The structure of communities and ecological diversity represent the fundamental manifestations of species interaction. Diversity constitutes a peculiarity of the ecosystem itself, the main parameter of the evolution process being, at the same time, the factor that acts according to the principle of opposite connection. The importance of studying biodiversity resides in the possibility of clarifying the mechanisms of communities structure formation and ecosystems. Communities differ in number and species composition. According to the hypothesis of the edge effect there is a tendency of increase in species diversity and density of populations on the borders of communities. The theoretical maximum of diversity is recorded in places where biotopes are large enough and is a total length of their borders. The more heterogeneous the living conditions are within the limits of the biotopes or the linear parameters of the ecological niches, the greater the species number is in this biocenosis [2]. In the event of substantial deviations from the optimum conditions of existence within the limits of the biotope, the number of species is reduced and their number will increase in accordance with the rule of compensation. Thus, the number of individuals and the number of species are in an opposite relationship. Investigations were conducted in “Plaiul Fagului” scientific reserve in various types of biotopes with different degree of heterogeneity [1]. Determination of specific composition and abundance of species of terrestrial vertebrates was performed using standard methods of relative assessment of the number - trap-nights. The following parameters were recorded in the captured animals: species, sex, age, physiological state and reproductive performance. The index of Patton edge effect [3] was calculated according to the formula: EI = T + P / 2Aπ, where TP - total surface area plus the length of the internal linear boundary of this area, A - surface and π = 3.14. The diversity of communities was determined using the Simpson index. Of great importance for communities of mammals are the climatic conditions. In 2015 it was a deficit of rainfall and temperatures exceeded the annual average. The reserve “Plaiul Fagului” is situated in the geobotanical district of durmast, oak and beech forests, in the center of Moldova and falls in the leafy woods category [1]. The investigations were carried out in two sectors. The first sector, with an area of 8.97 km2, was located in the eastern part of the reservation, on a slope to the northeast, and the second - in the south, with an area of 9.42 km2, which included areas of forest, ecotone and meadow. Natural and anthropogenic conditions have influenced the differentiated formation and distribution of existing vegetation, which, in turn, determines a high variety of mammal species. The presence of some species of mammals was recorded, of which the following are characteristic: Talpa europaea L., Sorex araneus L. Dryomys nitedula Pallas, Apodemus agrarius Pallas, A. flavicollis Melchior, Clethrionomys glareolus Schreber, Sus scrofa L., Capreolus capreolus L. , Cervus elaphus L., Vulpes vulpes L., Mustela putorius L. Examples of rare species are: Lutra Lutra L., Felis silvestris Schreber, Martes martes L. Species of small mammals are in the depression phase of the herd. The coefficient of catching constituted 2% in the ecotone and 10% in the sector №1 forest. In sector №2 this coefficient constituted 3.3% and 6% respectively. The dominant species was A. flavicollis with 100% - in №1forest sector, the share of males being 100% and 66.6% respectively in №2 sector, where the females were dominant. Out of 21 species of the ord. Rodentia, depending on the habitat, it was established that the largest number of species (33.3%) was recorded in the meadow, and the lowest number (4.8%) – in the ecotone. Comparing the values of the edge effect index for both sectors, it has been established that the ecotone values are higher for the №2 sector (1.88) compared with the №1 sector (1.63). This is explained by a larger diversity of flora and the length of internal linear borders of the №2 sector. The relationship between the number of species and the habitat area offers the possibility of appreciating the specific diversity with the help of the index of specific diversity concentration. This index is maximal in the forest ecosystem (0.88) and is minimal in the meadow ecosystem (0.66) and has intermediate values in the ecotone (0.79). It was found a positive correlation between the diversity index and the index of aridity in the forest ecosystem (r = 0.98) and the ecotone (r = 0.97) and a negative correlation in the meadow ecosystem (r = - 0.96). This is explained by the impact of stronger droughts in the meadow. As a result of the catastrophic drought the diversity of small mammals community diminished, which was anyway minimal in spring (0.143) to autumn (0.083) in the meadow while in the ecotone and in the forest the growth was insignificant. Analyzing the structure of communities of small mammals in “Plaiul Fagului” scientific reserve in autumn, we determined that the dominant species in the ecotone is A.agrarius, while in the forest – it is C. glareolus, followed by A.flavicollis. It was determined that the carrying capacity of the habitat was maximal in the forest ecosystem (7.78) and minimal (3.53) in the meadow. The work was performed within the fundamental project 15.187.0211F.