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<br />81 <br /> <br />deep chaBnels, high turbidity for long periods, and a shifting sand <br />substrate. Autochthonous inputs are primarily limited to planktonic <br />organisms. The ratio between priIll8ry production and ecosystem rE,spiration <br />is less than 1. The trophic resource is received as FPOM input from <br />upstream sources and large stream systems are therefore classified as <br />heterotrophic. <br /> <br />Distribution Pattern of Functional Feeding Groups <br /> <br />The longitudinal distribution of spec,ies within streaM ecosystems has <br />received considerable attention. Changes in distribution patterns have <br />been reported for benthic invertebrates (Ide 1935, Sprules 1947, Jonasson <br />1948, lIlies 1953, Harrison and Elsworth 1959, MaitlaBd 1966, Harrel and <br />Dorris 1968, Ulfstrand 1968, Minshall and Kuehne 1969, Charles and Sedell <br />1981, Minshall et al. 1983, and others), fish (Trautman 1942, Huet 1949, <br />1954, Kuehne 1962, Harrel et al. 1967, Whiteside and iflcNatt 1972, Tramer <br />and Rogers 1973, Horowitz 1978, Platts 1979, Hughes and Omernik 1980), and <br />special groups such as Trichoptera (Wiggins and Mackay 1978). Cummins <br />(1975, 1977) was the first to propose a "theoretical diagrammatic <br />representation" of Ill8croinvertebrate cOlDIDUllity structure along a stream <br />ecosystem continuum. Community structure ~'as describ,ed by roughly grouping <br />the stream continuum of conditions into he'ldwaters (orders 1-3), <br />medium-size streams (orders 4-6), and large rivers (orders greater than <br />6). As a consequence of physical and biological proc,esses, organic matter <br />particle size in transport becomes smaller as one pro,ceeds downstream. The <br />net effect of this change in food particle size stron:gly influences and <br />restricts functional group community structure. The ':onceptual model was <br />slightly revised to reflect the relationships between aquatic <br />invertebrates, types and locations of food resources and stream size <br />(Vannote et al. 1980). The relative dominance of the general functional <br />groups (shredders, collectors, scrapers, and predators) and their <br />dependence on the microbial biomass was superimposed on a hypothetical <br />stream continuum (Figure 4). Recently, Vannote (1983) predicted the <br />relative dominance of invertebrate functional feeding groups based upon the <br />qualitative changes in particulate organic inputs and primary production <br />along the stream contiuum (Figure 5). <br /> <br />Because a community pattern like that in Figure 5 includes numerous <br />species within the more broad functional group categories, community <br />pattern can be conceived as a complex population contlnuum formed by these <br />many, overlapping species populations. Species diversity was hypothesized <br />to increase in direct proportion to the variance of the physical system and <br />heterogeneity in the particle size composition of the food supply. <br />Community structure within the continuum was hypothes:lzed to be a function <br />of (1) rate and variance of physical energy expenditulre, (2) particle size <br />distribution of local organic input and the ratio of l~he terrestrial input <br />to autochthonous production (primarily alga,e), and (3) the qualitative <br />properties of organic matter in transport. <br /> <br />To date, most studies have focused on specific individual parameters <br />related to the River Continuum Concept. For example, the distribution and <br />