The population dynamics of Sclerocactus mesae-verdae were documented annually in 3 plots over 20 years in southwestern Colorado. Demographic studies of this duration are rare but they are necessary to gain a more accurate estimate of population trends in long-lived desert species. Sclerocactus mesae-verdae was federally listed as threatened in 1979; this study was initiated in 1986 with the objective of determining whether Colorado populations were growing, declining, or stable. Data collected yearly around 1 May included diameter, vigor, and flower/fruit counts for 1629 stems belonging to 659 plants. We used regression analysis to estimate the mean, variance, and average population growth rate for each plot. Observations made throughout the study suggested that the cactus responded strongly to climatic variables, especially precipitation and temperature, and that 2 of the 3 subject populations were subject to periodic devastation by a longhorn beetle (Moneilema semipunctatum). These observations led us to attempt to correlate significant mortality (beetle-caused) or recruitment events to climate variables. We found that the cactus populations appear to be roughly stable to increasing slightly, possibly due to several wet springs and warmer-than-average winter temperatures since 1992. However, the warming trend also favors beetle predation, which overall presents a concern for the species' long-term survival. We conclude that 20 years may not be long enough to clarify large-scale climate influences on the persistence of long-lived desert species.
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