Ecosystems are always changing, a big factor in ecologist's work. When an ecosystem changes, such as a pond dries out, or a new organism is introduced to the ecosystem, it affects the entire process, from the organism's food supply to the nutrients and chemicals available in the ecosystem.
Humans largely affect the biosphere, sometimes obviously, and other times, obliviously. Pollution in the ocean or the cutting down of trees for construction purposes can destroy an ecosystem and put organisms in grave danger. Ecologists study this and try to make society aware of the irreparable damage this causes the ecosystem and the survival of organisms.
The study of organisms affects the survival of humans. For example, if a farmer uses fertilizer on their soil, the nutrients in the soil may transfer to the oceans and kill the food supply of fish, a vicious cycle that affects other organisms, as well as humans.