In Western Polesie zoo-cenoses, about 1500 are invertebrate species and 346 are vertebrate species. Among the invertebrates identified, 50 species belong to a group of protected animals among which are to be found forms indicative mainly of peat-bogs systems, rushes, psammophylic systems, forests or even xerothem1ic systems, all of which are threatened with extinction in varying degrees. The Polesie region remains an area where new species of Polish fauna are described and the numbers of many unique species do not show any tendency to decrease. Of the national vertebrates, the best known are: reptiles – being 78% of the Polish fauna, amphibians – 72%, birds – 65%, mammals – 50% and fish – 43%. In each of these groups there are species at the utmost risk from the threat of extinction of which the most representative for the Polesie region are: the mud turtle, Phoxinus percnurus and the aquatic warbler (Acrocephalus paludicola). The numbers of most vertebrates are stable but in some cases are decreasing (mainly in peat ecosystems and agro-cenoses), while in some cases the trend towards an increase is observed. The largest expansion – and consequently the heaviest impact upon the environment is characterized by such animals as: the beaver, the otter and the fox. Their influence on the environment is considerable.