In ecology, a group of particular organisms which live in a community is called POPULATION.
For instance, all the number of lions which live in a community make up the population of lions in that community. Populations of different species of animals that are found in that area combine together to form the community.
Breaks down over time and becomes dirt
A biologist whose interest lies primarily in the study of plants or animals can be called a naturalist, although these days it's more likely she'll be called a natural historian, a botanist, or a zoologist.
