Lotic communities

The forces that shape community structure are those that determine which and how many species occur together, which species are common and which are rare, and the interactions amongst them. Thus, the topic of community structure involves a synthesis of all the environmental factors and ecological interactions influencing an assemblage of co-occurring species. For communities to exhibit predictable structure requires that their assembly is the outcome of nonrandom processes that result in repeatable patterns, rather than chance and dispersal. This leads us to expect that the same species, in roughly the same abundances, will be found in the same locale as long as environmental conditions do not change greatly, and that similar communities should occur wherever environmental circumstances are comparable.