Impact of two invasive succulents on native-seedling recruitment in Neotropical arid environments

Kalanchoe daigremontiana and Stapelia gigantea are two succulent invasive species with potential impacts on recruitment of native vegetation in a protected area of importance for conservation of arid environments in northern South America. We hypothesized that while K. daigremontiana has the potential for inhibiting recruitment of native plants through allelopathic effects, S. gigantea could facilitate recruitment of nurse-dependent native taxa. To explore these contrasting impacts, we designed a comparative study and a transplant experiment. Density of native seedlings and species richness were significantly lower in patches invaded by K. daigremontiana when compared to patches with native vegetation or invaded by S. gigantea, suggesting that native-seedling recruitment is negatively affected by K. daigremontiana and not affected or facilitated by S. gigantea. The experiment did not generate results concordant with those obtained in the comparative study. Seedlings' recruitment of two selected native species was facilitated by presence of nurse-plants (exotics and natives); however, the magnitude of this effect varied according to the type of nurse-plant, susceptibility to herbivory, and herbivory level. We identify K. daigremontiana as the invasive with top need for management actions, because it has the potential to alter composition and physiognomy of native-plant communities in tropical arid environments.