Here's something not a lot of people know: Hirta, the largest island in the St Kilda archipelago, once had two unique species of mice and one of them is still thriving.
Charles Foster explains in an essay for Aeon:
Evolutionary innovation happens at the edge of genetic orthodoxy, at the edge of an established population, and typically at the edge of a landmass: hence the exuberant biological creativity seen on islands, where new challenges are faced and old inhibitions relaxed.
Take the St Kilda archipelago, for instance, in the heaving green sea off the outer isles of Scotland. It once housed a community of embattled farmers and seabird hunters. They were all evacuated in 1930, leaving behind two species of mice, both unique to the islands.
The St Kilda house mouse, whose life depended on its coalition with the humans, went extinct within a few years. But the St Kilda field mouse, uninhibited by house mice, cats and humans, blossomed and changed.
It doubled in size and became an enthusiastic flesh-eater, prowling the beaches and headlands for dead birds. Edges were fecund on St Kilda – at least for field mice. They always are. Indeed nothing else is.
I've also read that now St Kilda has a seasonal population of scientists, the field mice have began to fill the niche the house mice once occupied and moved into the scientists' living quarters.
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