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(e.g., the P and C compartments of Figure 4) attempting to co-exist. Phenologies (life histories) are highly evolved and sensitive to environmental change. Consequently, disturbance events (e.g., floods, droughts, fires, disease epidemics, invasions by exotic spedies) reduce reproductive success and, hence, bioproduction; thereby, connectivity of lotic food webs is naturally decreased (Figure 5). Our main point is that for a particular species to survive, either as a resident of the catchment or as an immigrant, enough individuals must realize a net energy gain to meet phenological requirements which permit conservation of the gene pool (i.e., net positive contribution to riverine bioproduction). Bioproduction at the ecosystem level of organization is controlled by the same plethora of environmental factors, although in the case of riverine fishes, especially anadromous species, harvest by humans often is more pervasive than other environmental disturbances. The degree of structural (Figures 1 and 2) and functional (e.g., flux of organic and inorganic materials and energy between consumer groups, Figure 4) connectivity determines the most probable biophysical state of the ecosystem at any given point in time. For many scientists this implies that tightly coupled ecosystems are highly evolved, undisturbed and essentially in equilibrium. However, circumspection of equilibrium concepts is waning in contemporary ecology (Murdoch 1991), owing to the realization that natural and anthropogenic disturbances occur too frequently in most catchments to allow equilibrium models at any level of organization to be realistic (Resh et al. 1988). Disturbance events after structural and functional connectivity (Figure 5); the instantaneous biophysical status of ecosystems is usually more analogous to a quasi-equilibrium (sensu Schumm and Lichty 1956, see also Huston 1979). Human Disturbances and Loss of Ecological Connectivity How much disturbance can occur in a catchment before ecosystem resilience (i.e., the ability to recover from disturbance, Odum et al. 1979) is exceeded and ecosystem structure and function is permanently altered (Yount and Neime 1990)? How much is attributable to natural interannual variation? Such questions were articulated years ago but remain largely unanswered, which is, of course, problematic for researchers and especially for managers attempting cumulative impact assessments at the catchment level. We have argued (Ward and Stanford 1983b) that natural interannual variation in catchments is encompassed by Connell's (1978) intermediate disturbance . 8