Because seasonal changes in vole density followed the curve for rate of growth of the complex of grasses on the Reservation, and because years in which there was a sparse growth of plants due to dry weather showed a decrease in the density of voles, the relationships between productivity of plants and vole population levels on the two study areas were investigated. In both fields the composition of the plant cover was similar, and the differences were chiefly quantitative. In June, 1951, ten square-meter quadrats were clipped on each of the areas to be studied. The clippings from each were dried in the sun and weighed. From Quarry Field the mean yield amounted to 1513 +- 302 lbs. per acre; while from House Field the yield was 2351 +- 190 lbs. per acre (Table 1). Using experience gained in making these samples, I periodically estimated the relative productivity of the two areas. House Field was from 1.5 to 3 times as productive as Quarry Field during the growing seasons of 1951 and 1952. Although House Field, being more productive, usually supported a larger population of voles than Quarry Field the reverse was true at the time of the clipping (Fig. 5). TABLE 1. RELATIONSHIP BETWEEN YIELD AND VARIOUS POPULATION DATA ====================================================================== House Field Quarry Field ---------------------------------------------------------------------- Yield in June, 1951, lbs./acre 2351 +- 190 1513 +- 302 _Microtus_, June, 1951, gms./acre 3867 5275 Per cent immature _Microtus_, June, 1951 29.85 38.02 Ratio _Microtus_, June/March 0.73 2.63 _Sigmodon_, June, 1951, gms./acre 1376 746 Per cent immature _Sigmodon_, June, 1951 35.72 44.44 Ratio _Sigmodon_, June/March 1.40 2.25 _Microtus-Sigmodon_, June, 1951, gms./acre 5243 6021 _Microtus_ mean, gms./acre/month 2922 1831 _Sigmodon_ mean, gms./acre/month 802 335 _Sigmodon-Microtus_, gms./acre/month 3728 2166 ---------------------------------------------------------------------- Although no explanation was discovered which accounted fully for the seeming aberration, two sets of observations were made that may bear on the problem. In June, 1951, the population of voles and cotton rats on Quarry Field was increasing rapidly whereas in House Field that trend was reversed. The trends were reflected by the percentages of immature individuals in the two populations and by the ratios of the June, 1951, densities to the March, 1951, densities (Table 1). Perhaps the density curve was determined in part by factors inherent in the population and, to that extent, was fluctuating independently of the environment (Errington, 1946:153).