Indices of Grassland Biodiversity in the Chihuahuan Desert Ecoregion Derived from Remote Sensing

We used a relatively simple and direct remote-sensing approach to determine biodiversity values in arid ecosystems and thus identify potential conservation sites. We developed indices based on regression models between grass, shrub, litter, exposed-soil groundcover components, and Landsat thematic mapper satellite imagery reflectance values over a reference site in the northern Chihuahuan Desert in New Mexico. This site supports low-disturbance desert grasslands that have been excluded from livestock grazing for 55 years and moderate-disturbance grasslands that have been under a continuous grazing regime for over 100 years. Greater richness and abundance of noninvasive and nonruderal plant species were associated with the lowdisturbance grasslands that had lower shrub abundance, increased litter and grass cover, and lower exposed soil. Using the thematic mapper indices, we computed an additive grassland biodiversity index such that, as exposed soil and shrub values go down, litter and grass values go up, as does the biodiversity index. When the biodiversity index was applied to the reference-site landscape, grasslands previously identified for their high conservation value were detected. As a further test, we applied the indices to a site in Chihuahua, Mexico, that supports similar grasslands but for which there are few other data on condition and conservation values. The soil, grass, and shrub indices were moderately effective in describing the range of variation at the Mexico site, but the litter equation was not. Still, higher biodiversity value in terms of nonruderal plant diversity tended to correspond to higher grass cover and lower soil exposure and a higher overall biodiversity index. Some localized calibration with geologic substrate may be required along with an assessment of the temporal constraints, but generally the index shows promise for quickly and efficiently detecting desert grasslands of high biodiversity conservation value. Indices de Biodiversidad en Pastizales Derivados de Percepción Remota de la Ecoregión del Desierto de Chihuahua Resumen: Utilizamos una estrategia simple y directa de percepción remota para determinar valores de biodiversidad en ecosistemas áridos y posteriormente identificar sitios potenciales para conservación. Se desarrollaron índices basados en modelos de regresión entre pastos, hierbas, hojarasca, componentes de la cobertura de suelo expuesto y los valores de reflectancia de las imágenes de satélite del mapeador temático Landsat para un sitio de referencia en el Desierto de Chihuahua en Nuevo México. Este sitio sostiene pastizales de desierto de baja perturbación que han sido excluídos del pastoreo del ganado por 55 años y pastizales con perturbación moderada que han recibido pastoreo continuo por más de 100 años. La mayor riqueza y abundancia de especies de plantas no invasoras y no ruderales estuvieron asociadas con los pastizales de baja perturbación que tuvieron una abundancia arbustiva más baja, una mayor cobertura de hojarasca y pastos y una menor cantidad de suelo expuesto. Usando los índices del mapeador temático Landsat, calculamos un índice de biodiversidad aditivo para pastizales de tal manera que cuando los valores de suelo expuesto y arbustos disminuyen, los valores de hojarasca y pastos incrementan, al igual que el índice de biodiversidad. Cuando el índice de biodiversidad se aplicó al paisaje del sitio de referencia, los pastizales previamente identificados por su alto valor de conservación fueron detectados. Como una prueba posterior, aplicamos los Paper submitted April 5, 2000; revised manuscript accepted February 7, 2001.