ELI5: Why are there so many more poisonous/venomous animals in Australia than the rest of the world?

The answer lies in the energy-efficiency of envenomating and not engaging in a chase or a fight.

If you look a list of venomous animals in Australia, you will see that most of them are predators. Australian terrestrial venomous creatures are (almost) all unactive predators: snakes and spiders.

A common scenario for the use of venom, is the predator waiting in ambush. A fast moving mammal like a bush rat comes past. The predator will get one hit. Venom will decide if the predator will eat. It may not get another chance to strike at another animal for some time. Venom may decide it's survival.

Most of Australia is a pretty inhospitable desert with low productivity. Nutrients and food are hard to come by, so energy expenditure is at a premium. So whenever they do go hunting, they have to make it count - and what better way to guarantee a kill than by producing a highly-toxic venom?

Other places where similar venomous creatures abound share ecological features; being not only hot and dry places, but also old, glacially eroded lands that are low in the resources needed to support high energy animal activity.

Australia's snakes and spiders are thus highly specialised low energy attack weapons most suited to a desert environment.

I also wonder if the low basal metabolic rate of marsupials makes them harder to kill with venom than placentals, which could explain the power of the venom in Australia.

In Equatorial regions, toxicity is much more a defensive mechanism than a offensive one. For the obvious reason that by the time it takes effect, the foe may have prevailed, many venomous creatures have developed warning signals such as bright colours, a rattling or hissing sound or even bites that don't deliver venom.

Australia is also under populated for a continent and creatures that pose a lethal threat to man have not been destroyed as in densely populated continents. The most fatal spider in the world is not Australian nor is the most venomous snake.

/r/explainlikeimfive Thread