Padre Island is the longest of five barrier islands occurring along the Texas Gulf Coast. South Padre Island is separated from the northern two-thirds of the island by the Mansfield Channel. The composition and pattern of vegetation on South Padre Island are relatively well known, but data on the interrelationship of dune and vegetation stability are lacking. We hypothesized that (1) there should be an inverse relationship between elevation change and percent cover on transects across the nearshore dunes of South Padre Island; and (2) percent cover, species composition, and species importance should be most stable where elevation change was least. We tested these hypotheses using three study sites differing in vegetation abundance. Elevation measurements were taken at 1.0-m intervals along three transects at each site using survey-grade Global Positioning System equipment. Vegetation abundance was determined in 10-m intervals along each transect. No sites or topographic zones were devoid of elevation change. Even a site that had a mean percent cover of 65.1% had a mean elevation change of 15.6 cm. However, cover does not have to be great to provide considerable stability. There was no significant difference in elevation change at Site 1, where cover was 57%, and Site 2, where cover was only 12.5%. As hypothesized, there was a significant inverse correlation between elevation change and percent cover when analyzed over all transects and sites, but the relationship did not hold for all sites or topographic zones when these were examined separately. Lack of correlation may be due to differences among sites and zones in the number of different perturbations and their intensities and frequencies. Only Site 2 showed a significant difference in percent cover between the initial and final samples. Species composition and importance were more stable where elevation changes were low.
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Vol. 2008 • No. 244