a link in the food chain is probable broken. ( a species became extinct)
you can have a lack of food, urban space, fresh water sources, high taxes, too few hospitals, lack of police, a slack in the law, bad weather, poor entertainment sites, and so much more.
We cannot decrease air pollution, but there are many ways of decreasing the level that it is rising in, for example, recycling.
bigger population (?)
Presence of friction, incline and so on.