Demography and Habitat Associations of the Basalt Cactus Pediocactus nigrispinus

Cacti are iconic members of arid ecosystems. Pediocactus nigrispinus is a species of cactus endemic to the Pacific Northwest and has been listed as a sensitive species due to its vulnerability to habitat loss. Best conservation methods for this species are poorly understood, due in part to a limited understanding of its demography and habitat associations that may help define its occurrence. A demographic study from four plots in Washington State was implemented in 2016 to provide a baseline of the population dynamics for P. nigrispinus and to enhance conservation efforts for this sensitive species. Five years of size, fecundity, and survival information for the cactus were recorded. A stochastic analysis was performed to assess survival and fecundity rates in relation to different climatic variables. To better understand the cacti's limited habitat, biotic and abiotic associations were assessed in an additional 16 plots throughout Washington and Oregon by spatial analysis of soil and climatic variables and in situ assessment of vegetation and ground cover associations. These variables were then compared with population density, overall size of the individuals, and number of fruits to assess how changes in associations may correlate with changes in these traits. The demographic study found no juvenile recruitment and a declining population over the four years of analysis regardless of variation in annual precipitation. The association analysis found no correlation between population density or fecundity and biotic or abiotic associations but indicated higher fruit production in Oregon than in Washington. With the declining population and narrow endemism of the species observed, further monitoring and study for conservation efforts are recommended. Key Points Plant community and microbial associations were similar for P. nigrispinus throughout its range in Oregon and Washington. No juvenile recruitment and a declining population size were observed for P. nigrispinus over four years. A small increase in survival of adults in the medium size class could lead to stabilization of the P. nigrispinus population.