A Preliminary Analysis of the Physical environment of the Norway Rat

It is now generally recognized that the physical environment plays an important role in limiting populations of animals, but an understanding of the extent of this role depends largely upon the recognition and measurement of the major environmental factors involved. Much work has been done on the qualitative aspect of the problem, particularly descriptions of factors considered important, but such items as food and space have all too long escaped the yardstick. The object of this paper is to present the results of a series of observations and experiments attempting to quantify the physical environment of a feral population of Norway rats (Rattus norvegic us) in Baltimore, Maryland. A more detailed investigation into the evaluation of domestic garbage as food for rats is being published separately (Schein and Orgain 1953). Information relating to the rat's environment was obtained by environmental surveys of experimental residential areas, in cooperation with the Rodent Control Division of the Baltimore City Health Department. The "experimental areas" have been under observation since 1944 and records of rat populations have been kept by the Division of Vertebrate Ecology of the Johns Hopkins University School of Hygiene and Public Health. This information made it possible to select blocks in which the rat population was relatively stationary. In order to measure the environmental factors in terms of numbers of rats, it was necessary to select only those blocks with rat populations close to the carrying capacity of the block. Hence, if a factor could be determined to be the limiting one for a particular population, a measurable reduction in this factor should result in a reduction in the rat population, which is also measurable. In this way it should be possible to determine the unit of factor corresponding to a unit of rat population. The unit of area selected for survey purposes was a city block bounded on all 4 sides by paved city streets, which Davis et al. (1948) report form fairly effective natural barriers to random rat movements. The Baltimore city block is characterized by 2or 3-story brick structures arranged in solid rows facing the streets, and having narrow back yards which extend to an alleyway which usually runs the length of the block. Solid wooden fences constructed along the property lines generally separate the yards in the rear. Portions of many yards are paved with concrete or brick, which, when defective or improperly constructed, form excellent harborage for rats. A survey form had been developed by A. H. Holloway (unpublished ms.) that contained all the principal environmental factors. These factors were further divided into sub-classifications to fit the environmental conditions encountered throughout the survey area. This form was later revised by Charles Snyder (unpublished ms.), who introduced weighted values for the various factors. The form has been further consolidated by the senior author, and the weighted values were altered to fit the particular environment under study. In addition, minor ' This investigation has been accepted in partial fulfillment of the requirements for the degree of Master of Science in Sanitary Engineering for the senior author. 2 Currently with the Maryland State Department of Health, Baltimore, Maryland. 3 Currently with the U. S. Department of Agriculture Livestock Experiment Station, Jeanerette, Louisiana.