Habitat Characterization and Pilot Reintroduction of Star Cactus (Astrophytum asterias)
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Astrophytum asterias is federally listed as endangered and in the United States is found only in Starr County, Texas. The species has a priority ranking of 2 by the United States Fish & Wildlife Service which means it has high recovery potential. One means to achieve recovery is by reintroduction. To establish a successful reintroduction, it is important to know the species' habitat. Therefore, this study characterized A. asterias habitat by conducting vegetation transects and collecting soil samples in 15 subpopulations. The top five plant species with greatest dominance included: Varilla texana, Prosopis glandulosa, Acacia rigidula, Opuntia leptocaulis, and Castela erecta subsp. texana. Astrophytum asterias has been found in the following soils: Catarina soils; Garceno clay loam; Jimenez-Quemado association; Maverick soils, eroded; Montell clay, saline; and Ramadero loam. Of the 15 subpopulations sampled, 9 were classified as saline-sodic; 2 saline; 2 sodic; and 3 non-saline, non-sodic. In this study a pilot reintroduction was also established to test the feasibility of reintroducing A. asterias. Seeds and seedlings were used as propagules for the pilot reintroduction. Four treatments were established: seeds planted in the spring; seedlings planted in the spring; seeds planted in the fall; seedlings planted in the fall. Each treatment consisted of 120 individuals. Overall less than 4% of the planted seeds produced seedlings. Seedling survivorship of the spring and fall treatments was 55% and 72.5%, respectively. Mortality of seedlings was due to desiccation, herbivory, infestation by weevils, burying by Mexican ground squirrel, and other miscellaneous and unknown causes. Twenty-eight candidate models were developed to assess the potential influence of season of planting; average state of the seedlings per subquadrat; environmental variables of monthly precipitation and average monthly ground temperature; average seedling diameter at the time of planting per subquadrat; and percent bare ground of each subquadrat on percent survivorship of the reintroduced seedlings. In addition to the 28 candidate models, a temporal model was also included which assessed passage of time as the only factor influencing survivorship. Of the model statements analyzed, the temporal model was the top model (AICc = 1129.3094, w1 = 0.99999854) indicating that of the factors analyzed, passage of time had the greatest influence on seedling survivorship. Based upon the research of this study, a draft reintroduction plan for A. asterias was developed to guide future reintroductions.
CitationBirnbaum, S. J. (2009). Habitat characterization and pilot reintroduction of star cactus (Astrophytum asterias) (Unpublished thesis). Texas State University-San Marcos, San Marcos, Texas.
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