A group of individual organisms of the same species
in one area comprise a population, such as a pod of lobsters.
A species is a collection of similar organisms that can interbreed, such as lobsters.
Populations of several species interacting in one area comprise a community; a marine community may consist of lobsters, anemones, clams, fish.
The biological community and the non-living elements in their environment (habitat) make up an ecosystem, such as an ocean.