A model to optimize trap systems used for small mammal (Rodentia, Insectivora) density estimates

Abstract

The environmcnt found in the upper and lower Padane Plain and the adjoining hills isn't very homogeneous. In fact it is impossible to I'ind biotopes extended enough 10 satisfy the ne- cessary criteria for density cstimnte of small mammals bascd on the Kemoval rnethod. This limitation has been partially overcome by adopting a reduced grid. counting 39 traps whose spacing dcpends on the studied species. Aim of this work was to verify - and eventually measure - the efficiency of ;I sampling method ba- \ed on a -'reduced number of catch points. The efficiency of I8 trapping cycles. realized from I00 I tu 1993. was evaluated as percent bias. For each of the trapping cycles, 100 computer siinulatioris we- re pcrformed. so obtaining a Monte-Carlo estimate of bias in density values. Then later. the elliciency of different trap arrangements was examined by varying the critcriu. The nunihers of traps ranged from 9 to 49. with trap spacing varying from 5 to 15 m and a trapping period duration lrom 5 to 9 nights. In this way an optimal grid system was found both for dimensions and time duration. The simulation processes involved, as a whole. IS 11 different grid types. for 11347 virtual trapping cycles. Our re- sults indicate that density cstimates based on retluced grids are affected by an average - 16% bias, that is an underestimatc. and that an optimally cized grid iiiiist consist of 6x6 traps scltiarc, with about X.7 In spacing. and bc in operation for 7 nights. Key >twdT: Renioval method, Monte-Carlo sirnu1;rtions. Small maminals, Population density.