Ontogenetic Specialists (=Parasites!) in Food Webs

I was reminded today of a really cool paper that I should write about: a 2011 Ecology Letter’s paper called “Stage structure alters how complexity affects stability of ecological networks.”

First, I should define secondary extinction.  A secondary extinction occurs when you remove one species (or several) from a food web, and then a second species goes extinct as a result.  For example, if you remove the one host species that some parasitoid species relies on, that parasitoid should then go extinct.  Considering secondary extinctions is one good way to consider food web stability; a robust food web is one that has few secondary extinctions per primary extinction.

A few years ago, Lafferty and Kuris (2009) found that though parasites increase the number of linkages in food webs, they actually decrease food web robustness/stability.  (There is a lot of literature out there about the relationship between complexity and stability.  Here’s a tiny taste.)  That decrease in robustness occurs because parasites (in this case, trematodes) are more prone to secondary extinctions than other organisms, especially if you’re removing their hosts.

Rudolf and Lafferty (2011) very cleverly showed that this is likely the result of the complex life cycles of trematodes (or other parasites).  They call them “ontogenetic specialists,” meaning that at different life stages, individuals of the same species utilize different resources.  Parasites are, of course, not the only organisms that are ontogenetic specialists.  In fact, Rudolf and Lafferty (2011) point out that Werner (1988) estimated that ~80% of taxa are ontogenetic specialists.

One cool ontogenetic specialist. Tadpoles eat algae, while frogs are carnivores. Should they be one food web node?

Rudolf and Lafferty (2011) made theoretical networks with various levels of niche overlap among ontogenetic stages.  They then analyzed the robustness of those networks by removing 30% of the species and then quantifying secondary extinctions.  They also did the same stuff with a particular empirical example – a salt marsh food web with the parasites included.

They found that on average, 35-80% of resources overlapped within species in empirical systems.  Note that they didn’t find something close to 100%, which is what we would expect if  ontogenetic specialization didn’t occur.  This is important because most food web models consider the species as the unit, meaning that all individuals in that species are modeled as eating the same average stuff (100% overlap).  But if that species has three life stages and each stage uses different resources, than all individuals aren’t doing the “average” thing.  In other words, we’re treating our species like generalists, when no individuals are generalists.

So, ontogenetic specialists like parasites increase the complexity of food webs, but decrease the stability.  This negative relationship counters the complexity-increases-stability hypothesis, and the negative relationship occurs because ontogenetic specialists are especially prone to secondary extinctions.  Cool!

Further Reading:

Rudolf, V. H. W., and K. D. Lafferty. 2011. Stage structure alters how complexity affects stability of ecological networks. Ecology Letters 14:75–9.