Food webs -- diagrams of who eats whom in an ecosystem -- are among the first aspects of ecology that most of us encounter in school. However, much is unknown about the rules governing the structure, formation and dynamics of these networks.
A large part of the reason is that reasonably complete food webs are extremely difficult to produce. A moderately large web can involve from tens to hundreds of species (even though many communities have far more) and it is necessary to trace feeding interactions for every single one of them. This can involve such pleasant tasks as examining the stomach contents of fish -- lots of fish. Tracking the feeding interactions of insects isn't easy either. And parasites, especially those that feed on other parasites? Forget about 'em!
That may not be a good idea, says a new study published in Proceedings of the National Academy of Sciences. (It is freely available here.) The authors took four published food webs that included parasites and examined the effects of removing them.
While much of the paper is devoted to methodological issues, the bottom line is that parasites are important. "Parasites dominated food web links; on average, a food web contained more parasite–host links than predator–prey links." Including parasites raised food web connectivity, which, computer models suggest, makes extinctions less likely to propagate.
The authors repeatedly return to the question of proper and improper methods of including parasites in food webs. However, I wonder whether they should be included at all. The problem is that many parasites have complex life cycles, requiring several hosts to mature and reproduce. In normal trophic interactions, having more than one food source makes an organism less vulnerable to extinction. But for a parasite, all links may be crucial. In other words, a predator-prey arrow doesn't have exactly the same meaning as a parasite-host arrow. To avoid confusion and develop robust theory, it may be best to distinguish between predator-prey and parasite-host interactions, at least where complex life cycles are involved.
Dunne, Jennifer A., Richard J. Williams and Neo D. Martinez. (2002) Network structure and biodiversity loss in food webs: robustness increases with connectance. Ecology Letters 5:558-567
Lafferty, Kevin D. Andrew P. Dobson and Armand M. Kurls. (2006) Parasites dominate food web links. Proceedings of the National Academy of Sciences 103:11211-11216