How many worms is too many?

In disease ecology, we divide parasites into two groups: microparasites and macroparasites.  I have a previous post about the differences between the two groups (spoiler: size isn’t everything).  But to recap: microparasites tend to cause density-independent pathology, while macroparasites tend to cause density-dependent pathology.  In other words, the more macroparasites a host has, the more likely the host is to die or suffer reduced fitness.  Here is a graph of this concept that should make intuitive sense to everyone:


The relationship isn’t necessarily linear.

Why is it worse for hosts to have more macroparasites?  Because each one takes some energy from the host; each one steals some host resources.  So, more macroparasites means more stolen energy/resources.

But of course, hosts don’t get to choose how many macroparasites they have, and it turns out that macroparasites are not evenly distributed among hosts.  In fact, most hosts have no macroparasites, while just a few hosts harbor the majority of macroparasites.  This is called an “aggregated” distribution, and it is described by an even fancier statistical entity:  the negative binomial distribution.  One day, I’ll post about why we see this aggregated distribution of parasites among hosts, but for now, just know that aggregation of parasites is pretty much ubiquitously true in macroparasite systems.


Ok, so, some hosts are super unlucky and accumulate many macroparasites, and those hosts tend to have lower survival and fitness than other hosts.  What if instead of looking at organisms that are strictly parasitic, we look at symbionts that don’t harm their hosts but don’t help them either.  These are the commensalists (or stowaways) that I talked about last week.  Imagine, for instance, that an insect is carrying around one phoretic mite – a mite that needs to hitch a ride on another animal for dispersal.  The mite doesn’t benefit the insect, but it doesn’t hurt it, either.  Now imagine an insect completely covered in phoretic mites.  Are they still causing no harm?

phoretic mites on Sexton Beetle - Poecilochirus

That’s a lot of mites! Source: BugGuide.

And finally, what about mutualistic symbionts?  As I mentioned last week, branchiobdellidans are little worms that live on crayfish.  They can benefit their crayfish by cleaning the crayfish gill chamber, thereby presumably increasing gas exchange.  But they might also take bites of the gill tissue, which is not particularly mutualistic!  And perhaps they’re more likely to start snacking on gill tissue when other resources are low – like when there are so many branchiobellids that there isn’t enough other food to go around?

Brown et al. (2012) experimentally showed that “normal” branchiobdellid densities increase crayfish growth relative to crayfish with no worms.  But high branchiobdellid densities actually decrease crayfish growth relative to crayfish with no worms!  The relationship between branchiobdellidans and crayfish switches from mutualistic to parasitic with increasing worm density!  Very cool.

The dotted gray line represents crayfish growth in the absence of any branchiobdellids.  I humbly suggest that the authors name this THE PIRATE THRESHOLD.

The dotted gray line represents crayfish growth in the absence of any branchiobdellids. When worm densities get too high, crayfish growth actually decreases relative to the controls – we’ve crossed THE PIRATE THRESHOLD.

Like Goldilocks, crayfish need to find a worm density that is just right for them.  Next week, I’ll tell you how crayfish regulate how many branchiobdellids they have.  Stay tuned!


Brown, B.L., R.P. Creed, J. Skelton, M.A. Rollins, and K.J. Farrell. 2012. The fine line between mutualism and parasitism: complex effects in a cleaning symbiosis demonstrated by multiple field experiments Oecologia 170:199–207.

Ecological interactions: is parasitism under-represented in the ecological literature? [UPDATED]

I read a lot of papers about parasite ecology.  I try to stay up-to-date on the literature, and I also try to blog mostly about “new” papers, so that my posts are interesting to people in my field as well as my general audience.  When I’m scrounging for awesome new papers to blog about, I find myself wondering: why aren’t there more parasite ecology papers?!  When you consider that at least 50% of all organisms are parasites, and 100% of species have at least one parasite species, it seems like more ecology papers should be about parasites.  Is parasitism under-represented in the ecological literature?

This post is a quick analysis.  I feel like I’ve seen something like this in the literature before, but I couldn’t find it.  Please link me in the comments if you know of something published! [UPDATED:  See below*]

I used Web of Science to calculate the number of papers related to parasitism, predation, mutualism, and competition that were published in various popular ecology journals since the year 2000.  That’s four key words: parasit*, predat*, mutual*, and competit*.  Papers might end up in searches for multiple terms, but we’ll assume that there are relatively few of those papers.  Here are the raw data:


Parasites certainly seem under-represented in some journals, like Ecology.  But in others, like PNAS, parasites (almost) rule supreme! I’m not sure whether that is just completely idiosyncratic or if it actually means something.  However, I did notice that higher impact journals tend to have relatively more parasite papers:


Finally, to look at overall trends, here are the data lumped together across all journals:


Mutualisms need some TLC.  And while we’re at it, let’s focus on mutualisms beyond plant-pollinator and plant-mycorrhizal fungi systems!

Parasitism does appear to be under-represented!  When you consider that at least 50% of all organisms are parasites, it seems like more than 21% of ecology papers should be about parasites.

I suppose a large number of “parasite ecology” papers may end up in non-ecological journals, like Parasitology, International Journal for Parasitology, etc., whereas there is no Predatorology journal.  Do you think that might account for the under-representation of parasitism in the popular ecology journals?

***Someone very kindly linked me to a TREE paper by Raffel et al. (2008) that did something somewhat similar.  You can access the PDF here.  Raffel et al. (2008) were reviewing the ways that we can use concepts from predator-prey ecology to inform research in parasite-host ecology.  They also looked at the number of ecological papers regarding parasites and predators, and found that about 10% of ecological papers were about parasites, while about 20% were about predators.  I didn’t look at proportions of total papers like they did – I just did straight numbers of papers, so my “percentages” aren’t exactly comparable.  However, it looks like we both found that parasitism is underrepresented in the ecological literature.