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16 BIOLOGICAL REpORT 29 1 Chapter 3. Ecological Underpinnings of IFIM IFIM is based on the analysis of habitat for stream-dwelling organisms under alternative management treatments. One could logically ques- tion why habitat was chosen as the decision vari- able in IFIM when there are so many other factors (such as stream productivity or fishing mortality) that can potentially influence fish populations. The simplest reason for basing the analysis on habitat is that IFIM was designed to quantify environmental impacts, and impacts to habitat are the most direct and quantifiable. The more germane reason for basing IFIM on an analysis of habitat, however, is the progression of ecological studies that have implicated or di- rectly shown that habitat is an important determi- nant of the distribution and abundance of fishes and aquatic invertebrates in streams. Though re- cent studies have concentrated on the microhabi- tat requirements of individual species and life stages, stream habitat studies have their origins in community ecology. Detailed information about life history requirements has led to the identifica- tion of key physical features of their habitat (e.g., depth, velocity, substrate material), quantification of their importance, and methods to estimate how they change as a function of stream flow. Longitudinal Succession Some of the earliest works relating the distribu- tion and abundance of stream .fishes to their habi- tat were conducted by Forbes (1907) and Shelford (1911). Shelford (1911) introduced the idea of 'lon- gitudinal succession,' which he compared to a con- temporary geologic theory developed by Davis (1909). Davis conceived the idea that the landscape develops systematically through erosional stages of youth, maturity, and old age. Headwater streams were considered 'young' and charac- terized by high energy and erratic behavior. A youthful stream was steep, had a relatively straight channel with large bed material, and typi- cally exhibited a highly variable flow regime. In contrast, a mature stream had a lower gradient, a meandering pattern in smaller bed material, and a less variable hydrograph. Longitudinal succes- sion was based on the observation that species distribution and abundance also graded up and downstream, corresponding to Davis' stream-age classifications. Refer to Fig. 3.l. Later authors tried to determine possible mechanisms or associations relating the faunal differences along the longitudinal profile with spe- cific characteristics of individual locations. Traut- man (1942) and Huet (1959) found that gradient was a good predictor of faunal regions, whereas Burton and Odum (1945) emphasized the effect of temperature along the headwater-to-lowland con- tinuum. Going downstream in small watersheds, species were'added to the assemblages rather than replacing other species. In contrast, species re- placements occurred where distances were suffi- ciently large to create temperature barriers or where specific types of habitats were not present. However, studies of longitudinal succession were distinctly one-dimensional in that they did not attempt to distinguish the effects of temperature from the associated effects of habitat structure and complexity. Habitat Segregation The study of stream habitats on a two-dimen- sional scale developed as investigators observed that species tended to segregate by habitat type within the same longitudinal zone. Thompson and Hunt (1930) were among the earliest researchers to document the use of different habitat types by different species in short lengths of stream. Their study documented that fish communities tended to segregate by habitat type based on velocity, depth, substrate material, and cover type. Hubbs (1941) postulated that the morphological and behavioral characteristics of stream-dwelling fishes were reflections of the habitat type in which the species most typically occurred. He