system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:If the Australian fauna is so dangerous and poisonous, how come that harmless species of rabbits and toads managed to invade, thrive and wreck the ecosystem so easily?
gpt
This is a two part question. 

**First you need to understand Australia as an ecosystem.**

We are an island, and we are very far away from anything else. Our ecosystem is incredibly delicate, and very unique. Our native animals aren't found in neighbouring countries and our whole ecosystem hangs in a delicate balance as it's unlike anything else in the world. This is due to millions of years of being unreachable by the rest of the world. 

The fauna is "so dangerous and poisonous" because it's competing with itself in a harsh environment. The majority of Australia is harsh desolate desert, and a lot of the coastal area is bush, which will go up in flames every 12 or so months. As Australia is such a harsh place to live, the fauna has adapted and evolved over time. 

If you were stuck on a gladiator island of death for a few million years, you'd evolve to be a bit fiercer too. 

**Second, is the reason they're still a pest in spite of our native animals**

Cane toads (the "harmless species of toads" you referenced) were artificially introduced in an effort to reduce the number of bugs eating artificially introduced crops. 

In Australia, cane toads have no natural enemies. Their toxin can kill most native animals that normally eat frogs, as they are not one of the animals that has evolved to be part of this gladiator island of death over time. They therefore pose a risk to both native fauna and pets such as cats and dogs. Cane toads can use a wide variety of habitats and thrive in urban and disturbed areas. They have a voracious appetite and can eat a lot of different foods. They also breed quickly, allowing them to rapidly colonise and dominate an area.

These abilities give cane toads a competitive advantage over native species. Native frogs control insects that may otherwise become pests and are in turn food for many native animals. Populations of many native frogs have declined severely in recent times and some species are threatened with extinction. Spread of the cane toad may increase these threats.