50%-ish of life = Parasites

Ecologically Oriented very kindly linked to my blog today.  While I was over there visiting, I realized that I haven’t blogged about the ubiquity of parasites yet!  If I’m going to talk about parasite ecology, I should probably emphasize how common parasites are.  Though this is a topic that might well span multiple blog posts, a good starting point is this 2008 PNAS paper.  The paper is pretty much a gold mine of interesting tidbits, so I highly recommend reading it (the link is to the full-text).  Here are some of my favorite points:

Dobson et al. (2008) start the paper by pointing out that if you want to know the proportion of the world’s biodiversity that is parasitic, you probably need to know how many species there are in the world.  Unfortunately, we don’t know how many species there are.  A temporary answer is:  a lot, and a lot more than we have currently described.

There are estimates that something like 40-50% of organisms are parasites.  I don’t know what percentage of organisms are “predators,” or “prey,” or “predators AND prey” – is that review paper out there? – but 50% is probably comparable.  Parasitism is a common strategy!

Don’t forget that those 50% of organisms actually have to parasitize other organisms.  So, how many species are parasite hosts?  Well…all of them, probably.  Even parasites have their own (hyper)parasites!

I like this line from the Dobson et al. (2008) paper:  “In the best-studied taxa, an average mammalian host species appears to harbor two cestodes, two trematodes, and four nematodes, and an acanthocephalan is found in every fourth mammalian species examined.”  You’ve probably spent your life striving to do better than average, but maybe in this case you’ll settle for falling behind the curve?

So, pretty much every organism is either a parasite or has parasites or both.  That is a wonderful reason to frequently read a blog about parasite ecology!  😛  But why else should we care about parasites and parasite biodiversity loss?  Here are some reasons outlined by Dobson et al. (2008).  First, parasites are important in host population regulation.  Second, they’re important parts of food webs; I’ll blog about that separately in the future.  Third, helminth parasites bioaccumulate pollutants.  Again, that’s a story for another blog post, or you can check out the paper for more details.



Dobson, A., K.D. Lafferty, A. Kuris, R. Hechinger, and W. Jetz. 2008. Homage to Linnaeus: How many parasites?  How many hosts? PNAS 105(S1):11482-11489.

8 thoughts on “50%-ish of life = Parasites

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