Bulgaria is inhabited by 20 amphibian species, which makes the amphibians the least diverse class of vertebrates in the country. They include seven species of newts and salamanders from a single family, Salamandridae, as well as 13 frog and toad species from five families—Bombinatoridae, Bufonidae, Hylidae, Pelobatidae and Ranidae. The most recently classified species are the northern crested newt, identified in 2005, and the Macedonian crested newt, identified in 2007. Bulgaria provides various habitats for amphibians. The country falls within six terrestrial ecoregions of the Palearctic realm. Bulgaria has varied topography. From north to south the main geomorphological regions are the Danubian Plain, the Balkan Mountains, the Sub-Balkan valleys, the Rila–Rhodope massif to the south-west, the Upper Thracian Plain and the Strandzha mountains to the south-east. The country has a dense river network but with the notable exception of the river Danube, they are mostly short and with low water flow. (Full list...)