5 - Body size in streams: macroinvertebrate community size composition along natural and human-induced environmental gradients

Summary

Introduction

Communities contain a diversity of species and a spectrum of life forms. A consequence of evolutionary processes is that lists of species from different communities tell us almost nothing about how similar they are ecologically. On the other hand, by quantifying the life-history traits represented, we can discern similarities and differences among communities and gain an understanding of the functional relationships between traits and habitats. In response to Southwood's (1977) contention that habitat provides the templet upon which evolution forges characteristic life-history strategies, we are interested in the extent to which life-history traits of macroinvertebrates can be directly mapped onto stream habitat axes. More recently, stream researchers have used the metaphor of environmental filters (Poff, 1997) that can eliminate certain traits and produce similar trait compositions in similar habitats (Statzner, Dolédec & Hugueny, 2004). Trait-specific selective forces along environmental gradients may act over evolutionary time, as Southwood (1977) contended, or over an ecological time scale, selecting for successful strategists from the potential pool of colonists.

The earliest categorization of species traits in stream ecology related to trophic role. Cummins (1974) identified grazer-scrapers, fine particle collectors (gatherers or filterers), large particle shredders and predators. Stream ecologists now use this trophic categorization to focus on the similarities and differences of communities in different parts of the world (e.g. Winterbourn, Rounick & Cowie, 1981; Thompson & Townsend, 2000; Fenoglio, Bo & Cucco, 2004), in different parts of the river continuum (e.g. Vannote et al. 1980; Grubaugh, Wallace & Houston, 1996) and in different kinds of terrestrial setting (e.g. Thompson & Townsend, 2000; Woodward & Hildrew, 2002).