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FIGURE 10.5 The proportion of sampling occasions that included each taxon over 24 years of study in the Broad-stone Stream, United Kingdom. Numbers along the lower axis refer to individual species. The core community consisted of eight taxa that were always present. These included: 1, Nemurella pictetii; 2, Leuctra nigra; 3, Plectrocnemia conspersa; 4, Sialis fuliginosa; 5, Pentaneurini; 6, Ceratopogonidae; 7, Heterotrissocladius marcidus/Brillia modesta; 8, Polypedilum abicorne. The pH optima for individual taxa are indicated where known. (Reproduced from Woodward et al. 2002.)

distributions usually are locally abundant as well (Gaston and Blackburn 2000), as Heino (2005) demonstrated from a comparison of stream insects collected within one drainage with distributional data from a countrywide data set. Although such a relationship can be an artifact of sampling, it is likely that the explanation has an ecological basis in organism-niche relationships, dispersal abilities, and population growth rates.

When one takes into account the many factors that influence regional diversity, the myriad environmental conditions that make a locale more or less welcoming for particular species, and the variable extent of sampling effort, it is hardly surprising that reported diversity at the local scale varies enormously. Because most species are minor components of ecological webs, the completeness of the species list may not matter greatly. Nonetheless, the few exhaustive inventories are of interest because they give a sense of just how biologically diverse stream communities can be. Perhaps the most complete species lists come from long-term studies of a small German stream, the Breitenbach (Table 10.1), and of the Broadstone Stream in the United Kingdom (Schmid-Araya et al. 2002). More than 1,000 invertebrate species have been collected from the Breitenbach, and because this list was compiled using aerial as well as aquatic collections, it is uncertain what fraction derives from habitats other than the stream, including a small impoundment and other standing water habitats,

TABLE 10.1 A total of 1,085 species of metazoans reported as of 1989 from a 2 km stretch of the Breitenbach, a small stream near Schlitz in northern Germany. (From Zwick 1992.)

Insecta

Number

Other Metazoa

Number

Diptera

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