“Bouncers keep the riffraff out”

by Jo Ohm


Dr. Tom Miller, Dr. Amy Savage, myself, and three fellow Rice undergraduates Marion Donald, Olivia Ragni and Rande Patterson at Sevilleta Wildlife Refuge, the home of many ant-tended cacti.

Aggression is a double-edged sword. Elbowing our competitors out of the way to forward self-interest offers several rewards but often comes with the risk of jealous friends, angry co-workers, a messy Bay of Pigs or competitors making mountains out of, well, ant hills. The same costs and benefits aggression offers to human aggressors, is found in ant-plant communities, with similar patterns and similar consequences. The solution to both is found in conflict resolution.

A study published in the Journal of Evolutionary Biology recently presented evidence that interactions between ant-tended plants, their pollinators and the ant-tenders, involve many competing conflicts of interests, giving rise to complicated community interactions. The article, “Ant aggression and evolutionary stability in plant-ant and plant-pollinator mutualistic interactions,” highlighted research conducted by Leonardo Ona and M. Lachmann at the Max Planck Institute for Evolutionary Anthropology. Their work suggested that the interplay between ants, pollinators and plants cannot be simply classified as mutualistic; benefits for one community member are not always beneficial for the members with which they live. College students in dormitories, commoners of the suburbs: We all can relate. There are indirect consequences and negative effects that arise from multiple individuals and multiple species all furthering selfish goals in an environment with shared resources and overlapping niches.

Tom Miller, a population ecologist at Rice University who studies similar model systems says that we encounter the costs of conflicting interests every day. Ant-tended plants can be conceptualized as a bar with strict bouncers. The ants tending their resource-filled habitat serve as bodyguards to defend the plant, on which they rely for food, shelter and other resources. Ants are like bouncers, the plants, like the bars.

“Bouncers need to keep the riffraff out,” Miller says. But he also brings up the question of how ants interested in protecting plants from predators will discriminate predator from non-predator. In the research published by Ona and Lachmann, ant-tending became detrimental to plant hosts at certain ant densities because ants could not distinguish pollinators from herbivores, and thus had no means to avoid deterring organisms that were beneficial for plant fitness.

“What if a bouncer could discriminate between people who spend a lot of money at the bar versus not a lot of money. Is there any benefit to the bouncer in making this distinction?” Miller asks. “For ants, the answer is ‘no’.”

Ants tending plants are dependent on the plant as a habitat and food source. When the plant’s ability to successfully reproduce is not directly related to the success of ants, Ona and Lachmann suggest that both are evolving with significant conflicts of interests. The ant is interested in protecting the plant from herbivore damage at all costs and higher rates of aggressive behavior correlate with lower rates of herbivory, encouraging ants to become more and more aggressive. Escalating ant aggression however is not always only directed at herbivores, but at pollinators as well. Thus, there is potential for plants to have lower reproductive success when ants are more abundant and more aggressive. The only way for this conflict of interest to be resolved is for the ant to depend on plant reproductive output. This could occur by ants eating ripened fruits or preferring to colonize new recruits of their plant hosts.

O. imbricata flowers in full bloom

Miller says that looking at the options for resolution in the ant-plant system can be applied to other model systems and have very broad applications.

“This type of research can be generalized to the idea of conflict resolution. Often a conflict of interest always exists when your dealing with community interactions and facultative mutualisms,” Miller said. “We see this all the time in cases of antagonism. An organism may help other organisms but never out of the goodness of its heart. Organisms always have self-interests motivating their involvement. Researching these systems helps us understand how these types of conflicts resolve.”

The broader insight Miller offers for Ona and Lachmann’s work is a lesson not foreign to human interactions with our own competitors. Conflicts resolve when we work for common goals. If ants and plants both become invested in plant reproductive success, conflicts of interest would be eliminated, and peaceful coexistence could emerge. Perhaps there’s a lesson here about finding our own peaceful coexistence. Like ants, we too could likely find commonly shared goals to lessen aggression and to evolve our communities into more stable states of coexistence.

Works cited:
Ona, L, Lachmann, M. Ant aggression and evolutionary stability in plant–ant and plant–pollinator mutualistic interactions. 2011. Journal of Evolutionary Biology 24(3):617-629.


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