Threatened species may be critically endangered, endangered, vulnerable. Rare and indeterminate. Rare species are with naturally small populations, either localised or thinly scattered, which are always at risk from pests / pathogens / predators or exotic species.
Great Indian bustard (Ardeotis nigriceps) is a rare species of birds found in parts of Gujarat and Rajasthan. Due to destruction of natural habitat and poaching, it has become a rare species.
Vulnerable species are those for which presently the population is sufficient but is undergoing depletion due to some factor or factors so that is facing risk of extinction in medium term future.