On summer evenings in the Midwest, the muggy air comes alive with a chorus of crickets, cicadas, and frogs — especially bullfrogs. Their booming mating calls sound like something between a foghorn and a didgeridoo.

As far as we know, summer here has always sounded like this. Bullfrogs are native to most of the Eastern US, from the Mississippi River to the Atlantic Coast. They evolved here. They belong here. I, for one, adored them as a kid growing up in Iowa, and spent countless summer days trying to catch them to get a closer look.

What’s unusual is that a few states west — into Colorado and on to California — summer nights are similarly marked by the iconic call of the American bullfrog. But here, they don’t belong. They’re unwanted. And they threaten the very existence of some of the West’s other amphibious animals, such as the Oregon spotted frog, which is found only in the Pacific Northwest.

American bullfrogs are not native to the Western US. Humans brought them to the region more than a century ago, largely as a food source. And in the years since, the frogs — which are forest green and the size of a small house cat — have multiplied dramatically, spreading to countless ponds and gobbling up everything that fits in their mouths, including federally threatened and endangered species. Conservation scientists now consider them among the most dangerous invasive species in the Western US, and in the 40-plus other countries worldwide where they’ve been introduced.

That leaves bullfrogs in an unusual position. Invasive species are typically brought in from other countries — Burmese pythons in Florida and spotted lanternflies in New York City come from Asia, for example — but American bullfrogs are, as their name suggests, American. They’re both native and invasive in the same country. And the difference of just a few states determines whether we treat them like pests or as an important part of the ecosystem.

It’s easy to hate bullfrogs. They do cause a lot of damage and, like other non-native species, they’re leading to what some researchers call the Starbucksification of the natural world — you find the same thing everywhere you go, which can make ecosystems less resilient. Yet bullfrogs themselves aren’t the main problem, but rather a symptom of a much bigger one.

How bullfrogs took over the West

One reason is that people enjoy eating them. Or more specifically, their legs.

In the 1800s, as the human population in the West surged amid the Gold Rush, so did an appetite for frog legs, which were associated with fancy French cuisine. To meet that demand, people........

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