The transects were 1 mile long on a north-south axis parallel with the beach, and extended 

 north from the PAIS ranger station to the Chevron Oil Company installation. A 

 1-meter-diameter hoop was tossed at 10-pace intervals from the previous sample. Frequency 

 and cover-class data were taken. One hundred samples along each transect were obtained 

 each year. An importance index was calculated for each species by multiplying percent 

 frequency by percent cover (Dyksterhuis, 1946) for a maximum possible value of 10,000. 



These transects were located in the central part of the South Bird Island Quadrangle. The 

 mid-island prairie transect extended from 500 feet west of the PAIS ranger station north to 

 500 feet west of the Permian Tank-main road junction. This area is termed an older 

 deflation plain by Hunter and Dickinson (1970). The backshore transect was in the center 

 of the 4-mile-long pedestrian-only beach. The two foredune transects were along a 

 continuous and well-developed foredune ridge. Representative photos of parts of the four 

 transects are shown in Figure 7. 



Results of the vegetative survey are shown in Tables 7 and 8, and generalized in Figure 2. 

 There was little difference in frequency of most species between the 2 years, but generally a 

 consistently iiigher cover value for these same species in 1971 (Table 8); tiiis apparently 

 represents differences in observer bias in the estimation of cover classes, rather than a 

 decrease in cover. Several species frequently found in the 1973 mid-island transect were 

 scarce or absent in the 1971 transect, e.g., a sedge, (Firnbristylis caroliniana), yellow-eyed- 

 grass (Xyris Jupicai), Centella asiatica, pencil -flower (Stylosanthes viscosa), and fleabane 

 (Erigeron myrionactis). Species composition of the mid-island grasses changed little in 

 2 years, but backshore vegetation changed dramatically during the interval. Hurricane 

 Beulah (1967) planed off the beaches on northern Padre to the foredune line. Since 

 Hurricane Beulah, the backshore vegetation at the pedestrian-only beach has rapidlv 

 advanced seaward. 



Species diversity increases inland (Table 9), with the greatest number occurring on the 

 mid-island transect (old deflation plain). Although similar transects were not made within 

 the younger deflation plain or Laguna Madre flats, species diversity also decreased in these 

 areas. The total number of vascular plant species on Padre and Mustang Islands (including 

 roadside waifs and some exotic plantings, such as five species of salt-cedar) is about 370 

 (Fred B. Jones, Corpus Christi botanist, personal communication, 1973). 



Of particular interest is the vegetation of the backshore and the foredune foreslope, and 

 the natural succession of plants from a barren, hurricane-planed backshore to a continuous, 

 mature foredune ridge. The following sequence of these events was based on information 

 from vegetative surveys, observation of areas of natural formation, and examination of 

 sequential aerial photos dated 1937 to 1974. 



Sea purslane, one of the first species to reappear on the denuded backshore. is 

 vegetatively dispersed by wave and wind action. Clumps of sea purslane trap sand, forming 

 small dunes which usually rise only a few feet above the beacli surface. Beacli 



37 



