Ecology was the study of the distribution of lifeforms throughout the galaxy, and how those lifeforms interacted with other lifeforms and their environments. It was a multi-disciplinary science which focused on a broad range of studies relating to living organisms. Ecology was generally considered to be a branch of biology.
Typically, several ecologies would develop independently on a class M world. These were usually separated by large distances, although there were some rare occurrences where vastly different ecologies co-existed significantly close to one another. Some examples of this included the planet Lactra VII and the Genesis Planet. (TAS: "The Eye of the Beholder"; Star Trek III: The Search for Spock)
In 2151, several crew members of the Enterprise NX-01 made an ecological survey of an M-class planet which would later become known as Archer IV. (ENT: "Strange New World")
In 2369, the USS Enterprise-D brought relief supplies to Tagra IV in the Argolis Cluster. The ecology of this planet had been devastated by the Tagrans' industrial activity. (TNG: "True Q")