Individual ecosystems are always less stable and less tolerant of change than a regional set of ecosystems as a whole. Furthermore, species ecology is more steady and conservative than spatial distribution which, for many species, can change very rapidly under the influence of human activity. Conservation at the ecosystem level it thus crucial. Ecosystems unify living organisms and function as an integral evolving system. Populations develop and reproduce within an ecosystem, and these processes ensure the conservation of species diversity. Plant associations have been termed Y-diversity. This is the level where limiting factors first become apparent. At a regional level they become noticeable much later. At the ecosystem level, sets of species form the basis for biodiversity estimation. These sets, their structure and inter-specific relations are brought out by the Braun-Blancquet floristic classification. This classification reflects Y-diversity, and such species characters as ecological amplidude, degree of stability, and abundance. The name and rank of syntaxon are determined by its distinctive and typical species. A preliminary estimation of syntaxon diversity in Ukraine suggests that its plant communities belong to 95 orders of 54 classes. Each syntaxon has a specific place in the system of ecological co-ordinates: soil humidity, trophic conditions, salt conditions, acidity, amount of nitrogen, humus and carbonates, climate conditions etc. The relation of a species to ecological factors characterizes its niche. The niche helps determine the species' abundance and reproductive ability, factors important for conservation.