Area structure of the peripheral populations of larks in the zonal ecotone “forest - steppe”

The transition zone "Forest - Steppe" under conditions of the south of Eastern Siberia is the limit of spread of all forms of larks at species level or their subspecies. The only horned lark of the peripheral species takes almost the entire territory of the region, including isolated relict steppe of the Eastern Baikal area. The southern part of the Eastern Siberia includes the entire area of the field lark, but here, on the area periphery, there is one of its subspecies. The spread of other species is limited by the southernmost areas of Transbaikalia. Complex orography, especially the transmission and impact of air streams and some other phenomena is among the main factors that determine the uniqueness of the spatial spread of larks, as well as other animal species, in the south of the Eastern Siberia. They cause an extremely heterogeneous and mosaic complexes of abiotic and biotic conditions of their habitat. The analysis of the nature of larks spread within the studied ecotone area "Forest - Steppe" allows reveal three groups: 1) species inhabiting almost the entire ecotone territory in various steppe and other open biotopes (horned and field larks); 2) species, spread in the significant part of ecotone area, but inhabiting within steppe landscapes (Mongolian and small larks); 3) species coming by the area edge from the south to the ecotone zone, that inhabite intrazonal arid ecosystems, only within steppe landscapes (grey and saline larks). The representatives of the first group are spread relatively even around the territory and take majority of suitable habitats, but in comparison to the central parts of areas the mosaic picture of spread is due to the inhomogeneity of environmental conditions in the region. The species spread only within northern juts (tongues) of the zonal steppes in Mongolia without leaving its scope are referred to the second group. The third group of the species related to the intrazonal ecosystems of arid territories is at the area’s periphery, as well as in the depths of areas it is spread around the territory in mosaic order and enters Transbaikalia along its southernmost outskirts. A very important habitat condition for both species (gray and saline larks) is not only the nature of vegetation, but also orographic and edaphic factors In comparison with the central populations in the edge populations of the larks in the area the distinct changes in topical preference are not marked. The birds proved to be rather conservative to the environmental conditions. This is probably due to the old age of the representatives of this group.