Beta diversity and similarity among cactus assemblages in the Chihuahuan Desert

We sampled the cactus species in 23 localities along a longitudinal transect in the southeastern segment of the Chihuahuan Desert. Using Wilson and Shmida's formula we calculated beta diversity values among contiguous and non-contiguous sites. As a complement, Jaccard's index of similarity was also calculated among all sites. A total of 61 species were found along the transect, although most tended to be infrequent, and only 27 surpassed the mean value of the frequencies (mean f=28.3%f=28.3%). In most cases, relatively high to moderate β   diversity values were obtained. The average value for all the site combinations was β=0.52β=0.52 (S.D.=±0.17, range=0.12–0.93). The highest similarity value was ISJ=0.79ISJ=0.79, corresponding to two non-contiguous localities. The high diversity of cactus species in the study area may be explained, in part, by a relatively high to moderate level of turnover among the different species assemblages. However, β diversity is mainly a product of the intermittent pattern of spatial distribution of the cactus species in this region, rather than a real species turnover.