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r stream is proportional to velocity. Chapman (1966) suggested that the defense of territories exhibited by salmonids was a surrogate for com- petition for food: "Social status confers definite benefits on individuals. . . despotic fish in hierar- chies or successful territorial fish grow more rap- idly than subordinates or refugees." Chapman also postulated that a similar principle operated at the invertebrate level, thereby forming a link between habitat and the fish food supply. These are important concepts relating to the argument over food limitations or space limitations in stream populations. Chapman's (1966) studyindi- cates that in territorial animals, food limitations may be shown to be actually space limitations in the form of competition over larger and higher quality microhabitats. The energetically advantageous behavior of feeding from a 'low energy' location with forays into 'high energy' locations is not confined to sal- monids. Lobb and Orth (1988) reported a nearly identical feeding strategy in the bigmouth chub. They suggested that the microhabitats selected by the chub not only presented the greatest energetic advantage, but also minimized the risk of preda- tion by birds and fish. Stream-dwelling fish that ambush their prey, including many centrarchids and esocids, tend to select low energy areas asso- ciated with complex structural cover (Haines and Butler 1969; McClendon and Rabeni 1987; Mona- han 1991). These species tend to select cover types that provide a shelter from the current and accen- tuate a contrast in lighting; ambushes succeed better if initiated from a dark area into a well-lit area (Helfman 1981). Probst et al. (1984) noted that smaller smallmouth bass often were observed occupying positions adjacent to moderate velocity, as if feeding on drifting invertebrates. Some species (such as darters, daces, sculpins, and madtoms) tend to occupy microhabitats where their food supply is greatest (Hynes 1970). Morphological and behavioral adaptations are also common in stream-dwelling macroinverte- brates. Some species of mayflies exhibit extreme dorso-ventral flattening, which allows them to creep around in the laminar sublayer on top of and between rocks in torrential currents. Some spe- cies have suction devices to hold them in place on the substrate material. Still others, notably the stoneflies, must live in a current because their gills are immovable and cannot exchange oxygen in standing water (Usinger 1956). Needham and Usinger (1956) found that aquatic insects were THE INSTREAM FLow INCREMENTAL METHOLDOLOGY 19 distributed along gradients of depth and velocity in a riffle composed of uniform substrate material. Minshall and Minshall (1977) suggested secon- dary feedback mechanisms between velocity and substrate material in providing greater surface area for habitation on the streambed and in deter- mining the distribution of food materials for macroinvertebrates. Although many species select and compete for microhabitats that optimize foraging energetics, prey species often select microhabitats that reduce their risk of predation. Whereas territorial expan- sion often occurs when a dominant competitor is removed from the system, a similar phenomenon has been observed in prey species when a predator is removed. Gilliam and Fraser (1987) proposed that animals do not select microhabitats by maxi- mizing energetics or reducing predation hazard independently. Rather, locations are selected that minimize the ratio between mortality rate and foraging rate. In essence, the most favorable mi- crohabitat for an organism would be a place where it could increase its energy input with the least risk of predation. Lewis (1969) concluded that stream trout populations were determined largely by the quality of the habitat; velocity was important as an energetic mechanism, but cover was related to a photonegative response and to predation avoid- ance. Power (1984) observed that armored catfish avoided shallow areas during the daytime, al- though their food is abundant there, and suggested that these areas were avoided because of suscepti- bility to avian predators. Habitat Bottlenecks Wiens (1977) coined the term ecological bottle- neck to describe mechanisms by which communi- ties of organisms are regulated by temporally variable, environmentally induced phenomena. An ecological bottleneck has the effect of depress- ing potentially competitive populations well below the carrying capacity. Once the restriction is re- lieved, competition is reduced or eliminated be- cause adequate resources are available for the standing crop that remains. A habitat bottleneck is similar to Wiens' (1977) definition, but refers solely to habitat limitations that affect popula- tions of individual species (refer to Fig. 2.4), rather than the community as a whole. In contrast to longitudinal succession and habitat segrega- tion, which focus on spatial distributions, the pri- mary dimension embodied by the habitat bottle- neck concept is the element of time.