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The results of long-term stationary and field research (1958–2019) of the population ecology of the birch mouse related to its habitat on the northern periphery of the range are summarized. In the study region (Eastern Fennoscandia), this species is distributed only in its southern half, and is not numerous in most places. In the collections of small mammals, it accounts for 5.9 % (the sixth largest number). The average long-term accounting index was 0.02 ind. per 100 trap-days (the share in catches 0.07 %) and 0.72 per 10 ditch-days (1.3 %). It lives in different biotopes, but tends to deciduous stands, adhering to sparse areas with a rich grass layer and a sufficient number of shelters in the form of old stumps, piles of dead wood, fallen trunks, etc. Winter hibernation lasts from late September to early May. The activity is polyphasic, with a shift to daylight, typical for the northern limits of the range. The basis of the food of the birch mouse is forest insects (on average 94.3 % of the cases), arachnids (65.2 %) are in second place, and plant food (31.2 %) is in third place.

The article is based on long-term (1958–2017) expedition and stationary research in the Kivach, Kostomuksha and Lapland nature reserves and Karelian field stations – Priladozhsky, Kaskesnavoloksky and Gomselsky covering a vast part of taiga of the North-West of Russia. The article analyzes the population and ecological features of the equal -toothed shrew (Sorex isodon Turov) associated with its habitat near the North- Western borders of its range. It was found that in these conditions, the species was characterized by a generally low level of abundance compared to the optimal for the range (an average, 0.02 per 100 trap days and 0.1 per 10 trench days), distinct transition to stenotopy and obvious fixation on elements of the anthropogenic landscape. It is also characterized by the peculiarity of the territorial structure, the shift of reproduction to a later date and its general extension until mid –September. During the reproduction season, wintering adult females bring two litters: the first – in late May-early June, the second – in July. The litter size ranges from 2 to 11 per adult female (an average, 7.7 + 0.3). The share of mature and offspring-bearing profitable animals in general is small (up to 5-8 %) and varies by year depending on the state of the population and weather conditions.

As a result of long-term monitoring (1958–2017) of the status and dynamics of small mammals’ population in Eastern Fennoscandia we analyzed the general regularities of felling impact on the structure of habitats, ecological features, number and territorial distribution of 11 regional small mammal species. In the course of the fifty-year complex studies, a definite specificity of the trends in changing the number and dominance of small mammals after clear cutting of various types of native forests was revealed. In continuous open clearings a temporary increase in the number of rodents and insectivores and redistribution of species in favor of open stations representatives is observed. The forest clearings are very dynamic by their environmental conditions. Small mammal population structure is influenced by the age and type of felling, configuration and area of cutting, the species composition of the cut down stands. The clearings are characterized by a less stable population of animals compared with mature forests, although the average censuring rate for clearings is higher. It was found that concentrated felling leads to disruption of population reproduction. The total animals number decreases, and the population dynamics is characterized by short sharp rises and long deep depressions. Silvicultural operations at felling significantly affect the nature and speed of successional processes occurring in the biotopic groups of small mammals. Reforestation is characterized by a change of dominants in small rodents’ and insectivores’ communities. In general, the population waves of small mammals in the forest clearings and in pine forests are quite synchronous, although the years of definite peaks and depressions may not coincide. On the basis of modeling the processes of forest exploitation impact on small mammal populations the possibilities of long-term forecasting and prevention of negative anthropogenic impacts were determined.

Long-term research in Karelia (weight indicators of 1,080 animals were analyzed) detected the regular seasonal age-related changes in body weight of the common shrew. During the first months of life young animals reach about the half of the size of adults, then their weight growth stabilizes, and after that there is a distinct winter depression. In the spring, a "growth spurt" takes place, and in the autumn, after the end of the breeding season, a new weight loss and the completion of the life cycle are observed. These processes perform the important adaptive function and are aimed at providing optimal conditions for wintering and successful reproduction of the population.

According to the long-term (1965-2013) stationary and expedition studies in the Easten Fennoscandia, different species of shrews react differently to clear-cutting and the formation of transformed anthropogenic landscape. The dominant species (common shrew) increases in the number in these conditions, however, the population becomes unstable, and the number fluctuates severely from year to year and from season to season (Kurhinen et al., 2006), but the other species - pygmy shrew (Sorex minutus L.) and masked shrew (Sorex caecutiens Laxm.) – respond to these changes otherwise. The first one reduces in the number, especially in highly transformed habitats, but in general, its populations acquire the necessary stability and sustainability, while the masked shrew is affected by the massive lumbering negatively. Nevertheless, the latter is regularly found in the newly formed coniferous plantations emerging after the radical forest devastation.

The history and current state of the problem of biological diversity is discussed. The origin of the problem , its gnoseological, methodological and terminological basis as well as its significance and pjsition in the system of modern environmental knowledge are analysed. "The concept of biological diversity" adopted in 1992 in Rio de Janeiro is discussed from modern point of view,. Following A.M.Gilyarovym (1992, 1996, 2001), the author considers that the main reason for the confusion and contradictions of modern concepts of "biodiversity" lies in a typical mythological thinking characteristic for pseudo-scientific circles and the artificial boom around the term created on this base.

Microevolutional phenomena proceeding in the peripheral populations of polytypic animal species were analyzed. It was shown that the structural and population rearrangement of species population typical for the periphery of the natural habitat and the sharp and irregular number fluctuations contribute both to the enhancement of genetic turnover and to the increase of hereditary variability. Along with more strict selection, more frequent chromosomal recombination and the enhancement of the mutation and geneticoautomatic processes it activate the evolutionary changes leading to the gain of new territories by the species, the change of an ecological niche and the formation of new populations and even species. Peripheral populations are the most important outposts of the species evolution. It is here that the major evolutionary events unfold, resulting in the adaptive morphogenesis and opening the way for further moving of species.