DENSITIES, PLANT SIZES, AND SPATIAL DISTRIBUTIONS OF SIX WILD POPULATIONS OF LOPHOPHORA WILLIAMSII (CACTACEAE) IN TEXAS, U.S.A

ABSTRACT Lophophora williamsii (Cactaceae) is thought to be threatened by habitat loss and overharvesting. However, basic demographic and habitat information to evaluate its conservation status has been lacking. We surveyed six wild populations of this species, three in South Texas and three in West Texas, to begin to address this gap. We found high levels of heterogeneity in plant presence and density at multiple spatial scales. While plant densities were not consistently different between South and West Texas, plants were significantly larger in West Texas. The two regions differ strongly in precipitation, temperature, elevation, and topography, all of which are correlated at the regional scale. Therefore, it was not possible to identify which of these variables, or other factors such as competition and human harvesting, may be responsible for the regional differences in plant size. However, our results provide initial information for determining the conservation status of this species. RESUMEN Lophophora williamsii (Cactaceae) se considera amenazada por la perdida de habitat y cosecha excesiva. Sin embargo, se carece de informacion demografica y ambiental basica para evaluar su estado de conservacion. Para abordar este deficit, examinamos seis poblaciones salvajes de esta especie (tres en el sur y tres en el oeste de Texas respectivamente). Encontramos altos niveles de heterogeneidad en la presencia y densidad de plantas en multiples escalas espaciales. Las densidades no son consistentemente diferentes entre el sur y el oeste, pero las plantas son significativamente mas grandes en el oeste. Las dos regiones difieren notablemente en precipitacion, temperatura, elevacion y topografia. Todas estas variables estan correlacionadas a escala regional, por lo que no es posible identificar cual de ellas (u otros factores como la competencia y la cosecha humana) causan las diferencias regionales observadas en el tamano de la planta. Nuestros resultados proporcionan informacion fundamental para determinar el estado de conservacion de esta especie.