Relation of Population Density to Sodium Availability and Sodium Selection by Microtine Rodents

Abstract

A COLLECTION of 55 published records of population densities of rodents of the genus Microtus reveals a strong correlation between levels reached at population peaks and local soil sodium-levels as reported in soil surveys or deduced from indirect evidence. In areas with low soil sodium the highest density reported was 230 animals per acre and populations exceeded 30 individuals per acre in only 12 (19 per cent) of 63 reports. In regions of intermediate soil sodium, Microtus populations ranged up to 400 with 16 of 22 (72 per cent) reports listing more than 30 per acre. In regions of high soil sodium there were 14 records in excess of 1,000 animals per acre and 30 of 33 (91 per cent) records were more than 30. Cyclic population fluctuations seem to occur in the animals regardless of sodium-levels, but the densities reached at population peaks are characteristically many times higher in regions of high soil sodium than in regions of low soil sodium. The correlation could be due to other ions associated with sodium in the soil. However, the possibility that it reflects a functional relationship between availability of sodium and population regulation suggested a series of laboratory experiments with meadow voles (Microtus pennsylvanicus) from a laboratory colony established with wild animals trapped at Madison, Wisconsin.