Potential Impact of Removal Trapping on the Delayed Outcome of Ongoing Monitoring. Small Mammals

Abundance assessment is one of the important components of ecosystem monitoring, carried out to track changes in the state of populations of individual species and community structure. The general requirement for monitoring studies of this kind is to obtain data with the possible minimization of impact on the observed object. The main methods for obtaining data on the population size of small mammals are traditionally snap-trapping and pitfall trapping (trenches/fences), i.e. removal trapping. By default, it is believed that removal has an insignificant effect on the state of their populations due to rapid recovery because of dispersal. In capture-mark-recapture (CMR), the captured animals that would have been removed during the snap- and pitfall trapping remain in place. The relationship between the number of new-born (yearlings) and the number of overwintered individuals that remained at the marking site from the previous year and moved in after the winter was assessed. Long-term CMR data for bank voles, common and Laxman’s shrews (10 years), and pygmy shrews (19 years) were analyzed. In the regression models, the total number of overwintered individuals, both survivors and moved in, was found to influence the abundance of young-of-the-year individuals in common shrews and bank voles. In all species, except for the Pygmy shrew, the number of individuals that overwintered at the marking site was found to influence the abundance of yearlings. The relation with the number of immigrated overwintered individuals was weaker or absent. In the Laxmann’s shrew, the number of overwintered individuals at the marking site is of decisive importance for the abundance of yearlings, which is completely unrelated to the number of overwintered individuals that moved in, and ultimately depends weakly on the total number of overwintered individuals. In the Pygmy shrew, the abundance of new arrivals depends only on the number of overwintered individuals that moved in after the winter. Thus, removal trapping can be expected to significantly affect species for which the contribution to reproduction of individuals marked in the previous year is significant and, taking into account species differences in the rate of the influence of local and alien overwintered animals, removal will distort the community structure determined in the local sample.