There is no hiding how much I admire the leafcutter ants. As soon as I learned how their societies worked, I knew I was in the right field. Seeing these forces of nature with my own eyes in Costa Rica has been a transcendental experience to say the least. They really do function like ecological black holes, sucking in plant material with more strength, the closer plants grow to the colony epicenter. In some of the bigger colonies, you can literally see the event horizon; the distinct circular perimeter just outside the area of maximum likeliness of colony plant selection. In a previous post, I mention how ants are more likely to select water stressed plants, as that material requires less physical work to move, and there is a higher nutrient to weight ratio. In this post, I’ll talk about a study carried out in 2010, that identifies an additional selective force at play. One that has to do with fungi growing outside of the nest.
These completely massive underground cities can reach a density of 6 million individuals. Their overwhelming size is a result of their ability to cultivate edible fungi in giant subterranean galleries. Worker ants work day and night, forming a trail to a suitable plant. Individuals cut small fragments of leaves and flowers to bring back to their nests. Inside the nests near the fungal farms, smaller ants designated to tend the garden chew the plant material into a pulp and place it meticulously on the growing mass of fungi. Being supplemented by a steady supply of plant material, the fungal cultivar breaks down the recalcitrant plant carbon into more simple sugars that the ants can utilize. Over millions of years, evolutionary processes have shaped how ants select suitable plants that will feed their fungal gardens, that will in turn, feed themselves.
To explain another possible selective force at play, Leonora Bittleston and her team designed a laboratory choice experiment. Individuals from the leafcutter species Atta colombica had to choose between two seedlings of understory tropical plant, Cordia alliodora. One of the plants had a low density of fungal endophytes growing on them, while the other seedling had 5.5 more endophytes growing on it.
The choice experiment churned out significant results, with ants cutting over 2.5 times the area of plants with the low endophyte treatment. Ants recruited more workers to plants with fewer endophytes, clearly showing this species has an engraved selective motivation, evolved to enhance colony fitness. Fungal endophytes could start breaking down plant tissue, making it more nutrient poor before it makes its way to the fungal garden, but the leading hypothesis for this selection is the assembly of contaminants the endophytic rich plant material could bring to the fungal gardens.
Leafcutter ants have been combating contamination on their fungal garden since they first figured out how to function as an agricultural society. Interestingly, leafcutter specialist Cameron Currie found a substance covering ant individuals that was later identified as bacteria. These bacteria growing on the bodies of ants tending the garden were then isolated and tested for antibiotic properties. As it turned out, this ant-fungal symbiosis was actually a three-way symbiotic relationship. The bacteria covering the ants gain a place to live and feed, while the ant’s fungal cultivar gains resistance towards pathogenic bacteria. With this in mind, one can realize that controlling a fungal contaminant is more troublesome. A metabolite that reduces the growth of the fungal contaminant likely reduces the growth their own garden, so antifungal production is kind of out of the question. Instead, ants have evolved a preemptive tactic to reduce fungal contamination. Atta colombica selects plants with fewer endophytes. This gives its colony a better chance of maintaining clean gardens, rid of fungal contaminants.
This is interesting from the plants perspective as well. Enhanced fungal endophyte density can actually relate to higher plant fitness. Before this study, fungal endophytes were seen as parasites, as their growth intercepts sunlight and reduces the plant’s photosynthetic output. But now, this interaction becomes slightly more complex, with endophytes reducing leafcutter selection. From a previous post, I had learned that drought resistant plants are less selected by leafcutter ants. Putting these two ecological ideas together, I can confidently say that in ecosystems where leafcutter ants persist, drought resistant plants with abundant fungal endophytes are much fitter than plants with fewer endophytes that are water stressed. These interactions involving ant selection thereby drive community assembly in the American tropics.