Eva-Liis Tuvi, Ain Vellak, Ülle Reier, Robert Szava-Kovats, Meelis Pärte (2011) Establishment of protected areas in different ecoregions, ecosystems, and diversity hotspots under successive political systemsBiological Conservation 144 (5): 17261732. http://www.sciencedirect.com/science/article/pii/S0006320711001029