Threats to Biodiversity

Drive along the highway west of Sydney and notice the cleared paddocks stretching to the horizon, broken by only a few scraggly gum trees — that is habitat loss in real time, where dense forest once stood and koalas, gliders and echidnas once lived. When forests are cleared for farms, suburbs spread into bushland, or wetlands are drained, the animals that lived there have nowhere to go.

• Land clearing for cattle, crops or housing — Queensland and NSW have very high land-clearing rates.
• Habitat fragmentation — even when habitat is left, if it's chopped into tiny pieces by roads and farms, animals can't move between patches to find food or mates. A 100 hectare reserve is far worse than one 100 hectare reserve.
• Urban spread — Western Sydney's growth has cut into critically endangered Cumberland Plain Woodland (now less than 10% of its original area).

Koalas, gliders, gang-gang cockatoos and the iconic eastern quoll have all been hit hard by habitat loss in NSW.

A warming climate shifts where species can live and the conditions they need.

• Range shifts — species move toward the poles or higher up mountains to find cooler temperatures. Eventually they hit the coast or the top of the mountain and have nowhere to go.
• Coral bleaching — when ocean temperatures rise even slightly above the normal range, corals expel the algae living in them and turn white. The Great Barrier Reef has had multiple mass bleaching events since 2016.
• Bushfire intensity — Australia's Black Summer fires (2019–2020) burned over 24 million hectares and killed or displaced an estimated 3 billion animals.
• Mismatched timing — some flowers now bloom before their pollinators are awake, so pollination fails.

Climate change is special because it makes every other threat worse. Habitat loss + a hotter climate is far more deadly than either alone.

An invasive species is a non-native species that arrives, spreads and damages the local ecosystem. Australia, because of its long isolation, is unusually vulnerable.

Why is the damage so bad? Native species evolved over millions of years with predators they "know". An invasive species is something none of them have seen before — they don't know how to escape it, fight it, or avoid eating it.

Overexploitation means taking organisms from the wild faster than they can replace themselves.

• Overfishing — Australia's southern bluefin tuna nearly collapsed in the 1990s; now strictly quota-managed.
• Illegal wildlife trade — pangolins (worldwide), shingleback lizards (NSW/SA, sold as exotic pets in Asia), seahorses.
• Logging of old-growth forest — slow-growing trees can take centuries to replace.

Pollution adds harmful chemicals or rubbish that harm life directly.

• Plastic waste — sea turtles eat plastic bags mistaking them for jellyfish.
• Chemical runoff — fertiliser from sugar-cane farms washes onto the Great Barrier Reef, fuelling crown-of-thorns starfish outbreaks.
• Microplastics — now found in every ocean and in the bodies of most marine animals (and humans).
• Light pollution — disorients turtle hatchlings, which crawl toward streetlights instead of the sea.

Threats almost never act alone. A koala population stressed by habitat loss is even more vulnerable to drought (climate), to attacks by dogs (introduced predator), to bushfires and to chlamydia (disease). Multiple threats stack.

Since European settlement in 1788, Australia has lost roughly 30 mammal species — about 10% of our mammal fauna and more than any other country in modern times.

For most of these, the cause is a combination of invasive predators (especially cats and foxes) and habitat change. The Bramble Cay melomys, lost in 2016, is officially the first Australian mammal driven extinct by climate change — its tiny island home was inundated by rising seas.