Sorry, cat people: According to some ecologists, your feline friends are terrible for the planet. And the problem is even worst with unowned cats, like those who cluster on one particular street in San Juan, Puerto Rico. But as Carrie Arnold unpacks in this essay, solutions are tough to come by.

Each year, cats collectively kill billions of birds, rodents, insects, reptiles and amphibians. They routinely make lists of the world’s worst invasive species. Free-ranging cats have been implicated in the extinction of Lyall’s wren in New Zealand and contributed to the extinction of 33 other species , and are considered a major threat to others, especially on an island like New Zealand — where birds, otherwise, have no natural land predators. To conservationists, it’s a major crisis.