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2007

Scientist Employs 'Circuit Theory' to Protect Endangered Species
Wired.com, December 10, 2007
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When we enter the wilderness, we like to leave the nonstop whir of electronics behind. The worlds of the mountain lion and of the integrated circuit seem to have nothing in common. But in fact, they are similar in some profound ways. Over the years, as mountain lions migrate and mate, their DNA flows across the landscape like electrons flowing around a circuit.

By borrowing some engineers' insights about how circuits work, ecologists now have a promising new tool for helping conserve mountain lions and other threatened species.

Ecologists are now using "circuit theory," thanks in large part to a scientist named Brad McRae who works at the National Center for Ecological Analysis and Synthesis in Santa Barbara, California. McRae designed electronics for printers before completing a Ph.D. in forest science at Northern Arizona University. He realized how striking the parallel was between the circuits he had worked on as an engineer and the species he was now trying to understand.

In a circuit, for example, resistance slows down the flow of a current; the flow of genes can be slowed down as well. Two populations of a species may be linked by a narrow corridor, lowering the odds that any animal will move from one population to the other. One way to reduce the resistance in a circuit is to add extra wires. Likewise, the flow of genes increases with extra corridors.

Over the course of 150 years, electrical engineers have developed a set of equations that let them predict how a circuit will behave even before they build it. McRae reasoned that by adapting those equations, he might do a better job of predicting how a species' genes flow across its range than with more conventional methods. He and his colleagues tested circuit theory on two endangered, well-studied species: big-leaf mahogany trees in Central America and wolverines in Canada and the United States.

They transformed the ranges of both species into grids of five-kilometer cells -- 31,426 cells for the mahogany and 249,606 for the wolverines. Then they calculated the gene flow resistance from cell to cell. If the gene flow was high, there would be few genetic differences between the populations. If there was a lot of resistance to gene flow, the populations would become genetically distinct.

The scientists compared their predictions about these differences to actual studies on wolverines and mahogany. As they reported last week in the Proceedings of the National Academy of Sciences, circuit theory beats popular gene-flow models. It not only works -- it works well.

Mapping gene flow can help preserve species from extinction. The fragmentation of a species range can reduce its gene flow in much the same way ripping out wires can reduce the current moving through a circuit. Populations that don't get enough immigrants bringing fresh genes with them can become inbred, suffering from diseases and infertility. By mapping gene flow, conservation biologists can identify populations at risk and make smart plans to restore the flow.

McRae and his colleagues are using circuit theory to help conserve mountain lions in southern California, sage grouse in the western United States and jaguars in South America. Circuit theory allows them to test what would happen if new corridors were added between populations or old ones were taken away. They've discovered a choke point, for example, in the range of mountain lions between the San Jacinto and San Bernardino mountain ranges in California. If the corridor is blocked -- by a new tract of houses, for example -- the entire network of lion populations in southern California could be at risk.

The success of circuit theory in the natural world may conflict with romantic notions that life is somehow above the reductionist simplicity of engineering and physics. But in fact, it doesn't drain the life out of life. Underlying the workings of a cellphone or a population of mountain lions is the same beautiful mathematics. It's just a coincidence that electrical engineers discovered much of that math first. Now it's time for conservation biologists to discover it too -- before it's too late.

Copyright 2007 Carl Zimmer

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