The Ecology of Selerocactus polyancistrus (Cactaceae) in California and Nevada

Abstract

Several key aspects of the ecology of Sclerocactus polyancistrus (Cactaceae) are described based on a 15 year study by the author. Highlights of the study, funded in part by grants from the Cactus and Succulent Society of America and the United States Navy (China Lake Naval Weapons Center), include a record of growth rates and the impact of predation and infestation. The study also includes the analysis of the carcass remains, identification of predators, the benefits and offsetting effects of predation, and the resulting morphological abnormalities. Using microhystological analyses to determine the relative density of discerned fragments of scats found within the carcasses, Neotoma lepida was found to be the chief predator of this species at elevations below 1500 m (5000 ft). At higher elevations, infestation primarily by the cerambycid beetle Moneilema semipunctatum, is the dominant cause of mortality. Returning each spring over a 15 year period (1976-1991) to six study sites in the Mojave Desert, the author recorded apical growth measurements of over 350 stems and correlated these growth rates to rainfall and other key climatological factors. The results show that microhabitat is a major factor in this species growth rate and that the oldest plants within a given population are in excess of 50 years in age. As expected, there is a direct correlation between seasonal rainfall and the growth rate and number of flowers produced. When surveying eastern Nevada and the Canyonlands National Park areas of Utah, the author has found similar predation and infestation in S. parviflorus and S. spinosior, which indicates that some of the data collected in this study may also be applicable to other small stem cacti of the southwestern deserts, particularly within the same genus.