Osipov V. V. RussiaPenzaNature Reserve «Privolzhskaya lesostep», 12a Okruzhnaya Str., Penza 440031, Russiaosipovv@mail.ru Author's articles ON THE CASTOR FIBER (CASTORIDAE, MAMMALIA) ACTIVITY INFLUENCE ON THE ECOSYSTEM BIODIVERSITY OF SMALL RIVERS IN THE FOREST-STEPPE ZONE