Human activity affects ecosystems in a wide variety of ways, but it primarily does so through agriculture, habitat destruction, water use and fishing. Whenever humans enter a habitat, they tend to reshape it to fit their own needs, destroying the resources that other animals use, which drives them out. The overuse of water drains natural aquifers and alters the local water table, and pollution can negatively affect wildlife populations.
Fishing is one area in which human activity can have massive effects on an ecosystem. Humans often put extreme pressure on apex predator species, such as tuna. When the numbers of these predators dwindle, the lesser predators that they consume grow in number, which then puts pressure on the species further down the food chain. Additionally, industrial fishing operations can damage habitats and kill unrelated species that are caught along with the targeted fish.
Humans also affect the environment through pollution, and these changes can be widespread. Increases in carbon dioxide and other greenhouse gases have raised global temperatures, and these increases can alter sea levels and weather patterns. Just a few degrees of change can damage fragile ecosystems by wiping out key flora or altering animals' reproductive cycles, and the fact that these gases can affect the environment for decades if not centuries ensures that their impact is long-term and widespread.