Abundance and Diversity of Surface-active Collembola along a Watershed in the Northern Chihuahuan Desert

Abstract

Jackknife diversity estimates (N2 = inverse Simpson diversity index) for surface-active Collembola were examined along a watershed in the Chihuahuan desert of southern New Mexico. Chord distance measures indicated low similarity in collembolan species composition among the six sites. However, assemblages at the top (black grama grassland) and bottom (mesquite fringe of playa) of the gradient were similar to each other, possibly reflecting available soil nitrogen. There was no significant correlation between Collembola and either weekly or long-term precipitation patterns. Populations of truly epigeic Collembola appeared unaffected by rainfall but were significantly, though weakly, correlated with long-term temperatures. INTRODUCTION Temporal and spatial distributions of soil microarthropods in desert ecosystems have been studied by several workers (Wood, 1971; Wallwork, 1972; Santos et al., 1978; Franco et al., 1979; Greenslade, 1981; Santos and Whitford, 1983; Steinberger and Whitford, 1984; Kamill et al., 1985). Considerably less work has been done examining the distribution and activity patterns of epigeic microarthropods in arid environments. Greenslade and Greenslade (1973) reported that some epigeic Collembola (e.g, Entomob7ya spp.) in semiarid Australia were active on sand dunes throughout the hottest part of the year. Comparing the taxonomic similarities of, and adaptations by, collembolan faunas in arid areas of Australia and Africa, Greenslade (1981) found several genera which were resistant to desiccation and concluded that populations of many species are underestimated in arid or semiarid areas. Pefaur (1981) showed that the abundance of epigeic microarthropods varied along an altitudinal gradient (300-1000 m) in a Peruvian desert, though there was little correlation between elevation and abundance of each species. Pefaur (1981) hypothesized that the abundance of epigeic species populations should increase in response to the increase of surface litter, which would provide greater shade and humidity. Except for Hypogastruridae (= Poduridae), he collected most Collembola from moist areas. In the Chihuahuan Desert of southern New Mexico, differences in soil type and drainage patterns produce distinct plant communities. Some of these produce more organic litter than others. Our objectives in this study were to examine the composition of epigeic collembolan communities associated with different plant communities along a watershed and to determine the seasonal activity of epigeic Collembola in this system. MATERIALS AND METHODS The study was conducted at the Jornada Long Term Ecological Research Site (LTER) located 40 km NNE of Las Cruces, New Mexico. The LTER area encompasses a watershed extending from the base of Mt. Summerford down a drainage slope (bajada) to a temporary lake basin (playa) 2850 m distant. Soils vary from coarse gravels on the bajada to sandy loam on the playa edge and clay silt on the playa basin. Pitfall traps were placed in six plant communities distributed along the watershed. From the bottom to the top of the drainage slope, these were: 1) playa, dominated by