Animal IQ

We Could Stop Invasive Species by Editing Their Genes. But Should We?

Invasive species are a big deal. In North America, the snakehead fish from China set wildlife rangers on edge with its voracious appetite, the 75,000 eggs it lays per year, and —oh yeah — the fact that it can literally crawl across land to find the next river or lake to live in. So what if we could monkey with their genes and keep them from spreading any further? Mathematical models suggest that that strategy could work pretty well ... maybe a little too well.

Stopping Invaders In Their Tracks

We tend to think of rats as being everywhere that people are. But for a long time, the island of New Zealand got along just fine without any mammals aside from the occasional bat. That changed sometime around 1200 C.E., when Polynesian settlers brought some fuzzy little stowaways in the form of kiore (AKA "Pacific rats") along for the ride. A second wave of rodents came with European colonizers in the 18th and 19th centuries. These included both Norway rats and ship rats, which are both larger and more aggressive than the frankly adorable kiore.

The result? Absolute devastation. The rats have wreaked havoc on New Zealand's ecosystem, driving several species of birds, frogs, and tuatara (a lizard-like reptile not found anywhere else in the world) to near-extinction. It's been a couple of centuries, but the invaders still pose a threat to the ecosystem. And if you ask ecologist James Russel, "Either a bird is going to be killed by a rat that we brought here, or we're going to kill the rat. And I would rather humanely kill the rat than have the rat inhumanely kill a bird."

That humane method of eradicating rodents that Russel mentioned doesn't involve traps or poison. It involves monkeying around with DNA. Enter CRISPR. Basically, the plan would be to engineer a population of rats that passes on a gene for infertility and release that population into the wild. Over the course of a couple of decades, the gene would propagate among the existing wild rats until it finally produced a final generation — one incapable of reproducing at all. The key to making this plan work is something called a gene drive. Whereas natural selection usually grants only a fifty-fifty chance of passing on any given gene, a sequence souped up with a gene drive will always be passed on.

Here's an example: Mickey is a modified rat, and Minnie is au naturel. They have kids, and both Mickey Jr. and Minnie Jr. are guaranteed to carry both the modified gene from their father and the natural gene from the mother. The natural gene is dominant, so both of them can still have kids. You know how it goes: Mickey Jr. meets Melissa, and Minnie Jr. finds Mortimer, and soon enough Mickey III and Minnie III are born — both with the dominant natural gene and the recessive modified gene. Eventually, Mickey III and Minnie III get together. Now, because the modified gene will drive through on both sides of the family tree, Mickey IV carries two copies of it. And because that gene is for infertility, that makes him the end of the lineage. After a couple of generations, the infertility gene is guaranteed to spread throughout the entire population. The problem is, it might be too effective.

Children of Rats

So here's the issue: remember how the rats got to New Zealand by hitching a ride with people? Yeah, they're still doing that. And that means that if we release these genes to eradicate the invading species, there's a very high chance that the genes will spread back to places where the species are originally from, ending their lineages too. That's not just idle speculation, either — it's based on precise mathematical models developed by the same people who proposed the solution in the first place.

Dr. Kevin Esvelt was part of the team that developed CRISPR, and in 2013, he championed the technology as a means of eradicating invasive species and controlling the spread of diseases like malaria. But now, speaking with the New York Times, he calls the idea "an embarrassing mistake." According to his newest research, the gene drive system wouldn't just be an effective way of ridding New Zealand of rats, it would be almost guaranteed to rid the world of rats. And that would be a very bad thing. The little omnivores fill a vital link in the food chain in places where they belong, and the same is true of pretty much every other species that ever found itself thriving far from home.

That doesn't mean that gene editing is dead as a method of controlling harmful species, but it does mean that we'll need to do a lot more work to make its threats less aggressive and inevitable. One possible solution? A gene drive that automatically self-destructs after a few generations. That way, it might have time to wipe out the population in a closed system like an island, but not enough to take on larger populations on the mainland.

What if there's another way to look at species that thrive far from their homelands? Check out Fred Pearce's "The New Wild" — he argues that, although there are many examples of displaced organisms damaging an ecosystem, there are as many or more examples of alien species integrating seamlessly into their new food chain. We handpick reading recommendations we think you may like. If you choose to make a purchase through that link, Curiosity will get a share of the sale.

The World's 11 Worst Invasive Species

Written by Reuben Westmaas January 5, 2018

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