Density-dependent regulation within a natural population of Sigmodon hispidus by artificial predation



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A natural population of cotton rats (Sigmodon hispidus) was investigated for changes in fecundity, survivorship, and migration rates due to changes in density. A 10.3 hectare field four miles north of Hitchcock, Galveston County, Texas, was divided into three areas. The population of cotton rats in two of these areas (area II and area III) was reduced over a period of seven months by removing a portion of the capture during each trap period. A third area (area I) served as a control, and a conventional mark and recapture excercise was conducted on the resident population to provide a base on which to compare the populations from the treated areas. The dissection of female cotton rats removed from the treated areas during each trap period revealed two distinct reproductive periods; the first occurred in early spring and terminated in early summer and the second began in late summer and terminated in the fall. The occurrence of lactating females from the control area coincided with lactation in females from the treated areas indicating that the pattern of reproduction was the same in all areas. The mean in-utero litter size in area II was 4.2 in the spring and 4.7 in the fall. The means for area III were 4.4 in the spring and 6.0 in the fall. The mean for area I in the fall was 4.3. The high mean of 6.0 from area III is attributed to the intensive removal of cotton rats from that area. Male cotton rats were discovered to immigrate to the treated areas at a significantly greater rate than were female cotton rats. One reason for this significant difference in migration rates may be due to the fact that male cotton rats range over greater areas than do female cotton rats. This was indicated in the present study by finding the mean distance between recaptures to be significantly greater for males. Migrants did not appear to belong to any specific weight class. Adult survivorship rates considered separately for males and females did not differ significantly between areas. The proportion of juveniles found in the treated areas was numerically greater than the proportion of juveniles found in the control area at the .10 level, possibly indicating an increased probability of survival for juveniles from areas where cotton rats were being removed.



Population biology