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Two population census methods (relative indexes), food consumption and gnawing evidence on wooden stakes, were compared in a large 32 by 32-m outdoor enclosure of wild free-living Norway rats (Rattus norvegicus). Wooden survey-type stakes were driven partway into the ground with a portion left projecting above ground as a gnawing surface. Gnawing frequency was measured on a yes-or-no basis at each stake laid out in a 9 by 9 grid pattern. The food consumption census method employed paraffinized food blocks with consumption measured by weight. It was found that both census methods reflect population changes, and a relatively good correlation could be made in relation to the predetermined size of the rat population and subsequent changes. The correlation between the two census methods themselves was significant although not highly so (r = 0.453, P < 0.001), which is not entirely surprising since the two methods measured totally different factors. The data were analyzed using a three-dimensional frequency distribution method. To further assess the two methods, a nonparametric technique was also used to measure space utilization (area occupied by rats) in the outdoor enclosure. The space utilization pattern obtained by the gnawing index was substantially similar to the space utilization pattern indicated by the food consumption index.
Norway rats, Rattus norvegicus, rats, rodents, rice, census methods, gnawing behavior, food consumption, population monitoring
Assistant scientist, Eureka Laboratories, Inc., Sacramento, CA
Specialist in vertebrate ecology, Wildlife and Fisheries Biology, University of California, Davis, CA
Extension wildlife specialist (animal damage), Cooperative Extension, University of California, Davis, CA