The Ants and the Wasp: Fables Aesop never told

Getting along together is tough. In ecology, the problem is often stated the way the great G. E. Hutchinson did in a famous paper "Why are there so many kinds of animals?" In principle, a given species at some location should be a little better at getting food than others, and it ought to outcompete any similar species, driving them to extinction. If that were true, we would expect there to be very few species anyplace. Instead, there are lots of species all over.

The explanation Hutchinson offered 50 years ago is the foundation of modern community ecology – each species is different enough from others that it is best at something. Sometimes similar species feed at different times of the day, or have specialized on one food source. I described how competition between pollinators has driven insect diversity yesterday, and a similar dynamic drives, or at least permits, diversity in other ecosystems.

A paper in the March issue of American Naturalist illustrates a fascinating example of a community that maintains its diversity through the different tradeoffs each species has to make.

In Maintaining Diversity in an Ant Community, Adler, et al. study a community of ants in an Arizona desert. The researchers put out small crickets and large crickets for the ants to scavenge. The large crickets would require a team of ants, while the small crickets could be carried by a single foraging ant.

The different species of ants found their prey at different rates. Once they found the prey, they differed in how quickly they could carry it to a nest, and in how well they did at defending the prey once other species arrived. This trade-off between speed of discovery and effectiveness of defense represent a set of axes on which species can differentiate themselves. If only speed mattered, then the fastest species would drive the others to extinction, while if defensive ability only mattered, the best defender would starve the other species out. As it is, a smaller, speedier ant can recover some of its prey before it gets chased off, while the better defender doesn't have to get there first, since it can steal some of the crickets after other species have found them.

This model alone could explain how several of the six species could coexist, through tradeoffs in prey availability, speed of harvest and ability to defend prey. Two species managed to beat the tradeoff, both discovering prey and defending it well. How did those two species manage not to drive the others extinct?

The answer comes when we look at the ecosystem more broadly. Those two species are preyed upon by a wasp. The wasp is one which lays eggs in living ants, and whose offspring eat the inside of the ant like a parasite, until the adult wasp emerges from the ant, killing it. This combination of parasitic behavior ending in the host's death is called "parasitoid." These parasitoid wasps target the two species which would otherwise outcompete the other 4. When a wasp approaches the prey item, those two species have to hide, allowing the other species to compete for the prey.

Differences in the foraging behavior of those two species allowed them to persist with one another in the absence of their parasitoids.

I leave you to draw your own moral from the story.