The attention in the article is paid to the key species research importance. Their classification was discussed in our article published in “Studia Biologica” (Y. Tsaryk, O. Reshetylo, I. Tsaryk, Biol. Stud. 2019: 13(1); 161–168). The main emphasis was made on ecological key species, while other categories were analyzed as well (protective, social etc.). A row of potential key species in different ecosystems was proposed based on the original field research. So, such phytophagous insects like the alder leaf beetles Agelastica alni, Linaeidea aenea, and the mother of pearl moth Patania ruralis are proposed to be the key species in forest ecosystems. Altogether in terrestrial ecosystems such species as the common carder bee Bombus pascuorum and the buff-tailed bumblebee B. terrestris are tend to be the key species among the pollinators (excluding European honey bee Apis mellifera). Ants are important invertebrate key species as well: Formica – in forest ecosystems, Lasius, Myrmica and Tetramorium – shrub and meadow ones. Almost the same we can say about some species of amphibians (the common toad Bufo bufo, the common frog Rana temporaria, and the edible frog Pelophylax esculentus). Eurasian jay Garrulus glandarius, the great tit Parus major, and woodpeckers, the great spotted woodpecker Dendrocopos major in particular, are among the birds’ key species on our minds. European otter Lutra lutra is the possible key species in freshwater ecosystems. Zooplankton as a whole community might be considered a key “species” too. The great pond snail Lymnaea stagnalis is the key species among freshwater mollusks. The fish of Barbatula, Cobitis, Sabanejewia, Misgurnus genera, and Eudontomyzon representatives are the key species in streams and rivers. European beaver Castor fiber, European otter L. lutra, the white stork Ciconia ciconia, and reed warblers are thought to be the key species in semiaquatic ecosystems. The presented data about the key species need further research on their population and consortive organizations, as well as their functional role in the support of ecosystem stability under the environmental stochasticity, has to be clarified.