Inter–trap movement and estimating rodent densities



The area and inter–trap distance of small mammal live trapping grids can seriously influence observed movement patterns and estimates of animal densities. This paper examines inter–trap movement (ITM) according to inter–trap distance, time of the year, weather and numbers captured for the wood mouse, Apodemus sylvaticus, and the bank vole, Clethrionomys glareolus, in an oak wood in southern England. The analysis is based on live trapping data collected in three studies over an eight–year period. Trap spacing affected inter–trap movement, including the statistic, ‘Mean Distance Moved’ between successive captures (MDM), of different species and sexes of rodents in different and predictable ways. No association was found in MDM between males and females of the same species, MDM and time of the year or MDM and weather. However, there was a weak negative association between MDM and numbers captured, particularly in mice. Using an independent method of assessing effective grid area, the usefulness of the MDM in correcting grid areas for the estimation of animal densities is discussed.