Causes of individual mortality in the endangered star cactus Astrophytum asterias (Cactaceae): The effect of herbivores and disease in Mexican populations

Abstract

Little is known about the causes of mortality in natural populations of threatened cactus species. In particular we focus on the endangered Astrophytum asterias in four populations where most individuals of the species are found. At two sites one of each vegetation type (Tamaulipan thornscrub [TS] and Piedmont thornscrub [PT]) we measured individual mortality by three causes: the plant pathogen Phytophtora infestans, a cerambicid beetle and the terrestrial squirrel Spermophilus mexicanus. We found that damage by the combination of pathogen and herbivore meant a reduction of more than 50% in population size. From these factors, the most important cause of death was S. mexicanus followed by the cerambicid beetle in TS and P. infestans in PT. Mortality was concentrated in the small size categories, even though large size plants were also affected albeit to a lesser extent. More quantitative studies in field conditions are necessary to establish sound conservation efforts that could improve the recovery plans for this endangered plant species.