Spatial Network Structure of Wetlands in the Prairie Pothole Region
U.S. Geological Survey
Last modified: June 29, 2007
The Prairie Pothole Region (PPR) of the north central United States contains hundreds of thousands of depressional wetlands, constituting some of the most productive waterfowl habitat in North America. In response to regional climate cycles (drought and deluge), the extent of surface water within these wetlands tends to be highly dynamic, with many surface water-bodies disappearing during drought, only to reappear during periods of greater precipitation. As wetlands wink in and out of existence, wetland-dependent species (particularly birds and amphibians) disperse to suitable habitat, either from drying wetlands to more permanent water-bodies during drought, or from permanent wetlands to ephemeral water-bodies during deluge. This dispersal process can be conceptualized in terms of spatial networks, where individual wetland basins represent nodes and characteristic link lengths vary as a function of the dispersal ability of different species. Importantly, the underlying structure of these spatial networks varies as wetlands (nodes) appear and disappear, with potential implications for the long-term persistence of wildlife populations. I used satellite remote sensing to monitor wetland presence/absence within an approximately 30,000-km2 area in the PPR. Preliminary results comparing an extreme-drought year with a very wet year indicate that cumulative directed degree distributions from ephemeral wetlands to permanent wetlands fit truncated power-laws over a range of dispersal distances. For conservation purposes, this information allows us to identify permanent wetlands that might act as important refugia during drought or as sources for recolonization during deluge. During extreme drought, there was little correlation between the number of links from dry wetlands to permanent wetlands and the number of links from permanent wetlands to other permanent wetlands, indicating that few permanent wetlands were acting as stepping stones linking multiple dry and permanent wetlands. Similar analyses will be extended to years falling within these two precipitation extremes to examine how wetland network topology changes as the proportion of flooded basins varies along a continuum.