Relative influence of size, connectivity and disturbance history on plant species richness and assemblages in fragmented grasslands

Questions: What is the relative influence of size, connectivity and disturbance history on plant species richness and assemblages of fragmented grasslands? What is the contribution of small fragments to the conservation of native species pool of the region? Location: Tandilia's Range, Southern Pampa, Argentina. Methods: Cover of plants was registered within 24 fragments of tall-tussock grassland remnants within an agricultural landscape using modified Whittaker nested sampling. We analysed the influence of site variables related to disturbance history (canopy height, litter thickness) and fragment variables (size, connectivity) on species richness (asymptotic species richness, slope of the species–area curve) as well as on species assemblages by multiple regressions analysis and canonical correspondence analyses, respectively. Cumulative area was used for analysing whether small fragments or large fragments are more important to species diversity in the landscape. Results: Asymptotic species richness was significantly influenced by site variables, in particular by Paspalum quadrifarium's canopy height, but not by fragment variables. Species assemblages were also affected by site variables (12.2% of total variation), but no additional portion of the species assemblage variability was significantly explained by fragment size and connectivity. Sampling of several small fragments rendered more exotic and native species than sampling of few large fragments of the same total area. Conclusions: Our results agree with previous studies reporting low sensitivity of species diversity to size and isolation of grassland fragments in fragmented landscapes and high sensitivity of species diversity to local variables. The higher capture of regional native species pool by small grassland fragments than by few larger ones of equivalent accumulated area highlights the value of small fragments for conservation.