According to Peter Olson of the Natural History Museum in London, “All free-living organisms host one or more parasites”. This can be taken two ways, both of them generally true: a) that each individual multicellular organism hosts at least one individual parasite within its body, and b) that each free-living species plays host to at least one species of parasite that attacks it exclusively. Consider this second point for a moment. For each free living species there is one or more (usually several more) parasite species — that is, as a category (polyphyletic, obviously), parasites may very well be the most diverse types of organisms on the planet.
On the other hand, most parasites are much smaller than their hosts, and so it has typically been assumed that they contribute a negligible fraction to any particular ecosystem’s total biomass. Nuh-uh. In a report published in Nature this week, Kuris and colleagues presented five years’ worth of analyses of estuaries in California and Baja California in which they measured the amount of biomass made up of parasites and free-living organisms.
In sum, Kuris et al. (2008) examined 138 species of infectious agents, 199 species of free-living animals (including invertebrates as well as fishes and birds), and 15 species of free-living plants in their study. They found that plants contributed the most biomass to all three of the estuaries they studied, followed by groups such as snails, bivalves, and crabs. Parasites made up only about 0.2% to 1.2% of the animal biomass of each environment, and on average parasite groups had biomasses 1000 times lower than the average free-living group. However, as Kuris et al. (2008) report,
Certain parasitic groups dominated the parasite biomass, reaching levels similar to those of common free-living groups. For instance, the biomass of trematode worms was comparable to that of the fishes, burrowing shrimps, polychaetes or small arthropods. In all estuaries, trematode biomass exceeded bird biomass by threefold to ninefold.
In other words, parasites make up a larger fraction of the living matter in these environments than do the top predators. In particular, parasites that castrate their hosts (i.e., prevent them from diverting resources into reproductive effort) were the most abundant. The world is not fishy or feathery, it is fluky.
So, whereas the famous quote attributed to J.B.S. Haldane that if there is a creator he must have “an inordinate fondness for beetles” still applies, it may be that he has an even more pronounced affection for parasites. Especially the castrating sort.
Kuris, A.M., R.F. Hechinger, J.C. Shaw, K.L. Whitney, L. Aguirre-Macedo, C.A. Boch, A.P. Dobson, E.J. Dunham, B.L. Fredensborg, T.C. Huspeni, J. Lorda, L. Mababa, F.T. Mancini, A.B. Mora, M. Pickering, N.L. Talhouk, M.E. Torchin, and K.D. Lafferty. 2008. Ecosystem energetic implications of parasite and free-living biomass in three estuaries. Nature 454: 515-518.