Various mammals influence their environment greatly through digging and deposition of excreta. These functions play an important role in soil structure formation, particularly by changing the hydrothermic condition of the soil, and by enrich it with organic matter. These changes increase the biodiversity of soil communities and increase the stability of forest ecosystems located in the steppe zone, even in unfavourable conditions of human pressure. The first effect is a rise in the quality and quantity of microorganisms (the most important part of ecosystems). Digging by mammals in different forest ecosystems can raise the diversity of microorganisms by 12.3-38.4%. Diversity of decomposers is particularly enhanced. At the same time, the abundance of the microflora rises by 5-248%. Deposition of excreta by mammals is even more effective: it can cause an increase of microbial diversity of 21.7-54.6%, and of microbial abundance of 12-839%. Diversity of grass vegetation is also improved. Where digging and deposition of excreta by rodents are combined, grass diversity rises by 29-125%. Digging by wild pigs increases grass diversity by 25-50%. Digging is especially effective in increasing the diversity of the soil mesofauna. Digging by wild pigs enriches the mesofauna diversity in different ecosystems by 33-87%; similar activity by moles increases diversity by 25-160%; that of mole-rats by 12-150%; that of mouse-like rodents by 14-100%. Deposition of excreta by elks increases diversity by 25-50%; that of rodents increases diversity by 33-85%. Fresh rodent excreta increase diversity by 52%; one year old excreta by 33%; one and a half years old by 85%. At the same time there is aconsiderable rise in the abundance and biomass of the mesofauna (by 50-400%). Species numbers of saprophages and predators rise by 32-100% and 12-27% respectively.