Evolution by natural selection is not usually considered very peaceful—the “survival of the fittest” is usually assumed to come at the expense of competitors for food or shelter or other resources. But the “fittest” can also be those who recruit assistance from other individuals, or other species—and who provide assistance in return.
This was the perspective of Peter Kropotkin, a Russian prince and political anarchist who studied the wildlife of Siberia while working as an agent of the Czar’s government. In the harsh conditions of the Siberian winter, Kropotkin reported finding not a bitter struggle over scarce resources, but what he called “Mutual Aid” among species, as well as in the human settlements that managed to eke out a living.
Something like what Kropotkin described is documented in a new paper by Elizabeth Pringle and colleagues. Examining a protection mutualism between ants and the tropical Central American tree Cordia alliodora, Pringle et al. found that drier, more stressful environments supported more investment in the mutualism.
Many different species of plants recruit ants as bodyguards. In the case of C. alliodora, the relationship works like this: the trees support scale insects, which suck the trees’ sap and excrete sugary honeydew. Ants—specifically the species Azteca pittieri—tend the scale insects and collect the honeydew, and establish a colony on the tree, which grows specialized structures, called domatia, to shelter the ants. Once established on a tree, the ants also defend it against intruders, including herbivorous insects. So the tree trades some carbohydrates, which it makes via photosynthesis, to gain the ants’ protection against things that might eat the leaves it uses to conduct photosynthesis.
Pringle et al. examined populations of C. alliodora from across Central America, which experience a range of different rainfall levels. Let’s just go directly to the data, from Figure 3 of the paper:
At sites with lower annual rainfall, the trees hosted more scale insects (panel A); and there were also bigger ant colonies on trees at the drier sites (panel B). So the trees at drier sites invested more in the mutualism, and hosted more protective ants. And it looks like this increased investment paid off.
Panel C is a scatterplot of the relationship between “latency time” for ants—the time between a disturbance on the tree, and the arrival of some angry ants—and precipitation. At the drier field sites, ants were quicker to run off potential enemies. Panel D shows the concrete benefit of that increased vigilance. The team excluded ants from selected leaves on trees at three sites with different rainfall levels, and then tracked how much herbivore damage those ant-free leaves sustained, compared to leaves the ants were free to defend. And, while the ants significantly reduced herbivory damage at the driest site (Chamela, the white bar) and the site with intermediate rainfall (Huatulco, the gray bar), they had no effect at the wettest site (Santa Rosa, the black bar).
As Pringle and her coauthors explain, Cordia alliodora is a deciduous tree growing in tropical dry forests. During the rainy season, the trees can grow leaves, which photosynthesize a supply of carbohydrates. When the rains are over, though, the trees drop their leaves, and if they haven’t stored up enough carbohydrates to fuel the growth of a new crop of leaves, they’re in trouble. In dry conditions, photosynthesis doesn’t proceed as rapidly, and losing a lot of leaves to herbivores could mean that a tree ends the rainy season without enough carbohydrates to make it to the next year. So it’s worth spending extra carbohydrates on bodyguards.
The big unanswered question here is, how do the trees actually manipulate the density of scale insects they support? The authors have no data, but they speculate that since the scale insects suck directly from the tree’s phloem—the vessels that carry sap—the trees at wetter sites may reduce the nutrient content of their sap, or otherwise make it less attractive to scale insects. African acacias, which are also ant-protected, have been shown to stop supporting their ants when they’re protected from herbivory by fencing, so this is pretty plausible.
However it works, Pringle et al. also note that their data illustrates a positive feedback loop: ants on the more supportive trees provide better protection against herbivores, and better-protected trees are able to spare more carbohydrates to support the mutualists. That feedback helps to explain why, even though each species involved is really only working for its own best interests, the mutualists stick together. Some previous theory work has predicted that positive feedback may often help to stabilize friendly interactions between species.
And so, as Kropotkin proposed, natural selection seems to have resulted in Cordia alliodora trees that cope with the hardship of drier climates by recruiting and supporting more mutualists—getting by with a little help from their friends.
Pringle E.G., Akçay E., Raab T.K., Dirzo R., Gordon D.M. & Agrawal A.A. (2013). Water stress strengthens mutualism among ants, trees, and scale insects., PLoS Biology, 11 (11) e1001705. DOI: 10.1371/journal.pbio.1001705.s019