The changes caused by human activities have consequences for the biotic and abiotic parts of ecosystems. Changes have been particularly rapid in the last 50 years. Today, the greatest changes are taking place in developing countries.
On a small scale, a tree may be cut down to let more sunlight into a garden. The tree can no longer provide birds and insects, together with worms and organisms that live among the tree's roots, a place to live.
On a large scale, natural vegetation is removed and replaced with either by crops for food, the production of biofuels, grazing animals, or to provide room for the houses in which people live. This has altered the food webs and nutrient and energy cycles of ecosystems across the Earth.
Often, changes to ecosystems are unintended. For example, human waste (such as household and industrial waste) enters natural ecosystems, and sometimes this waste can build up to harmful levels.
Human activities such as harvesting plants and hunting animals can unbalance the flows and cycles within those ecosystems. Tree-felling may lead to soil erosion and a loss of habitat. Lack of shade and moisture in the soil can result in desertification. Over-fishing from oceans can cause species to become threatened and can harm food webs.
Introduced species are plants and animals that live in a place where they are not found naturally. They have been introduced, either deliberately or accidentally, by human activities. Often they thrive in their new ecosystems, surviving and reproducing better than native species. As a result, native species cannot get the resources they need, and their numbers tend to decrease. The European starling provides a good example:
It should be noted that not all introduced species cause problems.