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A wild mouse invesitgating his environment

Understanding Invasives With Mathematical Models

Biology professor Scott Nuismer publishes study complicating methods for predicting species invasiveness

By Tara Roberts

Imagine a family of mice from a tiny island that hop a ship to a distant country. When they arrive, they enter a new ecosystem. But will they wreak havoc or go extinct?

In recent years, scientists have increasingly used phylogenies – maps of evolutionary relationships among organisms – to forecast whether a species could become dangerously invasive. But a new study by University of Idaho biological sciences professor Scott Nuismer and his colleagues at Washington State University and Rice University indicates such predictions are less simple than hoped.

The tactic of using phylogenies to predict invasiveness has roots in Charles Darwin’s “naturalization conundrum.”

“The thing Darwin was grappling with there was whether distantly related species or closely related species would be more likely to become established,”Nuismer says.

Darwin settled on the side of tying success to distant relationships, because distantly related species would be less likely to compete for the same resources. However, recent studies using DNA sequencing and phylogenies have shown mixed results. Researchers have wrestled with various reasons why this might be so, and Nuismer and his team decided to “take a step back” and investigate the question without imposing preexisting expectations on it.

To do so, they turned to mathematical models of evolution, Nuismer’s specialty.

“Math makes you sit down and really be rigorous about what we’re assuming,” he says.

Using mathematical models, the team discovered that an invader’s relatedness to native species mattered less than the way various species in the invaded community interact with each other and evolve over time.

Earlier phylogenetic studies hadn’t taken into these variables into account, Nuismer says. “Everyone was implicitly assuming that interactions work in a very specific way.”

Scientists have defined two main modes of species interaction in the wild, but in most cases they can’t be sure which mechanism is at play among species. This complicates the idea that tracking genetic similarity alone is enough to guess whether an invasive species will gain a foothold in a new environment.

“There is no black-and-white prediction,” Nuismer says. “The prediction is, you can’t predict much.”

While Nuismer’s study doesn’t have good news for people hoping to use phylogenies to prepare for species invasions before they happen, Nuismer says it may prove useful to other scientists who are studying how species interact and evolve.

The paper, “Revisiting Darwin’s conundrum reveals a twist on the relationship between phylogenetic distance and invasibility,” was published Nov. 28 in the Proceedings of the National Academies of Science.