Figure 1. Factors controlling the community structure of stream and riverine communities. Factors are shown hierarchically in terms of general importance; however, factors do interact and take on different levels of importance depending on the regional and local context. For example, flood or drought magnitude may determine which species colonize an area, but if temperatures exceed their tolerances or the bottom substrate is too silty for them to feed, they may be absent.

Figure 2. Wood that falls into streams and traps leaves and other organic material supports biologically productive and diverse biotic assemblages. The fauna live both on and in the wood, as well as on the biofilm that covers submerged pieces.

(a) Microbes and meiofauna: http://www.microscopy-uk.org.uk/mag/wimsmall/smal3.html

(b) Elmidae (adult riffle beetle): http://entweb.clemson.edu/museum/misc/aqua/aqua22.htm

(c) Aeshnidae (dragonfly nymph): http://twri.tamu.edu/

(d) Hydropsychidea (caddisfly larvae): http://waterknowledge.colostate.edu/arctopsy.htm

(e) Simuliidae (blackfly larvae): http://www.fish.washington.edu/naturemapping/water/1invert.html

(f) Pteronarcyidae (stonefly nymph): http://waterknowledge.colostate.edu/pteronar.htm

(g) Peltoperlidae (stonefly nymph): http://www.dec.state.ny.us/website/dow/stream/plefamilypageone.htm

(h) Tipulidae (crane fly larva): http://www.chebucto.ns.ca/Science/SWCS/ZOOBENTH/tipulida.html

(i) Ephemerellidae (mayfly nymph): http://www.wlu.ca/~wwwbiol/bio305/Database/Ephemerella.htm

Figure 3. Community structure of stream fauna may be highly predictable over time or space depending on the frequency and magnitude (‘extent’) of dispersal and the constancy and predictability of flow. For communities with a high degree of predictability, the factors in Figure 1 may be useful in predicting the diversity and composition of communities.