Institute of Plant and Animal Ecology, Ural Branchof Russian Academy of Science, 8 Marta ul. 202. Ekaterinburg 620219.Russia

Abstract

The number of residential animals in daily samples is diminishing regularly duringprolonged catching. It might diminish according to exponential law if: (1) the probability to becaptured for each animal living on the sampling territory does not alter in the course oftrapping; (2) all animals have the same possibility to be captured. The number of captured non-residenlal (migrating) animals stays the same during the whole period of trapping, thusincreasing the number of animal captured at the end of trapping vs. one expected accordingexponential law. It is not correct however to consider the presence of migrants to be the onlyreason of the deviation from exponential decrease, because it is observed also in residentalanimals. (1) Home ranges of some animals overlap the trap line only by their edge. Thepossibility to capture such animals is 2-3 times lower than animals with central position ofhome ranges on the trap line. (2) The removal of peculiar demografic groups is not equal.Adult females are captured with highest intensity; young animals, on the opposite, stayuncaptured longer. It may be registered as increasing of proportion of young and subadultusanimals per an adull female with each day of removal. (3) Reproduction is intensive insummer. Young animals (up to age of 20 days) does not visit traps. Some of them becomeactive in the middle and the end of trapping period when a great share of older animals(including their parents) is already captured. Because of these reasons disturbed exponentialremoval exists even in residential animals, and we can not differ the real proportion of migrantseven in cases when the disturbance of exponential decrease per se is proved properly.