Linking environmental régimes, space and time: Interpretations of structural and functional connectivity

Connectivity as a concept has been increasingly part of discussions or explanations in hydrology, geomorphology and ecology. We address recent critiques of this approach by demonstrating how a refinement which distinguishes structural connectivity from functional connectivity can be used to explain patterns observed in very different environmental systems. These systems are found in linkages between surface and subsurface flowpaths and the hyporheos in the River Don, a temperate river channel in Yorkshire, UK; in surface and subsurface fluxes in agricultural land in the UK; and in vegetation and surface conditions in a degrading environment at the Sevilleta LTER site in the semi-arid Southwest USA. First, we demonstrate long-term geological and structural controls mediated by in-channel processes. Second, human organization of landscape elements is a significant control on runoff and erosion, so that similar events can produce very different responses (and vice versa). Third, linkages between the removal of grass vegetation and runoff and erosion produce non-linear and path-dependent feedbacks which control the subsequent degradation of the landscape, making the process difficult to reverse. As a result of these studies, we argue that even in cases where connectivity cannot be directly quantified (at least at present), this limitation does not prevent the concept from being a useful heuristic device for exploring responses of complex systems. Furthermore, this result implies that an increasing need exists for disciplinary connectivity to investigate such systems.