it

What does 'it' refer to?

Some species seem to have a stronger influence than others on their ecosystem. Take away the sea stars along the northwest coast of the United States, for instance, and the ecosystem changes dramatically; in the absence of these sea stars, their favorite prey, mussels, takes over and makes it hard for other species that used to live there. Sea stars are known as keystone species, because as top predators they determine ecosystem structure by their eating habits. If you chop down an aspen tree by a beaver pond, not much will happen; but if you take away a beaver, a wetland might dry out, changing the kinds of plants that live there and the animals that rely on them. Because beavers exert their influence by physically altering the landscape, they are known as ecosystem engineers.