Population dynamics and effects of large ungulate disturbances on Winkler cactus (Pediocactus winkleri K.D. Heil) were documented annually over a 20-year time span at one plot within Capitol Reef National Park, Utah. This cactus species was federally listed as threatened in 1998. The study began in 1995 to gain a better understanding of life history aspects and threats to this species. Data were collected annually in early spring and included diameter, condition, reproductive structures, mortality, recruitment, and disturbance by large ungulates. We used odds ratio and probability model analyses to determine effects of large ungulate trampling and weather on these cacti. During the study, plot population declined by 18%, with trampling of cactus, low precipitation, and cold spring temperatures implicated as causal factors. Precipitation and temperature affected flowering, mortality, and recruitment. Large ungulate disturbances increased mortality and reduced the probability of flowering. These results suggest that large ungulate disturbances and recent climate regimes have had an adverse impact on long-term persistence of this cactus.