Rare species

A rare species is a group of organisms that are very uncommon, scarce, or infrequently encountered. This designation may be applied to either a plant or animal taxon, and is distinct from the term endangered or threatened. Designation of a rare species may be made by an official body, such as a national government, state, or province. The term more commonly appears without reference to specific criteria. The International Union for Conservation of Nature does not normally make such designations, but may use the term in scientific discussion.
Rarity rests on a specific species being represented by a small number of organisms worldwide, usually fewer than 10,000. However, a species having a very narrow endemic range or fragmented habitat also influences the concept. Almost 75% of known species can be classified as "rare".
Rare species are species with small populations. Many will move into the endangered or vulnerable category if the negative factors affecting them continue to operate. Well-known examples of rare species - because these are large terrestrial animals - include the Himalayan brown bear, Fennec fox, Wild Asiatic buffalo, or the Hornbill.
They are not endangered yet, but classified as "at risk", although the frontier between these categories is increasingly difficult to draw given the general paucity of data on rare species. This is especially the case in the ocean where many 'rare' species not seen for decades may well have gone extinct unnoticed, if they are not already on the verge of extinction like the Mexican Vaquita.
A species may be endangered or vulnerable, but not considered rare if it has a large, dispersed population. IUCN uses the term "rare" as a designation for species found in isolated geographical locations. Rare species are generally considered threatened because a small population size is less likely to recover from ecological disasters.
Rare plants can be classified based on the size and distribution of their populations. Some species may be rare because they consist of only a few individuals, are confined to a limited geographic area, or both. Certain rare plants are found sparsely distributed across a wide area. Others might have a large number of individuals that are concentrated in a very small area, such as a single county or canyon. The rarest plants typically have both a small number of individuals and a very limited geographic range.
Assessments of the status of rare plants are conducted using the best available data and consider various factors, including:

Total number of occurrences and the condition of these populations
Population size
Extent of range and area occupied
Trends in population size and distribution over the short and long term
Nature, severity, and urgency of threats
Number of occurrences that are protected and actively managed
Intrinsic vulnerability of the species
Specificity to environmental conditions

A rare plant's legal status can be observed through the USDA's Plants Database.

Critically Endangered, Possibly Extinct

Common name	Scientific name	Image	Last sighting	Estimated population of mature individuals	Endemic geographic location
La Vega Racer	Hypsirhynchus melanichnus		1910	0-25	Hispaniola
Hispaniolan two-lined skink	Mabuya hispaniolae		1937	0-50	Dominican Republic
Hispaniolan Ten-lined Skink	Spondylurus lineolatus		1985	0-50	Hispaniola
Gloomy tube-nosed bat	Murina tenebrosa			0-1	Tsushima Island & Yakushima, Japan
Emperor rat	Uromys imperator		or prior	0-3
Guadalcanal rat	Uromys porculus			0-3
One-striped opossum	Monodelphis unistriata			0-10
Baiji	Lipotes vexillifer			0-19
Glacous macaw	Anodorhynchus glaucus			0-20
Angel Island mouse	Peromyscus guardia			0-49
Aru flying fox	Pteropus aruensis			0-49	Trangan, Indonesia
Cuban ivory-billed woodpecker	Campephilus principalis bairdii			0-49	Cuba
Imperial woodpecker	Campephilus imperialis		1956	0-49
Emma's giant rat	Uromys emmae			0-49	Owi Island, Indonesia
Ethiopian amphibious rat	Nilopegamys plumbeus			0-49
Garrido's hutia	Capromys garridoi			0-49	Banco de los Jardins y Jardinillos, Canarreos Archipelago, Cuba
Zuniga's melanomys	Melanomys zunigae			0-49	Lima Department, Peru
Dwarf hutia	Mesocapromys nana			0-50	Zapata Swamp, Cuba
New Zealand greater short-tailed bat	Mystacina robusta			0-50
Puebla deer mouse	Peromyscus mekisturus			0-50
Kouprey	Bos sauveli			0-50
Montane monkey-faced bat	Pteralopex pulchra			0-50
San Felipe hutia	Mesocapromys sanfelipensis			0-99	Cayo de Juan Garcia & Cayos de San Felipe, Cuba
Malabar large-spotted civet	Viverra civettina			0-249

Critically Endangered / Extinct in the Wild

Common name	Scientific name	Image	Last sighting	International Union for Conservation of Nature Red List of Threatened Species category and criteria	Present estimated wildlife population	Present estimated captive population	Present estimated total population	Endemic geographic location
Northern white rhinoceros	Ceratotherium simum cottoni		N/A	Critically Endangered A2abcd; C1+2a; D	0		'
Barbary lion	Panthera leo leo		N/A	Extinct in the Wild	0	98	98	Belfast Zoo, Ireland Rabat Zoo, Morocco Keystone Safari, United States
Yangtze giant softshell turtle	Rafetus swinhoei		N/A	Critically Endangered A2acd; D			'
Ivory-billed woodpecker	Campephilus principalis principalis			Critically Endangered D	1-50	0	1-50
Fernandina Island Galápagos tortoise	Chelonoidis niger phantasticus		N/A	Critically Endangered D	0-5	1	1-6
Northern Sumatran rhinoceros	Dicerorhinus sumatrensis lasiotis			Critically Endangered A2abd; C1+2a	0-9	0	0-9	Tamanthi Wildlife Sanctuary, Myanmar
Vaquita	Phocoena sinus			Critically Endangered A2a; C1+2a; D; E	6-10	0	6-10
Baishanzu fir	Abies beshanzuensis		N/A	Critically Endangered D	3	1-9	4-12
Bornean rhinoceros	Dicerorhinus sumatrensis harrissoni		N/A	Critically Endangered A2cd+3cd+4cd; C2a; D	0-14	1	1-15	East Kalimantan, Indonesia Sarawak, Malaysia
Wood's cycad	Encephalartos woodii		N/A	Extinct in the Wild	0	1-36	1-36	Kirstenbosch National Botanical Garden, South Africa
Loneliest palm	Hyophorbe amaricaulis		N/A	Critically Endangered	???	1	1	Curepipe Botanic Gardens, Mauritius
Hainan black crested gibbon	Nomascus hainanus			Critically Endangered A2acd; B1ab; D	36-37	0	36-37
Gobi bear	Ursus arctos gobiensis			Critically Endangered	30-40	0	30-40
Māui dolphin	Cephalorhynchus hectori maui			Critically Endangered A2cd+4cd; C2a; D	48-64	0	48-64
Colossal squid	Mesonychoteuthis hamiltoni		2015	Least Concern	???	0	???
Indonesian Javan rhinoceros	Rhinoceros sondaicus sondaicus			Critically Endangered D	76-80	0	76-80
Western Sumatran rhinoceros	Dicerorhinus sumatrensis sumatrensis		N/A	Critically Endangered A2cd+3cd+4cd; C2a; D	30-80	7-8	37-88
Ornate Ground snake	Erythrolamprus ornatus			Critically Endangered D	18-100	0	18-100	, Saint Lucia
Saola	Pseudoryx nghetinhensis			Critically Endangered A2acd+3cd+4acd; C2a	20-100	0	20-100
Alagoas curassow	Mitu mitu		N/A	Extinct in the Wild	0	50-100	50-100
Horrid ground-weaver	Nothophantes horridus		N/A	Critically Endangered A3c	1-99	1-9	2-108	, Plymstock, Plymouth, United Kingdom of Great Britain and Northern Ireland , Cattedown, Plymouth, UK , Plymstock, Plymouth, UK
Cat Ba langur	Trachypithecus poliocephalus			Critically Endangered D	130-150	0	130-150
Siau scops owl	Otus siaoensis			Critically Endangered D	50-200	0	50-200	Siau Island, Indonesia
South China tiger	Panthera tigris amoyensis		N/A	Critically Endangered	0	150-200	150-200
Devils Hole pupfish	Cyprinodon diabolis		N/A	Critically Endangered B1ab+2ab; C2a	100-200	1-9	101-209	Devils Hole, Death Valley National Park, United States of America
Loa water frog	Telmatobius dankoi		N/A	Critically Endangered B1ab+2ab	1-9	214	215-223	Small streams along the Loa River southwest of Calama, Chile
Bleeding toad	Leptophryne cruentata			Critically Endangered C2a	1-249	0	1-249	Mount Gede Pangrango National Park, Indonesia Mount Halimun Salak National Park, ID
Black-naped pheasent pigeon	Otidiphaps insularis		September 2022	Critically Endangered	50-249	0	50-249	Fergusson Island, Papua New Guinea
Ganges shark	Glyphis gangeticus			Critically Endangered A2cd; C2a	1-249	0	1-249
Pink-headed duck	Rhodonessa caryophyllacea		June 1935	Critically Endangered	1-49	0	???
New Guinea big-eared bat	Pharotis imogene			Critically Endangered D	49-249	0	49-249
Wimmer's shrew	Crocidura wimmeri			Critically Endangered B1ab	5-250	0	5-250
Archer's lark	Heteromirafra archeri			Critically Endangered A3bc; C2a	50-250	0	50-250
Spix's Macaw	Cyanopsitta spixii		N/A	Extinct in the Wild	0	200-300	200-300
Kākāpō	Strigops habroptilus		N/A	Critically Endangered A2be	0	250-300	250-300	Anchor Island, New Zealand Codfish Island / Whenua Hou, NZ Little Barrier Island, NZ Maud Island, NZ Sanctuary Mountain Maungatautari, NZ
Gurney's pitta	Hydrornis gurneyi			Critically Endangered A3c	50-300	0-5	55-305
Cross River gorilla	Gorilla gorilla diehli		N/A	Critically Endangered C2a	250-300	1-9	251-309
Philippine eagle	Pithecophaga jefferyi		N/A	Critically Endangered A2cd; C2a	250-300	10-20	260-320
Amur leopard	Panthera pardus orientalis		N/A	Critically Endangered C2a; D	60-125	217	277-342
North Atlantic right whale	Eubalaena glacialis			Critically Endangered C2a	350-400	0	350-400
Black softshell turtle	Nilssonia nigricans		N/A	Critically Endangered A4cd	1-499	50-100	51-599	Brahmaputra River floodplain, Assam, India Kaziranga National Park, IN Nameri National Park, IN Temple ponds of the Nagsankar Mandir, Sootea, Tezpur, IN Temple ponds of the Shrine of Bayazid Bostami, Chittagong, Bangladesh
California condor	Gymnogyps californianus		N/A	Critically Endangered C2a; D	300-350	150-200	450-550
Cumberlandian combshell	Epioblasma brevidens		N/A	Critically Endangered A1ce	200–1,000	?	200-1,000
De Winton's golden mole	Cryptochloris wintoni		N/A	Critically Endangered B1ab+2ab	1-999	1-9	2-1,008
Lord Howe Island stick insect	Dryococelus australis		N/A	Critically Endangered D	1-49	500–1,000	501-1,049	Ball's Pyramid, Australia
Wild Bactrian camel	Camelus ferus		N/A	Critically Endangered A3de+4ade	1,000-1,500	100-200	1,100-1,700
Key tree-cactus	Pilosocereus robinii		N/A	Extinct in the Wild	0	200–2,000	200-2,000	Royal Botanic Gardens, Kew, United Kingdom of Great Britain and Northern Ireland Missouri Botanical Garden, United States of America San Diego Zoo Global's Botanic Garden, US
Central rock rat	Zyzomys pedunculatus		N/A	Critically Endangered A2abce	500–2,000	1-9	501-2,009
Eastern lowland gorilla	Gorilla beringei graueri		N/A	Critically Endangered A4bcd	3,800-5,000	1-19	3,801-5,019