April 15th, 2015
In the living world, almost everything gets eaten eventually. That’s one of the things we all learn early on in our biology classes. Usually, the lesson is illustrated with food webs. Put simply, producers (or, with a fancier term, autotrophs) transform the sun’s energy into organic matter. They get eaten by consumers (or heterotrophs). These consumers, in turn, can get eaten by others, and so on. And then there are the decomposers (aka saprotrophs), who break down dead or decaying organisms.
But wait, what’s missing here?
In food webs as we imagine them, we tend to think exclusively of free-living species. And in doing so, parasites get ignored, despite the fact that they are a significant part of the earth’s biodiversity and can have quite substantial effects on host populations. Time then, to include them into the food webs we’re all so familiar with. Some have even suggested that their inclusion could dramatically alter the structure of these webs.
Is this the case?
Well, let’s test that, a research team spearheaded by Jennifer A. Dunne from the Santa Fe Institute and the Pacific Ecoinformatics and Computational Ecology lab in Berkeley, California, thought. Their results are published in the journal PLoS Biology. They started with seven well-known food webs and analyzed three different versions of each. Version A: no parasites, version B: parasites, and version C: parasites with concomitant links (taking into consideration that, when a predator eats its prey, it also eats its parasites) (see figure).
The analyses show that, when incorporating parasites, the food web, perhaps unsurprisingly, gets more complex (and even more so when including the concomitant links). But, for the claim that parasites have fundamentally transforming impacts on the structure of the web little support was found. Instead, what the researchers discovered was that the inclusion of parasite has similar effects as the inclusion of other free-living species in their place. In other words, the changes seem to result from an increase in the number of organisms, rather than from adding a specific type of organism.
However, there were two observations that might point to a special role of parasites. First, adding parasites alters some types of interactions (as they can get eaten by the predators of their host), and second, the feeding niches of the parasites are a bit different (complex life cycles, often with multiple hosts). Overall, when parasites enter the food web, it changes. But, those changes are mainly the result of adding more organisms, not because the newly included parasites have a unique effect.
parasites research,parasite food chain,evolutionary ecology of parasites