On the ninth day of Christmas –
Chalk it up to a complicated interplay of predator / prey relationships in the ecosystem
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Azteca ants build their nests in shade trees, and it’s relatively common to find other nests in trees nearby. But these clusters of ant nests are often separated by large sections of shade trees where there are no nests at all. ADecember overview paperin BioScience by scientists at the University of Michigan argued that there is now substantial evidence that this unusual clustering is the result of self-organizing behavior of the ants — not external factors like temperature or moisture. In fact, the mechanism at work is strikingly similar to a process described by the late Alan Turing in aseminal (paper)
Turing was attempting to understand how natural, nonrandom patterns emerge (like a zebra’s stripes), and he focused on chemicals known as morphogens. He devised a mechanism involving the interaction between an activator chemical and an inhibitor chemical that diffuse throughout a system, much like like gas atoms will do in an enclosed box. The BioScience paper draws an analogy to injecting a drop of black ink into a beaker of water. Normally this would stabilize a system: the water would gradually turn a uniform gray. But if the inhibitor diffuses at a faster rate than the activator, the process is destabilized. That mechanism will produce a so-called “Turing pattern: “spots, stripes, or, when applied to an ecological system, clusters of ant nests.
So you get spots, like on a leopard, or stripes, like on a tiger.
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“The same equations that Turing used for chemistry, we can use in ecology,”co-author John Vandermeer, an ecologist of the University of Michigan. “Those equations say you should get spots of predators and spots of prey in a system, and we’ve proven you do.”
Basically, any two processes that acts as activator and inhibitor will produce periodic patterns and can be modeled using Turing’s diffusion function. For instance, he has uncovered Turing-like features in how species are distributed in a given ecological system, including predator-prey models.
Vandermeer has been studying Azteca ants and the coffee farm ecology system in general for a good 34 years, and it noticed that ant nests tended to form patterns while working in the field. One of Vandermeer’s graduate students had done a study on parasitic phorid flies and realized the tremendous impact the fly had on the ants’ behavior.
“The predator was dispersed around by the wind, so it had a relatively rapid diffusion rate, compared to where the ant was, “he told Ars. “Biologically, we had a clear reaction-diffusion system.” In other words, they had the defining elements of a classic turing mechanism.
Vandermeer and his colleagues mapped the distribution of shade trees with nests of Azteca ants on an organic coffee farm in Mexico — roughly (trees out of a total of between 7, and 0 12, 01 0 trees overall. It’s an intricate ecosystem. For instance, when phorid flies find a cluster of ant nests, they plant their eggs in the heads of ants. Those heads will fall off once the larvae are fully developed, releasing new flies to venture out and find more ant hosts to implant.
That relationship between predator and prey, the authors contend, is the driver behind the emergence of ant-nest-clusters distributed in a Turing pattern. The ant nests serve as the activator in this system, increasing in size and number while forming spatial clusters. This kicks off a corresponding increase in the population of the flies, whose parasitic behavior acts as an inhibitor, decreasing the population of ants.
(****************************Enlarge Elements of coffee rust disease.
“At a very local level, the predator and prey form an unstable relationship, whereas adding diffusion to the mix may result in stabilizing the system, “the authors wrote.
There is also a disease called coffee rust, spread by spores on the wind. The same fungal disease that keeps scale insects under control can also target coffee rust. But sometimes a coffee-rust epidemic breaks out, such as the one thatdevastated cropsin Guatemala, Honduras, and El Salvador in 2012. The likely cause: in the preceding years, according to Vandermeer, there was a gradual deforestation of the affected areas, converting much of the forest to pasture and upsetting the delicate balance of the ecosystem.
“It’s a reasonable hypothesis to suggest that it was an indirect consequence of the gradual deforestation of the region leading to a critical transition, [such that] the disease suddenly turned into an epidemic, “said Vandermeer.
Said co-author Ivette Perfecto
There is still plenty of skepticism among scientists about whether or not true Turing mechanisms are at work in natural systems. “Distinguishing between a Turing pattern and other methods of pattern formation is not all that easy in a large-scale system like this,” Vandermeer admitted. “If you’re dealing with slime molds in a laboratory, you have much more control over the system.” Nonetheless, “My position is that Turing’s insight was so foundational, that all the assumptions that go into his qualitative insight are there in nature. It just seems right.”
DOI: (BioScience) ********************************************, (******************************************************************. 11. / biosci / biz (About DOIs) ).
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