(Yes, I'm late. Other deadlines and children are to blame. Carry on.)
When you envision the top predator of an ecosystem, what comes to mind is something like a thick-maned lion stalking prey on the African savannah, or a pack of wolves bringing down an elk. But a new study appearing in Nature this week suggests that in at least some ecosystems the parasites can give the more traditional top predators a run for their money.
A team led by Armand Kuris and Ryan Hechinger (both from the University of California at Santa Barbara) spent five years sampling the plants and animals in three estuaries on the Pacific coast of California – scanning, seining, and coring until they had an estimate of biomass for each of the 199 species of free-living animal and 15 species of plant they identified. At the same time, they dissected these animals and plants to survey the parasites living inside them.
When they were finished, they found that the 214 free-living species of plant and animal lived in association with 138 species of parasite. Individual parasites were small, and on average made up less than 2% of the weight of their hosts. But taken as a whole they were nevertheless a considerable chunk of the overall biomass in the ecosystem, comparable to the overall mass of the fish in these estuaries. More surprising still, the biomass of parasites was greater than the biomass of birds -- the ecosystems’ top predators. The graph below (from Figure 2, Kuris et al. 2008) 
shows a breakdown of parasites in the estuaries. Trematode worms are the largest segment of parasite biomass, probably due to their habit of castrating their snail host, forcing them to make thousands and thousands of baby trematodes instead of thousands and thousands of baby snails. Next come isopods and tapeworms, followed closely by nematode worms and parasitic copepods. That little bird icon on the far right shows the estimated biomass of all the birds in the ecosystem. Consider that for a moment – if you could catch all the birds in this estuary and put them on one side of a balancing scale, they would be outweighed by just the parasites living inside the snails the birds are probably hunting. No one has ever doubted that parasites play important roles in biological communities. These results suggest that they could be even more important than we thought. There’s more here, at Not Exactly Rocket Science.
Source: Ecosystem energetic implications of parasite and free-living biomass in three estuaries. Kuris, A. M. et al. 2008. Nature 454: 515-518.
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