Too many deer and they eat all the plants. Eat all the plants, and they starve. You gotta have balance. It has taken billions of years to fine-tune the life cycle. It is easy to perturb.
Coyotes eat chickens, lambs, and . . .
Because birds eat seed grain. Infestation of insects and a great many people starved. We cannot alter the food web significantly and expect mankind to thrive. Close proximity means more easily spread disease. Tundra, coniferous forests, deciduous forests, grasslands, deserts, rainforests, chaparral, etc .