What is hydrologic connectivity and why is it ecologically important?

Hydrologic connectivity (sensu Pringle, 2001) is used here in an ecological context to refer to water-mediated transfer of matter, energy and/or organisms within or between elements of the hydrologic cycle. Hydrologic connectivity is essential to the ecological integrity of the landscape, and reduction or enhancement of this property by humans can have major negative environmental effects. Some of these effects are immediate, localized and, therefore, obvious. For example, with respect to migratory fish, a given dam may act to reduce hydrologic connectivity (by preventing or impeding migration up or downstream), whereas interbasin river transfers enhance this property by allowing the dispersal of fish into river basins outside of their range. Less obvious, are alterations in hydrologic connectivity that exhibit a time lag and manifest themselves at geographic locations far from the source of disturbance. An example concerns the cumulative effect of dams on transport of the inorganic dissolved solute silica. Dams and associated impoundments can reduce the transport of this compound, which becomes deposited in the bottoms of reservoirs (Humborg et al., 2000). The cumulative effects of many dams along a river can potentially result in a reduction in the amount of silica delivered to coastal waters, with consequent negative effects on coastal food web structure that contribute to eutrophication (Justic et al., 1995; Turner et al., 1998). Management and policy decisions regarding land-use activities and hydropower development are often made in the absence of adequate information on hydrologic connectivity in the landscape. Our current knowledge of how this property maintains the ecological integrity of ‘natural’ ecosystems is poor due to: (1) the inherent complexity of water movement within and between the atmosphere and surface–subsurface systems; and (2) the extent and magnitude of human alterations, which often occur before we understand how hydrologic connectivity affects ecological patterns in the landscape (Pringle and Triska, 2000). Hydrologic connectivity is being altered at a rate unprecedented in geologic history, contributing to dramatic losses in global aquatic biodiversity and associated ecosystem integrity (e.g. Dudgeon, 2000; Pringle et al., 2000; Rosenberg et al., 2000). Humans have already appropriated one-half of the accessible global freshwater runoff and this could climb to 70% by 2025 (Postel et al., 1996).