TRACKING AS AN ACTIVITY INDEX TO MEASURE GROSS CHANGES IN NORWAY RAT POPULATIONS

Abstract

The efficacy of methods to manage Norway rat (Rattus norvegicus) populations, particularly the use of rodenticides, has been assessed with indirect census methods that measure changes in levels of rodent activity (Kaukeinen 1979, 1984). These techniques are less labor intensive and cause less disruption to study populations than direct census methods such as mark-recapture (Taylor et al. 1981), but they only produce an index of rat activity. Reductions in such indices may not necessarily reflect actual reductions in the size of populations, except where they are close to 0% or 100%. A further disadvantage of indirect methods is that they do not provide estimates of population size. This can be overcome by calibrating the index in situations where estimates of actual population size can be obtained by other methods (Sarrazin and Bider 1973). An index thus calibrated would be particularly useful in studies of factors influencing the size of rat populations. However, as yet no one has attempted to calibrate such methods by examining the relationship between indirect census indices with known rat population size. Tracking techniques are well established as methods of detecting the presence and movements of rodents (Lord et al. 1970, Marten 1972). A rat population living in and around farm buildings often occupies a habitat that may be altered daily or remain unchanged for months, depending on the type of agriculture practiced. The tracking technique should withstand these site disturbances without substantial loss of data. Furthermore, a method that estimates population size must be capable of detecting most, if not all, animals present. If some rats remain undetected because they are inaccessible to investigators, then estimates will be too low. Other factors likely to produce errors include adverse weather conditions that may reduce activity, changes in the behavior of surviving animals following control measures, and differences between observers if an arbitrary system of recording activity is used. A weather-resistant tracking plate has been tested in the United Kingdom (Shepherd and Greaves 1984). We examined the relationship between rat sign left on these tracking plates and rat population size before and after population reduction by trapping. We calibrate the use of such plates in terms of measuring both the efficacy of management strategies and actual population size for rats living on farms in southern England.