Using presence signs to discriminate between similar species.

The Lusitanian and the Mediterranean pine voles (Microtus lusitanicus Gerbe, 1879 and Microtus duodecimcostatus de Selys-Longchamps, 1839) are fossorial sister species and have an allopatric pattern of distribution in Portugal, which includes a potential sympatry area in the centre of the country. The present study aimed to determine the validity of using presence signs in the field for discrimination of the two species in an area of sympatry (Northern Alentejo) and the characteristics that achieve the best classification accuracy. A total of 175 trapping plots were sampled across the study area. Prior to the set up of traps, ten presence signs were randomly selected for measurements of four variables: proportion of soil mounds, mean diameter of mounds, proportion of burrow openings and mean diameter of burrow openings. On the basis of a classification tree analysis, results showed that presence signs can be used to discriminate plots inhabited by one or the other species in the studied sympatry area. The characteristic that most accurately enables species identification is the proportion of burrow openings: for every ten presence signs found in a plot, if more than eight have an opening, then it is inhabited by M. lusitanicus (i.e. mostly burrow openings with few or no mounds present); if eight or fewer have an opening, M. duodecimcostatus is present (i.e. mostly mounds with few or no burrow openings).