Of the five types of agricultural systems involved with obligate fungal cultivating ants, the most recently evolved, higher attine ants (hereafter leafcutter ants) have the largest ecological impact. Leafcutter ants are amongst the most organized, efficient, and voracious superorganisms ever to have evolved. Their fungal agricultural systems support the largest colonies on Earth, reaching up to 6 million individuals. That’s more than 23 times the population of Buffalo NY! You can imagine the caloric demands required for supporting a colony this size. Thanks to their fungal partners, these ants access nutrient pools that they otherwise couldn’t obtain. I have had the opportunity to view these species in their natural habitats twice already, and each time I come back home with an enriched admiration towards them. Through feeding their fungal gardens huge amounts of plant material, these ecosystem engineers both indirectly and directly alter their surrounding ecosystem. They don’t just select plants willy-nilly either. They are evolutionarily programed to choose plants that maximize their energy expenditure.
It all begins with winged sister queens dispersing during the beginning of the rainy season. Before they leave the nest, they head down to the maternal fungus garden. Here, she stores a small mass of hyphae in a specialized compartment located at the base of her mouth. She then mates with several males, where she accepts a lifetimes worth of sperm. Once she finds a suitable nest location, she drops her wings, excavates a shallow chamber, expels the fungal mass and immediately begins tending to the fungal cultivar. The fungal garden is, in essence, the life source for these species. She feeds the growing fungal mass with fecal droplets and unfertilized eggs. She continues cultivation duties until her first brood matures after 40-60 days. With a fleet of specialized worker ants, she resides in her brood chamber, directing all her energy towards the production of eggs. Workers that tend the brood move the eggs from the queen and imbed them into the fungal garden. Here, the eggs pupate, and the ant larvae solely feed directly on fungal hyphae. Compared to the ants obligate dependency on the fungi, the fungi too are entirely reliant on its ant partner.
The large Atta colombica queen, with her smaller workers and larvae on their fungal garden.
The fungi associated with leafcutter ants are unable to exist in a free-living form. The fungi gain a dispersal agent, a steady supply of plant material, as well as a manicured, pathogen controlled environment to grow and reproduce. Cameron Currie found a substance covered on ant individuals tending the fungal garden. Once under the microscope the substance was identified as bacteria. These bacteria 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 ants’ fungal cultivar gains pathogen resistance towards “bad” bacteria. When regions of the fungal garden become infected, you can find individuals rubbing their bacteria covered bodies in order to control the pathogen.
This manicured fungal garden receives not just plant material, but plant material that is chewed into a pulp. The mycelia can spread to the chewed material much easier than it could with unprocessed plant parts. This enhances fungal digestion which translates to more digestible carbohydrates becoming available to the ant colony. It’s important to note that these ants are generalists, that choose from a diverse assemblage of plants. Sometimes, ants unknowingly feed their fungal gardens plants they cannot digest. Undigested plant material is moved from the garden to garbage pits within the nest. Interestingly, ants visit these garbage pits, and learn the smell of undigested plant material. They then avoid selecting these plants as they simply take up space and may be vectors for diseases.
Foraging ants change the local plant community by increasing canopy openness. As ants remove leaves, more light reaches the ground. This directly increases the temperature of the forest floor, which enhances water loss. This is a positive feedback for the ants because leafcutter ants prefer foraging from water stressed plants. If you think about it, this makes perfect sense from the ants’ perspective. A water stressed leaf compared to a leaf of the same species with an adequate water supply contain more structural carbon and amino acids. Water stressed leaves also weigh less, so the energy cost is less for the material to be transported to the fungal garden. A plant being heavily foraged by leafcutter ants has an increased degree of water loss, as the cross sections of leaves are left with gaping, edge wounds. Without a waxy cuticle protecting the entire leaf from desiccation, plants heavily defoliated by leafcutters increased water loss by 45%.
Leafcutter's are ecosystem engineers. The ecosystem alterations extend outward from the central fungal garden upon leafcutter encroachment. These efficient agricultural systems permit the establishment of the largest ant colonies found on Earth. Their ecological achievements are owed to their fungal cultivars who widen their feeding niche, in return for having a dispersal agent and a continuous supply of plant material. In the Neotropics, these fugal farms act like ecological black holes, consuming massive amounts of plant material. Similar to black holes, these farms have a stronger effect on plant communities’ closer to the ant-fungal event horizon. With the establishment of a new, successful leafcutter colony, increased canopy openness radiates outward from the nest. Openness decreases the available water in a locale which enhances the degree of water stress in plants. Negative feedbacks occur in water stressed plants near these ant-fungal epicenters. Increased herbivory on these plants further increase water loss, making them even more desirable. These generalist species-complexes directly (herbivory), and indirectly (increase openness) shift the functioning of adjacent plant communities. For this reason, areas with abundant leafcutter colonies should shift the plant community towards more drought tolerant species. The way the forest floor functions is quite transient in the wake of a new leafcutter colony.
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