Half of 12 species of Rodents in Karelia is rather rare. Their part in catches is less than 0.5%. But our recent investigations have shown, that in some years and in several habitats their number reached the level of number of the main species.

In 1994 on dry meadows near v. Soanlachty the number of Microyus arvalis was 19.3% and their part in total catches was 97%. In 1975 in deciduous forests of the reserve "Kivatch" the number of Microtus oeconomus was 20-21%, the ratio of dominance was 75-100%. Apodemus argarius together with croplands are considered almost completely disappeared in Karelia. But in 1994 on the drained meadows of Lachdenpochja region its number was 10.7%, its part in total catches was 36%. The demands of Micromys minutus for a biotop are very high. But every autumn of 1994-1996 on fen and meadow landscapes of island of Kizhi (southern Zaonezhje) its number was 3.1%, the ratio of dominance was 77%.

On the territory of Karelia the number of these species is usually in the deep depression, because they live near the northern border of their area, which goes here. But seldom positive fluctuations of living conditions from time to time allow them to use their reproductive potential and to increase their population and their role in biocenoces very rapidly.