Changes in Norway Rat Populations Induced by Introduction of Rats

Abstract

The introduction of aliens into an existing population of mammals may be followed by unexpected effects that relate to social structure and population composition. These effects were studied by introducing alien rats into stationary and increasing populations of rats in city blocks. This work is part of a continuing study of the mechanisms of change in vertebrate populations using Norway rats (Rattus norvegicus) in residential areas in Baltimore as experimental animals (Davis, 1953). These rats inhabit back yards, basements, and garages and feed on garbage. The human sanitary conditions in general are poor and remain unchanged for months at a time, so that the food supply of the rats has only slight seasonal variations. Other environmental conditions are similarly subject to little change for many months at a time. The constancy of these factors permits experiments on populations in a relatively stable environment. Finally, the population of rats in each block is essentially discrete and isolated, as rats rarely travel from one block to another (see Davis, 1953, for references). METHODS AND PROCEDURES