Four biological communities, composed of interacting sets of species (or actually, populations of species; see populations above) are indicated by the clusters of squares.
Smaller clusters indicate simpler communities with fewer species (such as found in arctic lakes) and larger clusters indicate more complex communities (e.g., coral reefs or tropical rain forests).
Ecosystems include these communities individually or in linked groups, along with the flows of energy, chemicals, and nutrients that underlie and connect the living and dead matter in a region.
