Mice were introduced to an island before and after food was supplied in excess of food consumed. The population of mice on the experimental island with food in excess is compared with populations of mice on the same island before the addition of food, on an adjacent island, and on the mainland. Before food was supplied in excess, death rate (death and dispersal) greatly exceeded birth rate and the population failed. After food was supplied in excess death rate of the second introduction decreased and population increased to greatly exceed the density of mice on the control island.The increased density of mice on the experimental island is the result of a decreased death rate, particularly in mice from birth to approximately 1 month of age. Increased birth rate may also be a factor. Food supply regulates the abundance of white-footed mice by affecting death rate and possibly birth rate. The most important effect of food supply is on the survival of young from birth to approximately 1 month of age.Evidence that intraspecific strife for space occurs in relatively dense population is presented. Intraspecific strife for space may regulate population of mice beyond the level of abundance determined by the supply of food.
This study was designed to test the hypothesis that the level of summer mortality in young blue grouse is determined by the condition of the hen, as a reflection of her summer range, and that this mortality, in turn, determines the level of subsequent autumn and spring densities.Early mortality was studied in a series of field and aviary comparisons of chicks on, or from, two areas of Vancouver Island that were in different stages of vegetative succession following logging and burning. No differences were found in the survival of young between areas, but differences were found between years. There were always sufficient young produced into autumn to replace annual losses in the breeding population.The major conclusions are: (1) early mortality varied between years but not between different habitat types or areas with different breeding densities, (2) variations in early mortality between years appeared to be a result of undetermined parental influences which acted before laying, and (3) this mortality did influence autumn numbers, but was not involved in the regulation of spring numbers.
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