Wednesday, January 16, 2013

Riparian reptiles & dams

Regina septemivatta

Regulating water flow  through dams can be detrimental to riparian habitat. In the United States over 75,000 large dams disrupt the flow of rivers. Damming changes the water quality of a river system by reducing the sediment load downstream and increasing sediment load upstream, and by lowering dissolved oxygen levels in impoundments. Damming has been linked to population declines of aquatic organisms. Semi-aquatic species are sensitive to damming because flow regulation can fragment habitat by isolating the main river channel from adjacent riparian floodplains and flow regulation can also reduce periodic flooding, which can lead to population declines among organisms whose life histories are adapted to the river’s natural flow regime. Regulated rivers can also provide suitable habitat for invasive organisms that would otherwise not thrive under naturally variable flow conditions, and these species may displace native species. And of course,  high spate events caused by water releases from dams may displace individuals immediately downstream from dams.

In a new paper Hunt et al. (2013) note that riparian habitats are critical  for many reptile species, and that these habitats are frequently modified by anthropogenic activities. The study investigated the effects of two riparian habitat modifications – damming and urbanization – on overall and species-specific reptile occupancy patterns. They used time constrained search techniques to compile encounter histories for 28 reptile species at 21 different sites along the Broad and Pacolet Rivers of South Carolina. Using a hierarchical Bayesian analysis, they modeled reptile occupancy responses to a site’s distance upstream from dam, distance downstream from dam, and percent urban land use. The mean occupancy response by the reptile community suggested that reptile occupancy and species richness were maximized when sites were farther upstream from dams. Species-specific occupancy estimates showed a similar trend of lower occupancy immediately upstream from dams. Although the mean occupancy response of the reptile community was positively related to distance downstream from dams, the occupancy response to distance downstream varied among species. The percent of urban land use had little effect on the occupancy response of the reptile community or individual species. The results indicate that the conditions of impoundments and subsequent degradation of the riparian zones upstream from dams may not provide suitable habitat for a number of reptile species.

Hunt, S. D., Guzy, J. C., Price, S. J. Halstead, B. J. Eskew, E. A., Dorcas, M. E. 2013. Responses of riparian reptile communities to damming and urbanization. Biological Conservation, 157: 277 - 284.

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