Connectivity Conservation: Hydrologic connectivity: a neglected dimension of conservation biology

INTRODUCTION Hydrologic connectivity refers to water-mediated transfer of matter, energy, and/or organisms within or between elements of the hydrologic cycle ( sensu Pringle 2001). While this property is essential to maintaining the biological integrity of ecosystems, it also serves to perpetuate the flow of exotic species, human-derived nutrients, and toxic wastes in the landscape. All too often, we have acknowledged the importance of hydrologic connectivity in hindsight – as a result of environmental crises. Examples range from: the transport of exotic species that disrupt the food webs of rivers and lakes (e.g., Stokstad 2003); to the occurrence of extremely high levels of persistent organic pollutants (e.g., polychlorinated biphenyls PCBs) that bioaccumulate in global “hotspots” such as the Arctic (e.g., Aguilar et al . 2002); to effects of dams which impede riverine transport of essential elements such as silicon to coastal regions – which has been implicated in coastal eutrophication and the creation of coastal dead zones (e.g., Humborg et al . 2000). Management and policy decisions regarding land-use activities are often made in the absence of adequate information on hydrologic connectivity in the landscape. An important area of research is to understand how human alterations of this property (e.g., dams, stream flow regulation, water diversion, inter-basin water transfers, water extraction) influence ecological patterns on local, regional, and global scales. Half of the accessible global freshwater runoff has already been appropriated by humans and this could climb to 70% by 2025 (Postel et al . 1996).