For example, beavers may flood an area, allowing new species to immigrate.

These river and lake systems provided corridors that allowed many animal species, including humans, to expand their range across the Sahara.

This can allow certain species of birds to sleep without hindering migration and to prevent making frequent stops along the way.

Each of these has impacted on some of the trees and plants, but created additional habitats, allowing other species to become established.

This question points out the problem of allowing invasive species to take over.

Allowing species to become threatened or endangered results in long-term and costly recovery efforts whose success is far from guaranteed.

This allows species that normally are not found together to live close.

They "weed" their territories, allowing only selected species to flourish.

This allowed native species of birds which were no longer in the area to be re-introduced in 1999.

But other plants were allowed to develop because the unusual chemical composition of the soil did not allow aggressive species to take over.