New CitizAnts of the City

Authors: Craig D’Innocente, Elizabeth Leece, Patrick Jay Quizon

New CitizAnts of the City

By: Craig D’Innocente, Elizabeth Leece, and Patrick Jay Quizon

Did you know that ants organize their own systems similar to humans? Atta Sexdens, commonly known as leaf-cutter ants, organize their colonies into castes to structure their civilizations using abundant foliage. The phenomenon of agriculture was once marked as human only, but leaf-cutter ants are farmers just like us! Vegetation brought in from outside the colony is processed by the ants to provide a growing medium for mushrooms, most commonly of the Lepiotaceae variety. The fungus protects the colony from other insects, parasitic fungi, and provides a nutritious food source. This coevolutionary relationship is known as mycophagy. Originating in the tropics of South America, the range of leaf cutter ants continues to spread into agricultural lands and urban cities. With warming temperatures, A.sexdens heat tolerance allows for colonies to enter a positive feedback loop, increasing their foraging rate thus their colony size, reproduction rate, and range limit. The systems emerging from these colonies may be fascinating to observe in nature, however, as they extend into human centers their interactions may not be so pleasant. 

Caste System

In order to maintain a colonies’ garden, a caste system has evolved along with seasonal patterns. Within the colony, workers are divided into 4 major castes (1); commonly differentiated by head width (ranging from 0.6mm-1.2mm head width). Small workers commonly referred to a nurse ants (~1.0mm) watch over the garden to make sure that it does not become affected by pathogens or other sources of diseases. The nurses, as well as the slightly larger within-nest generalists (~1.4mm), are also in charge of managing the growth and breeding of the fungal gardens as well as the entire colony. The food for the garden is provided by even larger forager ants(~2.2mm). Forager ants travel to nearby vegetation and harvest the leaves so they can provide nourishment to the gardens. On their way, foragers trail back and forth from sources of leaves to the colony, exposing the colony to potential invaders. In order to protect the ants and the colony, soldier ants, called Defenders (~3.0mm) guard the nest. These ants are easily distinguished from the other ants because of their large head which contains massive teeth, which can easily intimidate any threats to the colony, its gardens, and its citizens. The most important role of the soldier ant is to provide protection to the queen, the most vital individual in any ant colony: their survival means the survival of the colony as a whole! Queen ants are easily the largest ants in the entire colony, as they can grow as big as 25.4 mm (1 in). Queens strictly create eggs to replace and expand the colony. Although this is a seemingly easy task, the amount of eggs a queen produces is positively correlated how much energy the colony creates in their gardens (2). Bigger gardens means more nutrients for the queen, which in turn means more members of the colony. In late Fall, the queen will begin to mate. With enough sperm, an exciting event takes place called nuptial flight, the queen flies away from her old nest, up to 11km away and burrows into the ground to begin egg-laying. Leaf-cutter ants are some of the few species of ants that practice polyandry, which means that they can have multiple queens in one colony. A.sexden queens are also the most long-lived and fertile insects, with 96-100%; creating a “large and complex society” (3). The changes in the reproduction of the species have massive effects to their overall population and fitness. 

1. sexdens are known to be an “aggressively territorial species” (1), expanding their colonies in all directions if given abundant foliage. The success of colonies depends on multiple factors including queen fertility and vigor, preservation of fungal garden, and temperature (2). As the temperature continues to rise, leaf-cutter ants are evolving to withstand not only these disturbances, but also the disturbances inflicted by human development.

Life in the City

As humans begin to urbanize many parts of the tropics, new towns and cities are becoming more common. These cities increase local temperatures since materials used for construction have high heat absorbance. What this means is that these materials (such as cement and asphalt) have a tendency to absorb and hold heat. Cities have become literal hotspots for both temperature and leaf-cutter ant colonies. Due to urban expansion into leaf-cutter habitat, colonies are found near city roads with increasing concentrations, profiting off of human disturbance (4). These colonies exhibit a higher heat tolerance following along the roads. Rather than invading human homes or dumpsters, leaf-cutter ants typically prefer outdoors spaces with abundant foliage.

Urban dwelling for leaf-cutter ants is due to their increased activity in warmer temperatures. Warmer temperatures increase foraging rates, which means that surrounding vegetation will become likely targets as nutrient sources for their fungal gardens (5). This acts as a positive feedback loop. Increased foraging leads to larger fungal gardens, which increases a colonies’ energy production; which allows the colony to produce more queens, and ultimately more offspring (2). Eventually, a colony will produce excess queens, who will leave their birth colony to start their own colony elsewhere (6). With increasing urbanization of the tropics, this means that it is more likely that queens have already begun their new colonies next to the cities. 

The People of the City

We already know what city living means for leaf-cutter ants, but what does it mean for the human inhabitants of the cities which they colonize? Leaf-cutter ants may not have as much of a direct impact on humans as other, more pesky ant species (like the ever present argentine ant), but they still manage to interact with humans. Rather than invading human homes or dumpsters, the threat leaf-cutter ants pose to human dwellings is outdoors, in our gardens and parks. Before going any further, this is not meant in any way to demonize leaf-cutter ants. Any damage they do to human establishments is due to large urbanization developments, and other anthropogenic effects which affect their nesting habits. Leaf-cutter ants might tend to their own garden and keep it safe, but that does not mean that they will protect and improve the ones created by people.

Agricultural Ants in Human Agriculture

That being said, leaf-cutter ants can pose a substantial threat to both local agriculture and agribusiness, as well as essentially anything else humans choose to grow. Leaf-cutter ants have been known to strip citrus trees of their leaves completely in 24 hours (7). This is like a garden competition, except sabotaging the work of others is allowed. 

Threat to agriculture does not go unnoticed by farmers, who have made a grand effort to protect their crops from destruction. Unfortunately, what this means for leaf-cutter ants and other flora and fauna is a deluge of pesticides. Pesticides are a huge threat to basically everything in the natural world, with their only benefit being protection for crops. As it stands, pesticides are likely the greatest direct threat to leaf-cutter ants as a species (8). And as habitat loss continues to be exacerbated by human development, leaf-cutter ants will continue to establish colonies next to cities and human establishments. This will lead to human produce being destroyed by leaf-cutter foraging, which will make humans search for ways of staving off defoliation. 

The True Enemy

The final threat to leaf-cutter ants, as is true for nearly every species, is climate change. Even though leaf-cutter ants have been shown to react well to high temperatures, there is undeniably still an upper limit to their heat tolerance. Indeed, because of their above average heat tolerance, conservation efforts may leave leaf-cutter ants behind. Regardless of how well they may be able to tolerate heat at this point in time,  they will still be affected by climate change just like every other species on Earth. 

What Can be Done

Atta Sexdens exhibit adaptations to respond to disturbances; however, we are still unsure of the long-term consequences of climate change and human disturbance. Although there is evidence for tolerance within observed populations, it is hard to assess the challenges within ant and human interactions. Creating natural habitats is important for all ecosystem functioning. The physiology of the species greatly depends on the ability to obtain resources within their environment.

People have begun movement to help mitigate the drastic changes in the world. Team trees began a campaign asking everyone in the world for donations (9). One tree will be planted for every dollar donated to the program. 20 Millions trees will lead to many changes to both the world and people (10). This program not only slows down the effects of climate change, it also provides and maintains new locations for leaf-cutter ant habitats and colonies.

Images belong to Wikipedia and Pixabay respectively

Citations

References

1. A. Byrne, “Atta sexdens” (Animal Diversity Web, University of Michigan. 2004)
2.  A. A. Moriera, L. C. Forti, R. S. Camargo, N. S. Nagamoto, N. Caldato, M. A. Castellani, V. M. Ramos, Variation in nest morphology, queen oviposition rates, and fungal species present in incipient colonies of the leaf-cutter ant Atta sexden. Tropical Zoology. 32, 107-117 (2019).
3. S. E. F. Envision, W. O. H. Hughes, Genetic caste polymorphism and the evolution of polyandry in Atta leaf-cutting ants. The Science of Nature, 8, 643–649 (2011).
4.  F. F. S Siqueira. J. D. Ribeiro-Neto, M. Tabarelli, A. N. Andersen, R. Wirth, I. R Leal, Leaf-cutting ant populations profit from human disturbances in tropical dry forest in Brazil. Journal of Tropical Ecology. 33, 337-344 (2017).
5. H. G. Fowler, S. W. Robinson, Foraging by Atta sexdens (Formicidae: Attini): seasonal patterns, caste and efficiency. Ecological Entomology. 4, 239-247  R. T.
6. Fujihara, R. S. Camargo, Luiz Carlos Forti, Lipid and energy contents in the bodies of queens of Atta sexdens rubropilosa Forel (Hymenoptera, Formicidae): pre-and post-nuptial flight. Rev. Bras. entomol. 1 (2012).
7. The Bug Master. “Leaf Cutter Ants.” The Bug Master, 6 Feb. 2017, https://www.thebugmaster.com/leaf-cutter-ants/.
8. J.S Britto , L.C. Forti, M.A. de Oliveira , R. Zanetti , C.F. Wilcken , J.C. Zanuncio , A.E. Loeck , N. Caldato , N.S. Nagamoto , P.G. Lemes, R.S. Camargo, Use of alternatives to PFOS, its salts and PFOSF for the control of leaf-cutting ants Atta and Acromyrmex. International Journal of Research in Environmental Studies 11-92(2016)
9. “Help Us Plant 20 Million Trees - Join #TeamTrees.” #Teamtrees, teamtrees.org/.
10. “Planting 20,000,000 Trees Will Actually Have This Impact”, Youtube, https://www.youtube.com/watch?v=-cPdImejxEQ&t=1s