Biology / Science

Why predators are a good thing

You’d think that a world without predators would be ideal, really. Everyone living in harmony, getting their food without harming anything sentient, so much less stress and less injury, too. No need to spend your time running away or worrying about shelter, etc, etc. You’d think it’d be great for everyone, right?

It turns out that when you take predators out of an ecosystem, things actually start to break down, though. The prey species aren’t being molested, so they do thrive… for a while. But as their numbers increase, so does sickness and scarcity. Most infectious diseases spread best in crowded conditions — for herd species, in particular, it can be devastating. We have actually got at least one good model for what happens when predators get scarce: Yellowstone National Park, in the US. Yellowstone was free of wolves for seven decades, until they were reintroduced to help manage the elk population. At that point, Yellowstone was being overgrazed due to the large herds of elk.

So wolves were reintroduced. Elk populations went down, and trees were able to grow again. Hurrah for predators!

Admittedly, this is a bit of a simplistic picture. Willows are one of the key trees in Yellowstone’s environment, and they’re not recovering as much as conservationists hoped. While the wolves were gone, beavers and elk were competing for food, and the elk won. Beavers are mostly gone from Yellowstone, and so has the habitat they create… habitat that promotes the growth of, guess what? Willows.

It’s a complex story, so maybe the take-home message should be that as with many things, it takes a village. Or rather, a food web. Pull out one strand and cut it, and the whole thing sags. Big predators are a part of that food web, and humans have been disrupting the predator end of food webs for a long time.

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