We suggest the concept of ecogeographic aggregations (EGAs) of plant species, according to which populations (ecogeographic units, EGUs) of each species are combined into spatially close groups (aggregations) if they are adjacent to each other and live under similar conditions. The specification of EGA is based on the analysis of thematic geographic maps (of soils, topography, etc.). The delineated areas occupied by multispecies ecogeographic aggregations can be candidates for the creation of new protected natural areas. This idea is illustrated by the example of 38 rare plant species of the Republic of Mari El. It is shown that the populations of these species can be assigned to 11 ecogeographic aggregations, only some of which are included in the existing protected natural areas of the Republic.