0. Extinct or probably extinct
Species whose wild populations have disappeared or probably disappeared from Estonia. Despite repeated searches they have not been encountered after 1950 (in case of water habitats after 1965), but their occurrence in 1850 1949 (1850 1964) has been reliably documented.
Species under strong threat of becoming extinct, whose numbers have declined to a critical level or whose habitats have been so drastically decreased that their survival in Estonia is unlikely if the influence of threat factors continues.
Species whose populations are declining due to overexploitation, or due to destruction or damaging of habitats, and whose numbers and distribution decrease quickly and which are likely to fall into the category Endangered (1) in the near future if the influence of threat continues.
Species which occur in Estonia within restricted areas or in few habitats, or are very sparsely, and which do not belong to the categories Endangered (1) or Vulnerable (2), but whose populations are easily threatened.
4. Care demanding
Species which do not belong to the categories Endangered (1), Vulnerable (2), or Rare (3), but whose status requires attention. Still relatively common species, but their numbers have decreased so drastically that they may fall into the category Vulnerable (2) if the influence of threat factors continues. Also species which belonged earlier to the previous categories but are now out of danger.
Species known to be Extinct (0), Endangered (1), Vulnerable (2) or Rare (3), but whose degree of being endangered cannot be specified due to insufficient data.