Does microhabitat segregation explain coexistence of globose cactus species?

Abstract

The factors determining cactus species richness and distribution at broad scales are well understood; however, the mechanisms explaining these parameters at local scales are poorly known, particularly in mesic regions. The aim of this study was to analyze if globose cactus species are segregated in terms of the microhabitats they occupy in Córdoba Mountains, central Argentina. In an area of about 40 ha, 19 rocky outcrops including all cactus species (a total of seven species) and covering the main vegetation and topographical conditions were selected. In each of the outcrops, 10 1x1 m randomly selected plots were established. The following variables were measured in each plot: cactus species presence; cover of graminoids, forbs, bare soil, and rock (%); rock size (small, medium and large); slope inclination (°) and aspect; soil depth (cm); and vegetation height (cm). A principal component analysis was performed with environmental variables. Then, segregation of the different cactus species along the three first axes of the PCA, which accounted for most of the environmental heterogeneity, was analyzed. Echinopsis aurea and Gymnocalycium mostii occurred at sites dominated by rock with low vegetation cover. Gymnocalycium bruchii, G. capillense and Parodia submammulosa were present at sites with low rock cover, deep soils and high vegetation cover. Out of the seven cactus species, Parodia submammulossa, Gymnocalycium bruchii and G. capillense did not differ significantly in microhabitat characteristics. Thus, these three species occupied sites with similar microenvironmental conditions, whereas the remaining species occupied sites with particular environmental characteristics. Four of the seven species inhabit unique environments (they do not overlap with the other species in the multivariate space). Moreover, these three species not only are present in similar environments, but also co-occur (they were found together in the same plots). Our results indicate that at the local scale, segregation in space is an important mechanism explaining plant coexistence and local species richness.