Minimising trapping effort without affecting population density estimations for small mammals

Abstract

Improving species community diversity studies needs population abundances to be calculated. Micromammal population densities are highly variable at small spatial scales. Mark-recapture methods based on grid trapping is the most reliable technique to study density in small rodents, albeit it is time-consuming because it necessitates increasing the number of spatial replicates. Here, we evaluated a live-trapping grid strategy to minimise field effort without decreasing the accuracy of small rodent population density estimations. We first computed spatially explicit estimates of population density using CMR histories from a large grid made by 100 traps set over 4 consecutive days and nights trapped twice per year between 2012 and 2015, and compared these estimates with those obtained from reduced session time and grid extent for two common rodent species: the wood mouse (Apodemus sylvaticus) and the bank vole (Myodes glareolus). We then compared density estimates to simpler catch-effort indexes of abundance for these rodents. Spatially explicit density estimates from capture-mark-recapture over 4 consecutive days from grids set with a single trap interspaced 5 m on a 10 × 10 square were highly correlated (R² = 0.945) with density estimates after 4 consecutive days on a 7 × 7 square arrangement. The best performance of catch-effort indexes relative to 4 days on 10 × 10 square grid spatial density estimates for these two rodents was found when using total trapping events recorded on 8 × 8 grids over 4 days or 9 × 9 over 3 days (R² = 0.947 and 0.945 respectively). Our results support the use of a reduced grid design over 4 days to obtain reliable spatially explicit density estimates. We also obtained a reduced duration of trapping at the expense of keeping larger grids when using catch-effort indexes.