Why Are Canopy Ants So Dominant?

It may be Friday the 13th, but this article is talking about some lucky ants. Canopy ants, in particular. Ants that forage in trees exhibit a high level of ecological dominance, and ants are usually the most conspicuous organisms running around on tree trunks and branches, especially in tropical forests. This begs the question: Why? Dr. Terry McGlynn and Erica Parra, in a paper published last year, set out to address this question.

Nest of Polyrhachis ants, a dominant arboreal genus. Photo: Alex Wild

The leading hypothesis explaining canopy ant dominance is that the carbohydrate-rich diet of ants (from plant-produced sugar exudes to Hemipteran cattle) somehow promotes their success in the trees (let’s call it the Antkins Diet Hypothesis, the antithesis of the popular low-carb diet). If the Antkins Diet Hypothesis is correct, then we should expect to see a correspondence between increases in carbohydrate use and some factor in a colony that would increase colony success relative to possible competitors. McGlynn and Parra test four specific mechanistic hypotheses in the omnivorous (and scary!) tropical ant Paraponera clavata (the infamous “bullet ant”):

  1. Aggressive Defense Hypothesis
    – Energy from carbs allows colonies to more aggressively defend nests
  2. Metabolic Fuel Hypothesis
    – Higher carb energy allows a colony to send more workers out to forage
  3. Foraging Success Hypothesis
    – Higher carb intake increases rate of foraging success
  4. Prey Acquisition Hypothesis
    – Higher carb intake increases rate of protein-rich prey captures

The researchers conducted a series of experiments designed to tease apart the different predictions of each mechanistic hypothesis which might explain the Antkins Diet Hypothesis (if it’s true). Through observations of foraging ants, nest response to disturbance (by poking nests with a stick!), and stable isotope analysis, the researchers were able to assess evidence for each hypothesis in the bullet ant.

"Paraponera clavata"
The fearless co-author of this study, Erica Parra. Photo: Alex Wild

Surprisingly, McGlynn and Parra only found evidence for the Foraging Success Hypothesis. The rate of success of ants in finding a food source increased as the proportion of carbs in a colony’s diet increased. So, the researchers found some support for both the Antkins Diet Hypothesis and an associated mechanistic explanatory hypothesis. What this finding suggests is that there is nothing particularly special about carbs as a food source that drives canopy success. Instead, it seems that dominant canopy ants have carb-rich diets merely because carbohydrate sources are proportionally more available in the canopy compared to protein-rich sources

Overall, this study by McGlynn and Parra was a nice investigation testing an influential hypothesis, using simple experiments that evaluate several explicitly defined mechanistic hypotheses. Their findings lay the groundwork for similar tests in other species and environments.