Reading some of the recent threads on zoology, the thought came to me: what are the rarest living vertebrate and tetrapod species, genera, families, orders, etc.? E.g., not just the rarest living species of bird or reptile, but the family among the vertebrates that is the closest to extinction, the rarest species of artiodactyl, the rarest family of rodents, etc.
I envision this, not as a single question that admits of a clear answer, but as a kind of catch-all for interesting facts related to the basic question which people have run into. I’m particularly interested in which higher-order taxa are “hanging on by a thread,” so to speak.
For birds, it’s probably the family Rhynochetidae. The single member, the Kagu of New Caledonia, is considered Endangered, with a world population of under 1000.
As far as the rarest species of bird, that’s a bit tougher, since when they get very rare it’s very hard to be sure just how many there are. However, rarest bird for which we may have a reasonable estimate of number is the Po’ouli (Melamprosops phaeosoma) of Hawaii, which seems to be down to three individuals.
This would also be the rarest genus of birds, since it’s monotypic.
The rarest species of snake is most likely the Antiguan Racer (Alsophis antigua), restricted to an island around 60mi[sup]2[/sup]. The rarest lizard species is likely the Giant Gomeran lizard (Gallotia gomerana), restricted to one of the small Canary Islands, numbering (accordong to best estimates) around 150 individuals. The entire order Rhynchocephalia, monogeneric and having two (possibly three) species, is restricted to a few islands of the southern coast of New Zealand.
For mammals, the rarest family is probably the Lipotidae, a monotypic family containing only the Whitefin Dolphin Lipotes vexillifer of the Yangtze River in China, which may be down to a few hundred individuals. (This is assuming that the Thylacine, family Thylacinidae, is extinct.)
The rarest order of amphibians is Gymnophiona, the Caecilians, numbering around 160 species. Many species are in competition for the title “rarest amphibian,” including the Majorcan Midwife toad (Alytes muletensis), Hamilton’s frog of New Zealand (Leiopelma hamiltoni) and the Axolotl of Central America (Ambystoma mexicanum).
The rarest Order of birds I suppose would have to be the Opisthocomiformes, containing only the Hoatzin (unless you include it in the Galliformes or Cuculiformes, as is sometimes done). However, the Hoatzin is widespread and pretty common.
Other small bird orders are the Loons (Gaviiformes) with 5 species, Flamingos (Phoenicopteriformes) with 5, and Mousebirds (Coliiformes) with 6. However, these all include at least some common and widespread species.
The order which contains coelacanths (Coelacanthiformes) contains only one genus (Latimeria), and it is thought that there are perhaps only about 300 individuals still around (as of 1995, anyway).
Although, I’m not sure that “rarest taxon” really has any meaning (above the species level, anyway), since there is no guidline that any given taxon must contain any specific number of lower taxa, and citing a list of species within a taxon doesn’t give any indication of the relative health of various species within that taxon. After all, there’s only one extant species in the genus Homo, for example, and it appears to be doing quite well for the moment.
You can get a general feel for the numbers of endangered, threatened, or otherwise vulnerable taxa in various groups here, though. But, again, those numbers tell us nothing of the relative health, or the total populations, of those groups as a whole.
The most likely candidates would be any “critical”, mono-specific groups.
My guess for the mammals would be the family Daubentoniidae, consisting of one species, the Aye-aye. Population estimates vary wildly, but I’ve seen estimates as low as 50 surviving individuals.
Here’s a list of the world’s rarest mammals. Looks like the Whitefin Dolphin is a contender for that, too. Other very rare species include the Vancouver Island Marmot (rarest rodent, order Rodentia), Seychelles Sheath-tailed Bat (rarest bat, order Chiroptera), and Javan Rhino (rarest even-toed ungulate, order Perissodactyla). The other species on the list will of course be the rarest species of their respective orders.
The most endangered Order of mammals is undoubtedly the elephants, Proboscidea, with three species, all of them declining in the wild. Although the order Tubilidentata, containing only the Aardvark, has only a single species, it is not in danger.
That figure is based on older estimates from the 1960s-1970s. It’s been found to be more widespread than previously thought, and more recent estimates give a range of 1,000-10,000.