Ecology, -noun.

1. the branch of biology dealing with the relations and interactions between organisms and their environment, including other organisms.
* Excerpted from the Random House Webster's Unabridged Dictionary 3.0.

From Wordsmyth: Dictionary and Thesaurus (on the web) we obtained the following:
1. the scientific study of the relationships between living things and their environments.
2. these relationships themselves.
Example of (2): "To understand the dangers of pollution, we must understand the ecology of humans."