The result, repeated across millions of years and thousands of island populations, is the extraordinary diversity of island-specific forms that biologists have documented from the Galapagos to Hawaii to Madagascar.

The tepui summits function as sky islands.

Instead of being surrounded by water, they are surrounded by cliffs — barriers that are as effective as any ocean at preventing most terrestrial organisms from moving between the summit world and the lowland world below.

A beetle adapted to life on the Roraima summit plateau cannot simply walk down the cliff face to the lowland forest.

A small frog cannot jump off the rim and survive the fall.