[Animals] Koalas are both endangered and so plentiful they're causing problems. How'd that happen?

[BirSaNN]

They are a poster child for imminent extinction, at risk from deforestation, climate change and bushfires. Yet, where I live in South Australia, they are so abundant they are in danger of eating themselves out of house and home.

Koalas present something of a paradox.

They are a poster child for imminent extinction, at risk from deforestation, climate change and bushfires. Yet, where I live in South Australia, they are so abundant they are in danger of eating themselves out of house and home. How is it that koalas can be simultaneously declared endangered in some areas and yet require population management in others? Despite the complex factors influencing koala populations, the ultimate cause of both problems may well be the same — habitat loss and fragmentation.

Koalas are native to — and entirely dependent on — the Australian eucalypt forests that stretch down the eastern seaboard of Australia in a hook from the tropical forests in the north down and around the bottom south-eastern corner coast of the country. 

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These forests, and their koalas, once stretched all the way across the south to the far west coast. But as the climate began to dry around 100,000 years ago, the forests retreated towards the eastern edge of the continent. The giant megafaunal cousins of the koalas, like the diprotodons, disappeared in this period and the koalas too retreated from the west and inland areas, and from most of South Australia. Genetic markers reveal that, around 30,000 to 40,000 years ago, koalas suffered from a catastrophic population crash, coinciding with the megafaunal extinctions around the world.

This was not the only time koala populations have crashed, nor the only time they have recovered. Species with small ranges are often more at risk of extinction. Although koalas are a relatively widespread species, they are also extremely widely dispersed and rarely found in high densities. This might put isolated populations at risk of extinction. Park managers typically consider three to four koalas per hectare the maximum sustainable number without risking forest damage, even in the most productive forests. In the more inland arid forests, there may be as few as one koala found every 247 acres (100 hectares). 

link: https://www.livescience.com/animals/land-mammals/koalas-are-both-endangered-and-so-plentiful-theyre-causing-problems-howd-that-happen