one major group of creatures that isn’t usually represented in food webs. Parasites. Researchers led by Jennifer Dunne from the Santa Fe Institute set out to remedy that omission.

The researchers analyzed three versions of well understood food webs. In one version, parasites were omitted. In the second, links between parasites and their victims were included but not concomitant links between parasites and the prey of other animals. For example, ticks were linked to rabbits but not to the coyotes that rely on those rabbits. The third version included all links.

You can see how complicated this all gets in the diagrams below.

Green: plants
Red: free-living animals
Blue: parasites
(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. 

First, parasites are sometimes unwittingly eaten by a predator that is feeding on that parasite’s prey. In the example above, a coyote might swallow a few ticks along with a rabbit. This sort of thing rarely happens with free-living species. Owls don’t inadvertently ingest lizards while they’re eating mice.

The other difference is that parasites often have unusually complex life cycles. For example, at different stages of its life, a trematode must use mollusks, invertebrates, fish or birds as hosts. Not too many predators are as particular. 

One thing is clear. If we want to fully understand ecosystems, we need to include parasites in our data sets. 

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.