Interactive effects of shading and disturbance on plant invasion in an arid shrubland: assembly processes and CSR-strategies.
Disturbance by small mammals and shrub canopies are ecological factors typical of arid ecosystems that may influence plant invasion through environmental and community changes. Whereas disturbance beneath shrub canopies may promote invasion by removing dominant species, disturbance in open areas may hinder plant invasion by increasing environmental harshness. However, we are unaware of studies explicitly addressing the interactive effects of disturbance by mammals and shading by shrubs on community assembly processes to understand plant invasion. In an arid shrubland, disturbance and shading were caused by the fossorial rodent Spalacopus cyanus and the shrub Flourensia thurifera, respectively. We used functional dispersion data (trait convergence vs. divergence) and Grime's theory (competitive, stress-tolerant and ruderal strategies, CSR) to gain insights into the underlying assembly processes. We compared environmental conditions, richness and abundance of native and exotic species, as well as functional dispersion and prevalence of CSR-strategies across four microsites (60 × 60 cm plots): undisturbed/open, undisturbed/shaded, disturbed/open and disturbed/shaded. We tested for functional differences between native and exotic species. Species richness was similar among microsites for both native and exotic species. Shading ameliorated environmental conditions, promoted trait divergence and increased prevalence of C- and R-strategies. Disturbance increased the abundance of exotic species and removed dominant species in shaded microsites. Exotic and native species were functionally different: exotics showed shorter life span, lower height, thinner stems and smaller leaves than natives. Synthesis. Disturbance, by removing plant biomass, favoured exotic species in shaded microsites, where shrub canopy ameliorated environmental conditions and-by relaxing habitat filtering-promoted niche partitioning processes as well as C- and R-strategies. We illustrate the value of linking trait-based ecology approaches to micro-environmental conditions, as it may provide insights into the underlying community processes when studying plant invasions at the local and microsite scales. Disturbed microsites beneath shrub canopies could be 'hotspots' where to concentrate efforts to manage plant invasions in arid ecosystems.