ABSTRACT
The effects of grazing by feral horses on vegetation and dune topography at Assateague Island National Seashore were investigated using color-infrared imagery, lidar surveys, and field measurements. Five pairs of fenced and unfenced plots (300 m2) established in 1993 on sand flats and small dunes with similar elevation, topography, and vegetation cover were used for this study. Color-infrared imagery from 1998 and field measurements from 2001 indicated that there was a significant difference in vegetation cover between the fenced and unfenced plot-pairs over the study period. Fenced plots contained a higher percentage of vegetation cover that was dominated by American beachgrass (Ammophila breviligulata). Lidar surveys from 1997, 1999, and 2000 showed that there were significant differences in elevation and topography between fenced and unfenced plot-pairs. Fenced plots were, on average, 0.63 m higher than unfenced plots, whereas unfenced plots had generally decreased in elevation after establishment in 1993. Results demonstrate that feral horse grazing has had a significant impact on dune formation and has contributed to the erosion of dunes at Assateague Island. The findings suggest that unless the size of the feral horse population is reduced, grazing will continue to foster unnaturally high rates of dune erosion into the future. In order to maintain the natural processes that historically occurred on barrier islands, much larger fenced exclosures would be required to prevent horse grazing.
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Acknowledgments
The authors thank Mark Duffy, Chris Lea, and Carl Zimmerman from the National Park Service, Wayne Wright, Robert Swift, and Bill Krabill from NASA Wallops, Ellen Raabe, Amar Nayengandhi, Christy Guy, and Russ Peterson from the USGS, the support of the Assateague Island National Seashore staff, Jack Kumer, Helen Hamilton, Mike O’Connell, Arte Rodriguez; and the NASA Wallops Flight Facility. The authors also thank Patrick Healy, Ginger Garrison, Chuck Holmes, and Christine Farris for reviewing this research, and Tony Orme and Stan Trimble for providing helpful discussion of coastal processes.
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De Stoppelaire, G.H., Gillespie, T.W., Brock, J.C. et al. Use of Remote Sensing Techniques to Determine the Effects of Grazing on Vegetation Cover and Dune Elevation at Assateague Island National Seashore: Impact of Horses. Environmental Management 34, 642–649 (2004). https://doi.org/10.1007/s00267-004-0009-x
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DOI: https://doi.org/10.1007/s00267-004-0009-x