Predator-Parasite Links in Food Webs

Predator-parasite links are common in food webs.  I posted previously about how ontogenetic specialists (e.g., many parasites) increase food web connectivity, but decrease stability.  That is, when you consider a species that requires different resources at different life stages as multiple “units,” instead of lumping all of the life stages together, the properties of the food web change.

When I posted before, I was really thinking about the resources of the “ontogenetic specialists.”  For a parasite species with a complex life cycle, those resources are the different host species.  But what if instead of looking at the ontogenetic specialists’ resources, we look at their predators?  Do different life stages get preyed on more than others?

Why, yes, yes they do!  Thieltges et al. (2013) classified predator-parasite links from eight food webs into one of three categories: concomitant predation, trophic transmission, and predation on free-living parasite stages.  Briefly, concomitant predation occurs when a parasite living in a host gets consumed along with its host by a predator, and the parasite is digested.  Trophic transmission occurs when a parasite living in a host gets consumed along with its host by a predator, and the parasite successfully establishes in the predator.  And predation on free-living parasite stages is just that – the parasites are being consumed when they are not in a host.  For trematodes, these free-living stages are eggs, miracidia, and cercariae.  (I mentioned previously that larval trematodes can be abundant in zooplankton communities, and may therefore represent a lovely food resource for aquatic predators.)

Break down of predator-parasite links in eight aquatic food webs. Mmm, parasite pie.

Turns out that concomitant predation is the most common predator-parasite link.  Neat!  There’s more cool stuff in the paper, but I’ll leave it to you to go check it out.

Have you seen this done elsewhere for other ontogenetic specialists?  For instance, do we expect larval organisms (e.g., tadpoles, larval insects, etc.) to be preyed on more than adults?

Reference:

Thieltges, D. W., Amundsen, P.A., Hechinger, R. F., Johnson, P. T. J., Lafferty, K. D., Mouritsen, K. N., Preston, D. L., Reise, K., Zander, C. D. and Poulin, R. (2013), Parasites as prey in aquatic food webs: implications for predator infection and parasite transmission. Oikos.

Wicked Cool Host-Commensal-Parasite System

I really like symbionts.  I really, really like interactions among symbionts, and I especially like it when commensals/mutualists eat parasites.  

So, it is with great pleasure that I introduce to you this urchin-crab-snail system.  The common pencil sea urchin (Eucidaris galapagensis) is a host for parasitic snails (Sabinella shaskyi and Pelseneeria spp) and commensal crabs (Mithrax nodosus).  And the crabs eat the snails!

Sonnenholzner et al. (2011) did some neat field and lab work to figure out how fishing for urchin predators affects parasitism of urchins by snails in this cool system.  Hilariously, they sum up their findings in the first line of the discussion by saying that they “found that the enemy (fisher) of the enemies (fish and lobster) of the enemy (crab) of the urchin’s enemy (snail) was the urchin’s friend.”  Swag.

Here’s the quick (and simplified!) version of their results, but I highly recommend checking out the paper!

I drew this fishing pole myself.

Do you know of any other host-commensal-parasite systems?  Bonus points if you guess my FAVORITE system of all!

Reference:

Sonnenholzner, J.I., K.D. Lafferty, and L.B. Ladah. 2011. Food webs and fishing affect parasitism of the sea urchin Eucidaris galapagensis in the Galapagos. Ecology, 92(12): 2276-2284.

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.