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:
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?
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.
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.