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Endemism is the restriction of a biological taxon's native distribution to a specified geographic region. A species can be endemic to one island, an archipelago or a continent; the region must be stated. Presence within a region is insufficient: the defining point is that its native range does not extend elsewhere at the stated scale.

The Seychelles warbler supplies a useful scale comparison. The research team's account identifies it as endemic to the Seychelles and describes conservation species translocations from the remnant Cousin Island population to other islands within the archipelago. Establishing populations on several Seychelles islands is compatible with endemism to the Seychelles; it is incompatible with describing its range as confined to Cousin Island alone.^[ref]

Charles Darwin used island distributions as evidence in biogeography. In the first edition of On the Origin of Species by Charles Darwin, he explained endemic forms as those found nowhere else and compared affinities of island organisms with nearby mainland organisms. His Galápagos and Cape Verde comparisons connected geographic barriers and colonization with evolutionary modification.^[ref]

Endemism is a pattern to explain, not a complete explanation of origin. A restricted range may reflect local speciation, limits to dispersal, or survival after extinction elsewhere. Nor does endemism by itself imply rarity or low local density: a common organism can be geographically restricted. This is the geographic use of the term; epidemiological endemicity concerns a different kind of persistence.

The Seychelles warbler, Acrocephalus sechellensis, is a small songbird of the Seychelles islands. Its natural geographic restriction is an example of endemism. Long-term field monitoring has made it a useful subject for studying cooperative breeding, in which additional individuals help care for young.^[ref]

In this species, older offspring can remain with their parents and help defend the breeding area or feed younger offspring. That observation raises a causal question: do helpers increase reproductive success, or do productive breeding areas simply produce and retain more helpers? Jan Komdeur's 1994 removal experiment addressed this by reducing groups and controlling comparisons for area quality, initial group size and breeder age.^[ref]

One additional caregiver improved parental reproductive success under the tested conditions. Two or more in medium-quality areas could instead reduce success relative to one. Competition over reproduction or resources were proposed explanations, rather than established universal causes. The experiment therefore shows why counting helpers alone cannot establish a steadily increasing benefit: the effect depends on ecological and social context.^[ref]