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20 BIOLOGICAL REpORT 29 Habitat bottlenecks are important, but some- times poorly understood. The basic premise of the habitat bottleneck is that populations of aquatic organisms are related to the availability of habitat through time. This definition has been commonly misinterpreted to mean that adult fish popula- tions must be instantaneously correlated with habitat. Such an interpretation logically requires a belief in instantaneous mortality and spontane- ous generation, or the ability of fish to move quickly among habitats, in order for fish popula- tions to increase and decrease at the same rate that habitat can change in a stream. In reality, habitat limitations affecting a population usually occur prior to the time when the population size is measured. Adult populations are frequently determined by recruitment, which is highly corre- lated with the amount of habitat available for early life stages of the species. Such 'habitat events' usually affect recruitment via habitat types directly related to the production and sur- vival of eggs, larvae, and fry (such as spawning habitat and young-of-year rearing habitat), or in- directly related to survival by the growth rates of age-O fish (such as temperature regime, young-of- year rearing habitat, or microhabitat for inverte- brate food supplies). These habitat bottlenecks typically occur 1 to 3 years prior to maturation, when their effects are detectable in the adult population (Nehring and Anderson 1993; Bovee et al. 1994). In addition, Bovee et al. (1994) found that 1. there may be several consecutive and inde- pendent habitat events that can affect adult populations (such as spawning habitat, fry rearing habitat, temperature regime, and adult feeding habitat); 2. limiting events frequently occur over variable time scales (such as acute events that limit fry survival versus chronic events, such as long- term crowding of adults during the summer); 3. habitat may be limited by both high and low flow events and by the rate of change of flow events; 4. the smallest amount of habitat available during the year may not necessarily be the limiting event (such as during the winter when fish are inactive); and 5. habitat types not directly utilized by the species (such as macroinvertebrate habitat as it affects food supply for fish) may be more important than the habitat directly used by the species. , Conclusion A common misinterpretation of IFIM is that it is only a 'trout model.' This misunderstanding is undoubtedly related to the origins of the technique for quantifying salmonid microhabitat (Collings et al. 1972), which formed the conceptual basis for the Physical Habitat Simulation System (PHAB- SIM). Although the modeling techniques underly- ing PHABSIM originated in salmonid streams, Bovee (1975) concluded that there were sufficient parallels in microhabitat use across stream com- munities that the same basic approach could be used in most riverine environments for essentially any riverine species. As discussed in this chapter, many concepts and components ofIFIM are rooted in community ecol- ogy and were developed from many stream set- tings. Students of IFIM should have little trouble recognizing the influence of the longitudinal suc- cession concept as a defining property of macro- habitat. Longitudinal succession has been con- firmed in many studies, in coldwater and warmwater streams, since Shelford's (1911) origi- nal hypothesis. 1 In IFIM, macrohabitat compo- nents such as channel structure and discharge are used to define sampling strata for the quantifica- tion of microhabitat. Temperature and water qual- ity are incorporated in IFIM to define the longitu- dinallimits where a species can or cannot survive. The treatment of microhabitat in IFIM using PHABSIM is consistent with the two-dimensional partitioning of microhabitats documented for stream-dwelling organisms ranging from algae to fish. Although the reasons for habitat partitioning may vary, such behavior is commonplace in stream ecosystems worldwide. One possible reason for the universality of this phenomenon is that streams provide unique but repetitive types of microhabitat niches no matter where they are, and there will always be one or more species adapted to filling those niches. PHABSIM has been criticized because it con- tains only a few variables, namely depth, velocity, and channel index (usually a combination of sub- strate material and cover). However, in nearly all of the studies conducted on habitat partitioning among stream-dwelling animals, these variables were consistently found to be important determi- nants of species distributions and abundance. The 1 The concept oflongitudinal succession has matured into what today is the river continuum concept (Vannote et a1. 1980).