If you kill an organism, the next organism below it on the food chain will multiply. This works across all species, from bears, wolves, and coyotes, to raccoons, deer, and squirrels, to birds, snakes, and insects, all the way down to bacteria and viruses. Remove one species, and it has a butterfly effect on all the others.
If there are insufficient plants and animals in an ecosystem, viruses and bacteria will multiply, attacking the dominant species: humans. This, coupled with the interconnectedness of humans through globalization, is why so many diseases are on the rise. Cities are infested with disease-carrying rats and suburbs are infested with disease-carrying ticks. The natural balance of the ecosystem has been decimated by humans, hurting humans in the process.
It is vital to protect the environment—not for the sake of plants and animals (though that would be nice)—but for the sake of humans.