The most simple definition of a community is a group of populations of different species that occur in the same space at the same time . This definition indicates that there are associations among species, and in fact many species do co-occur over wide ranges. A more functional definition of communities is the set of interacting species living at the same time and place . This definition implies that there must be some interdependency among the species for them to be included, but in reality it seems unlikely that two species in the same habitat at the same time would not interact in some way. Guilds and communities may not have clear boundaries, and often they are defined based on convenience for the researcher.
Communities are a true level of organization, just like the cell, organism and population.
Removing a species from a community will alter the density and composition of the community, which in turn may alter how it behaves and functions . This is much like removing an organ from an organism. For example, removing a predator from a community may allow some prey populations to increase rapidly in size. Just like organisms, communities take on distinct features based on the interactions among the constituent populations. Many of these features are just as distinctive as the adaptations we have talked about for organisms.