Small reserves as targets for alien species invasions: the case of Mykhailivska Tsilyna (Ukraine)

The issue of vulnerability of small protected natural areas to invasion by alien species and their occupation of leading positions within the structure of biotic communities is examined. Given the popularity of micro-reserves and the actual dominance of small-sized protected objects within Ukraine’s system of protected areas, this problem is considered using the example of the Mikhailivska Tsilyna Nature Reserve (202.5 ha). The reserve was designed to represent northern variants of steppe communities but is surrounded by large areas of cultivated fields and a system of forest belts. Changes in the native mammal fauna within the reserve and the invasion of alien species since its establishment in 1928 were studied. At the time of the reserve’s creation, fauna reconstructions indicate the presence of 17 mammal species, of which only 1 species (domestic horse) was conditionally alien. In 1980–1982, 31 species were recorded, including 17 alien ones (55%), of which two were domestic species, one was a ‘true’ alien (brown rat, Rattus norvegicus), and 14 near invaders. As of 2024, 35 mammal species occurred in the Reserve, including 13 aboriginal species and 22 species that invaded it later, including two domestic species, one introduced species (raccoon dog, Nyctereutes procyonoides), and 18 (51%) near invaders. Thus, for over 96 years, 38 mammal species have been recorded in the Reserve, including three domestic species, three ‘true’ aliens (brown rat Rattus norvegicus, muskrat Ondatra zibethicus, and raccoon dog Nyctereutes procyonoides), and 15 near invaders (40% of all species). The appearance of alien species may be influenced not only by large-scale human activities (ploughing of surrounding lands, planting forest belts, and creating ponds) but also by minor actions (stocking ponds, repairing dams, and building sheds). However, the latter not only attract newcomers but also facilitate their establishment. The small size of the Reserve has allowed forest species and species with a wide range of habitats to establish throughout its territory. The process of new species appearing in the Reserve’s ecosystem due to near invaders continues to this day. Further overgrowth of steppe areas and fallows on the new territory added to the reserve five years ago with trees and shrubs increases the opportunities for new settlers.