The small scale insects (green ovals) live on coffee plants at various distances from the shade tree where the voracious ant, Azteca, builds its nests. The big black beetles are insatiable predators of the scale insects. So when the scale insects are near a shade tree that contains an Azteca nest, scale insects benefit because ants keep their beetle predators away. See full caption below.

University of Michigan ecologists John Vandermeer and Ivette Perfecto used key concepts from complexity science to tell the natural history story of a common coffee pest, the green coffee scale insect. They published “Hysteresis and critical transitions in a coffee agroecosystem” in the Proceedings of the National Academy of Sciences July 9, 2019.

“As farmers seek solutions to perceived problems on their farms, agroecologists rightly wish to use the science of ecology to help,” states the paper’s abstract. “Unfortunately, basic research required to understand some of the vexing problems the farmers face is in its infancy. Yet ecology is complicated. Normative rules of thumb extrapolated from a few experiments or local traditions frequently fail in the face of such complications. Only through acknowledgment of the agroecosystem as a complex system may we gain full knowledge of the relevant ecological principles that in turn can aid agroecosystem management. Here we take a significant pest, the green coffee scale insect, as a model system and explore the utility of several concepts from complexity science to understand its regulation.”

“Perhaps the most surprising thing about the results is the way in which biodiversity and spatial patterns create conditions in which a potentially important agricultural pest is controlled,” said Vandermeer.  “At first it might seem that an obvious aggressive ant protecting the pest would itself be a pest (the friend of my enemy is my enemy).  Yet, that very ant interacts with a parasitic fly that attacks it to create a spatial pattern, one component of which acts as a restricted refuge for the scale insect, which is essential to keep the beetle predator from overeating and itself going locally extinct due to a lack of food.  Thus, all the components of this biodiversity interact in complicated ways that ultimately, and surprisingly, create complex interactions that result in the self-management of this potential pest.”

These results are currently being used to examine other pests in the coffee agroecosystem as well as other systems, agricultural and otherwise.  Any time an organism faces both a predator and a disease and lives in a diverse community, this basic mechanism of population regulation may be operating.  Generally speaking, the results may be important in future development of pest control strategies, reinforcing the generalization of many ecologists, entomologists and others concerned with food and agriculture issues. “With proper ecological management of agroecosystems, using basic ecological principles, ‘potential’ pests may never become real pests because of ‘autonomous pest management’ stemming from the complex interactions among all the components of the biodiversity in the system.”

Vandermeer is the Asa Gray Distinguished University Professor of Ecology and Evolutionary Biology and an Arthur F. Thurnau Professor. Perfecto is the George Willis Pack Professor in the School for Environment and Sustainability. They are partners in research and in life and have been studying coffee agroecosystems in Mexico for over 20 years.

Visit the EEBlog and search on Coffee in Mexico to see a series of blog posts by William Foreman and Mike Wood of Global Michigan from a 2015 trip to a coffee farm in Chiapas, Mexico with these devoted ecologists. Videos from the trip are on EEB's YouTube channel in the Research playlist.

Full image caption: The small scale insects (green ovals) live on coffee plants at various distances from the shade tree where the voracious ant, Azteca, builds its nests. The big black beetles are insatiable predators of the scale insects. So when the scale insects are near a shade tree that contains an Azteca nest, scale insects benefit because ants keep their beetle predators away. But when the scale insects are in the shaded tree refuge, there’s no predator to control their population and they reproduce rapidly, creating a dense local population, which becomes a focus for the proliferation of a fungus disease, symbolized in this cartoon as white ovals.

This combination of the beetle predator in one part of the farm, with the refuge for the scales, which allows the disease to build up, creates a complex spatial system that keeps the scale insect, a potentially important pest of coffee, under control. When scale insects disperse from far away from the refuge toward the refuge, they may or may not encounter a coffee bush that is patrolled by the ants (it depends on how close the coffee bush is to the refuge and where the ants happen to be foraging when the scale insects arrive). So some coffee bushes will have scales being eaten by the beetles, while others will be under ant protection (technically speaking, there are alternative equilibrium points in the system). Alternatively, when scale insects disperse from near the refuge to farther away, the point at which they are likely to encounter the predacious beetles in the absence of their ant protectors, occurs at a different point in space from the critical point when they are moving from far away toward the refuge. This difference between moving toward the refuge versus moving away from the refuge is known in the complexity science literature as hysteresis. This underscores the seemingly counterintuitive situation whereby changing a system in one direction gives a particular pattern, but returning it to its former state may not return to the original pattern.

Compiled by Gail Kuhnlein