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Genetic back burning: Fighting fire with fire

Speed read
  • Fighting invasive species means working smarter, not just harder.
  • Scientists argue for incorporating evolutionary knowledge into management techniques.
  • Genetic back burning uses evolutionary forces to blunt a species' ability to breach barriers.

When firefighters want to halt an advancing wildfire, they often opt for a technique called back burning. By burning more forest ahead of the blaze, firefighters create a line without fuel and the fire dies when it reaches this point. Geneticists from Australia wondered if this concept could work to halt other advancing lines. In the course of 100 generations of supercomputer simulations, what they’ve discovered points the way to a smarter defense against invasive species.

<strong> Feeling froggy.</strong> Ben Phillips found that computer simulations support the idea of incorporating genetic back burning into strategies for managing invasive species like the cane toad. Introducing slower members ahead of the advancing invasion front will, through interbreeding and forces of natural selection, render barriers more effective. Courtesy University of Melbourne.

Ben Phillips, senior lecturer at the University of Melbourne, has been long interested in evolution, particularly how space modifies its effects. As Australia grappled with a cane toad invasion, his interest was piqued when he noticed the toad invasion was speeding up. Whereas it used to encroach new territories at the rate of about 10 km (6 mi) a year, he observed the toad had sped up to nearly 50 km (31 mi) a year.

As he looked into the acceleration, he learned that rapid evolution on the leading edge of the invasion was the cause of the increasing speed.

“If you think about an invasion front, the individuals that are on that front each generation have to be individuals in the population that have moved the furthest,” Phillips observes. “The invasion front gets all the best dispersers in a population and puts them in the same place at the same time. They tend to breed with each other as a consequence, so their kids tend to be much higher than average dispersers.”

Meanwhile, back on the ranch, the dispersal rate in the core populations of the invading species remains about the same. Faced with the prospect of modifying valuable pastoral infrastructure to create a barrier against the invaders, Phillips' team was discouraged by the size of the area they would have to manage to stop a little toad—or at least the toads out front with the greatest ability to cross.

“We were a little bit despondent about the fact that this barrier had to be so wide, but then it occurred to us that it needs to be this wide because we’re trying to stop the invasion front,” says Phillips.  “That’s when we realized if we went and got those animals from where they were originally introduced, the low dispersing animals, and we put them in front of the barrier, they would spread back towards the invasion front and potentially prevent the highly dispersive individuals on the invasion front ever actually getting to the barrier.” (Read Phillips' account of how they came up with this idea.)

So, to halt an advancing species, Phillips considered a radical possibility: Would moving the slower members of the invading species out front blunt the invasion? Would mating slower moving members of the species with the faster work in the same way as burning ahead of a forest fire?

To answer these questions, Phillips went to the High Performance and Research Computing unit at James Cook University, a cluster with 876 compute cores, 70 TB of local data storage, 2576 GB of memory, and 40 Gbps of network connectivity.<strong> Harnessing evolution. </strong> New research shows that an invasive species' ability to overcome a barrier varies between core and frontal populations, and is eroded by rapid evolution. (a) shows the situation in non-evolved gene frequencies and (b) shows the situation in which gene frequencies evolve in response to a barrier. Each point is a replicate simulation (with overlapping points denoted by ‘petals' around each point), recording the breach time for that simulation. Courtesy Ben Phillips.

“High performance computing has enabled me to run individual-based models across a whole range of different possibilities and possible parameters, much larger populations, and much larger spatial domains. HPC has been incredibly helpful in that regard.”

Using 300 nodes at a time to track the movement and reproduction of individuals in the system, Phillips built a computer model to recreate a population on the computer to see how it spreads in a variety of scenarios. The simulations show that genetic back burning works, and that barriers to invasive species should be planned with evolutionary processes in mind.

Though amphibian invaders inspired Phillips' simulations, one of the more exciting insights is the range of applications that could arise. “Rapid evolution on invasion fronts doesn’t just apply to toads, it doesn’t just apply to invasive species, it applies to populations that are moving as a consequence of climate change, or it applies to tumors that are growing in the human body, it applies to lichens that are growing on the roof of your house,” he notes.

It remains to be seen if manipulating rapid evolution in invading fronts is effective within the human body, however. Phillips doubts genetic back burning would apply since human diseases spread too rapidly due to our long-distance travel habits, and few would want to volunteer to be infected. 

“Genetic back burning might have application in wildlife diseases, or in situations where human disease is spread predominantly by wildlife, but our understanding of how rapid evolution during invasion affects pathogens is still in its infancy. Clearly diseases spread, and the same selection pressures come into play. It’s interesting to see the possibility for novel management actions that arise out of our understanding that evolution can happen very rapidly on invasion fronts.”

Look here to learn everything you wanted to know about cane toads but were afraid to ask.

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