Some floristic characteristics of the northern Chihuahuan Desert: a search for its northern boundary

An index of Chihuahuan floristic affinity was constructed to characterize the floristic variation over a 5° latitudinal gradient in the northern Chihuahuan Desert, with an emphasis on evaluating northern boundary conditions. The index was based on evaluating the ranges of 494 species from 590 vegetation plots from Bouteloua eriopoda and B. gracilis grasslands and Larrea tridentata scrub, distributed among four sites along the gradient. The most northern site, the Sevilleta National Wildlife Refuge (34°N latitude), while maintaining a complement of primarily southern distributed species, had the lowest index values and the fewest Chihuahuan endemics (3%) and Southwestern desert species (19%) in general. At the intermediate sites (Jornada/Tularosa basins and Otero Mesa), one and two degrees further south, index values increased conspicuously along with number of Chihuahuan endemics (7%) and the Southwestern desert species (37%). At 29°N latitude, the Sierra del Carmen Protected Area in northern Mexico was the furthest southern site. It represented typical hot and dry Chihuahuan Desert conditions where Chihuahuan endemics comprised 29% of species complement and Southwestern desert species made up 55%. With respect to floristic boundaries, the Sevilleta was clearly transitional to the southern Shortgrass Prairie and Intermountain (Great Basin-Colorado Plateau) provinces. While it supported several grassland and shrubland associations that maintained their overall Chihuahuan character, there were also several associations with primarily northern affinities, and also a set unique to the Sevilleta, reflecting its ecotonal nature. The results show that sound biosystematics, biogeographical analyses and indices like those presented here can provide an important context for questions in synecology, plant ecology, and conservation biology

Negative relationships between species richness and temporal variability are common but weak in natural systems

Effects of species diversity on population and community stability (or more precisely, the effects of species richness on temporal variability) have been studied for several decades, but there have been no large-scale tests in natural communities of predictions from theory. We used 91 data sets including plants, fish, small mammals, zooplankton, birds, and insects, to examine the relationship between species richness and temporal variability in populations and communities. Seventy-eight of 91 data sets showed a negative relationship between species richness and population variability; 46 of these relationships were statistically significant. Only five of the 13 positive richness-population variability relationships were statistically significant. Similarly, 51 of 91 data sets showed a negative relationship between species richness and community variability; of these, 26 were statistically significant. Seven of the 40 positive richness–community-variability relationships were statistically significant. We were able to test transferability (i.e., the predictive ability of models for sites that are spatially distinct from sites that were used to build the models) for 69 of 91 data sets; 35 and 31 data sets were transferable at the population and community levels, respectively. Only four were positive at the population level, and two at the community level. We conclude that there is compelling evidence of a negative relationship between species richness and temporal variability for about one-half of the ecological communities we examined. However, species richness explained relatively little of the variability in population or community abundances and resulted in small improvements in predictive ability