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. 000740 .) Bruns & Minshall, 1985; Bruns, Hale & Minshall, 1987), In general, the number of filterers and overall taxa increase with increasing stream size (Richards & Min- shall, 1992), Ward (1986) reported an increase in macro- invertebrate density, biomass and total number of taxa from the mountains to the plains in the Saint Vrain River, Colorado. In our study, comparison of inverteb- rate density and number of taxa from high to low altitude in mountain sites did not show this general pattern, Sites sampled in the mountains in this study were comparable with the middle and lower altitude sites described by Ward (1986), Patterns of invertebrate community distributions observed in our study may have been different if seasonal data had been collected, Ward (1986) noted that total species richness would be less at middle and lower altitudes at mountain sites if only summer data were used because of the absence of stoneflies. In addition, species of other taxonomic groups, such as Chironomidae, Trichoptera and Coleoptera that occur during winter would be missed if only summer samples were collected (Ward, 1986), Furthermore, samples in this study were col- lected basin-wide rather than longitudinally along a single tributary, Thus, variability among streams within the mountains may mask longitudinal patterns. In contrast to Ward (1986), the results of this study indicated greater number of taxa per site in the moun- tains compared with the plains, The plains site used by Ward (1986) was located at an altitude of 1544 m, had hard substrates, and would be considered the transition zone between monntains and plains for this study; taxa at this site probably represent cold- and warm-water species, Plains sites in this study typically were located below this transition zone and had finer- grained, less stable substrates and higher nutrient concentrations than the plains site described by Ward (1986), These physical and chemical differences prob- ably resulted in lower number of taxa at plains sites in this study, Although invertebrate density and number of taxa differed between this study and that of Ward (1986), invertebrate community composition in the mountains and plains regions were similar, Ward (1986) reported that the orders Plecoptera, Trichoptera, Ephemeroptera and Diptera dominated mountain sites, and three orders of insect (Collembola, Odonata, Lepidoptera), Amphipoda, Hirudinea and Gastropoda were col- lected only in the plains sites, The data for this study indicate a similar pattern, with the exceptions that @ 1995 Blackwell Science Ltd, Freshwater Biology, 33, 439-454 Invertebrates of the South Platte River 451 Collembola were not collected at any sites and Lepi- doptera were collected only in mountain sites. The distribution of invertebrate families within insect orders also were similar to families reported by Ward (1986), Despite differences in the taxonomic composition among site groups, a notable feature is the similarity in the functional-feeding group composition between mountain and plains/braided channel sites, Although different in physical attributes, both site groups were less enriched (Le. lower surface-water nutrient concen- trations) compared with plains/tributary and plains/ downstream from point-source sites. Collector-gather- ers, collector-liiterers and shredders were the domin- ant groups, In contrast, the nutrient-enriched (plains/ tributary and plains/downstream from point-source) sites were dominated by collector-gatherers and collector-filterers and had a greater proportion of predators and scrapers compared with the mountain and plains/braided channel sites, Ward (1986) reported collector- gatherers and scrapers to be the dominant functional feeding groups at most mid- to lower-altitude mountain sites and the plains sites in the St Vrain River, The low proportion of scrapers at the non-enriched sites in the present study was interesting considering that periphytic algae were visible at most sites. i I Ecoregions and land use Results from the OCA analyses separated invertebrate communities based on the two dominant ecoregions, Southern Rockies and Western High Plains, Similarly, Whittier et ai, (1988) reported that the clearest differ- ences between biotic assemblages and physiochemical attributes measured were between montane and non- montane regions in Oregon, Corkum (1989) also reported distinct differences in benthic invertebrates between streams in mountain and plains regions of north-west North America, In contrast Quinn & Hickey (1990) could not readily group invertebrate communities into ecoregions for New Zealand stre- ams, and their results indicate that land use (i.e, catchment development) is more important than regional factors in determining the characteristics of river invertebrate communities. Within an ecoregion, land use can be an important large-scale factor affecting composition and structure of invertebrate communities, In the Southern Rocky