This Animal Lives Here and Absolutely Nowhere Else
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A species confined to one place and found nowhere else is the product of isolation, and the same isolation that produced it is what makes it so easy to lose.
What the term means
A species is described this way when its entire natural range lies within one defined area, whether an island, a mountain, a lake or a country. The term is relative to the area specified, so a species may be confined to one island and also to the country containing it, and a claim about it is meaningless without saying where. It says nothing about abundance, since such a species may be numerous within its range, and nothing about how long it has been there.
How isolation produces them
The mechanism requires a barrier and time:
- •A population is cut off from others of its kind
- •By sea, by mountains, by desert or by a change in climate
- •It accumulates differences that are not shared back
- •Selection pushes it towards local conditions
- •Eventually it can no longer interbreed with its relatives
- •Alternatively a once widespread species contracts to one refuge
Why islands are full of them
Oceanic islands carry a share of unique species enormously out of proportion to their area, and the reasons compound each other. Few lineages arrive, so competition is limited and arrivals diversify into roles occupied elsewhere by unrelated groups. Small populations change quickly. Predators are frequently absent, so defences are lost, which produces the flightless birds and fearless animals that island exploration repeatedly encountered. Distinct islands within a group isolate populations further, producing separate species on each, which is exactly the pattern Darwin was shown among the Galapagos finches.
The odd shapes isolation produces
Species confined to islands change in recognisable directions and the pattern is regular enough to have been named. Large animals arriving on an island tend to get smaller over generations, since food is limited and no large predators demand size, which produced dwarf elephants and dwarf hippos on several Mediterranean islands. Small animals tend to get larger for the mirror reasons, which produced giant rodents, giant insects and giant tortoises. Birds lose flight, since the cost of maintaining flight muscle is not repaid where nothing hunts them. All three tendencies reverse the moment a predator arrives.
Why they are lost first
Confinement to one place is the whole vulnerability. A single event affecting that place, whether a fire, a storm, a disease or a new predator, can remove the entire species with no population elsewhere to recolonise. Traits lost during isolation, particularly defences and flight, leave island species helpless against introduced rats, cats and goats. Their total numbers are frequently small to begin with. The great majority of documented extinctions since 1500 have been island species, and conservation planning concentrates on areas rich in them for exactly that reason.
The takeaway
A species whose entire range lies in one area is the product of isolation, whether by diverging in a cut-off population or by contracting to a last refuge. Islands hold a share far out of proportion to their size, because few lineages arrive, populations change fast and defences are lost. That same confinement means one event can remove the species entirely.