What is an environmental threshold?

An environmental threshold is a point at which the environment or ecosystem cannot recover. Once an ecosystem passes a threshold, it cannot return to its original state.

For example, if a large forest is being harvest over time and is becoming increasingly fragmented. When the first logging road cuts through the forest and one patch of forest is logged, the forest isn't greatly affected. Seed dispersal still occurs, some animals have to shift their habitat usage but they remain in the forest, runoff processes are only slightly affected. Enough trees surround deforested patches that soil erosion from wind and water isn't a large problem. Seeds are able to grow in the cleared plots. Overall ecosystem function largely remains intact.

Eventually, enough roads are added and enough trees are cut down that the forest crosses a threshold. Trees are removed at a rate that is faster than new trees are established and the sustainable yield is surpassed. So many trees have been removed that the healthy top soil is removed by wind and water erosion, no longer held in place by tree roots. The poor soil means new seeds can't establish themselves in this nutrient poor soil. The lack of trees means many seed dispersers living in the canopy such a birds move to different forests and other animals leave because their habitat is too small or poorly connected.

At this point, the forest has crossed a threshold and cannot sustain ecosystem processes and functions.