Islands of forest.

Step 1 of 6

A forest cut up

A species is threatened when it is likely to vanish if nothing changes, and extinct when its last individual dies. The biggest threat to species is the loss of the places they live. All three orangutan species, the red great apes of Borneo and Sumatra, are critically endangered, mostly because lowland rainforest is being replaced by oil palm plantations.

The Scene holds 400 km² of rainforest. With Forest left at 100%, untouched, $200$⁠ orangutans live in it, about one for every 2 km². Now it is at 35%, mostly cleared: what remains is cut into patches, and scraps too small to feed even two do not count. Breaking habitat up like this is called fragmentation.

Step 2 of 6

A century on

To an ape that lives in the trees, the plantation between the patches is a desert. A youngster crossing it gets lost easily, and may be killed. So each patch becomes an island, and each group must survive on its own. Years is at Year 0.

Step 3 of 6

Why small groups fail

In the typical run the two small patches have emptied, though their forest still stands. The extinction vortex is a spiral in which a small group shrinks, grows more fragile, and shrinks again.

Step 4 of 6

A matter of chance

The Scene shows one possible century. Switch Luck to Unlucky and Lucky: the same $66$⁠ orangutans dwindle to $7$⁠ or keep $44$⁠. The model runs $200$⁠ times, each with its own chance births, deaths and bad years. The lower chart counts the runs with no orangutan left after a hundred years: that is the risk of extinction. Working it out for a real population is called population viability analysis.

Step 5 of 6

Corridors

A wildlife corridor is a strip of habitat that animals can travel along between patches. Here, strips of planted forest. Without them, in the typical run $23$⁠ of the $66$⁠ orangutans are left after a hundred years.

Step 6 of 6

How a corridor helps

Along the Kinabatangan River in Borneo, forest is being replanted along the banks to join patches, and orangutans cross tributaries on rope bridges. In Canada, bears and wolves cross a highway on bridges and through tunnels.