The water bodies of the alas ecosystems of the Central Yakut Plain as the muskrat habitat are characterized. A typology of the lakes according to the origin of their hollows is provided. It is found that in years with sufficient humidification there is an increase in water abundance of the lakes as well as in the muskrat population density. This is accounted for by an improvement of food and protection conditions of this species habitat. The other factors have no substantial influence on the population dynamics of this rodent.Keywords: alas, thermokarst, hydroregime, habitat, muskrat, animal numbers.