system
Given the following post, respond as if you're commenting on Reddit:
human
Why were rabbits so invasive in Australia?

The general answer from wikipedia seems to be a lack of predators and a lack of parasites in Australia?   Is that the case, or is there something unique about rabbits?  Is there an animal that was outcompeted by the rabbit as well?
gpt
Ooh, nice question. It's the result of a long list of factors.

Due to Australia's highly variable weather patterns, the majority of the native fauna exists in boom-bust cycles. Animals will just barely hang on during multiple years of drought, then explode with the rains, unlike European fauna, which is used to breeding regularly every year (or sooner).

Australia had a number of large marsupial predators, but these became extinct with indigenous settlement. By the time that Europeans colonised Australia, the only predators above rabbit-size were the thylacine, the Tasmanian devil, a few large birds of prey, crocodiles, dingoes, and some pythons and monitor lizards, none of which are as rapacious as mammalian predators such as dogs and cats and foxes.

European settlement drastically modified the Australian environment. They dredged nearly all of the watercourses and swamps, logged forests and scrub, and demolished mallee habitat. All of this increased the extent of open grasslands. This was further compounded by the stopping of traditional burning practices, which increased the amount of fuel, which meant that fires spread further and burnt more.

So when European settlers introduced rabbits, their spread was virtually unchecked. Nothing could eat them quicker than they could reproduce. They were well adapted to thrive in grasslands and cropland, unlike the majority of their marsupial competitors, which rely on native bulbs/seeds/leaves/fruits/insects (the exception to this rule is kangaroos, which have also become a pest in these conditions for the same reasons). Marsupials have smaller litters less frequently, but rabbits pump offspring out like it's going out of style. 

Just to fuck things up further, the spread of rabbits encouraged the spread of other introduced predators such as dogs, cats, and foxes. These predators are now spread nearly throughout the whole country just because rabbits are everywhere to feed them. However, they also eat native fauna whenever they can-- but while rabbits can shrug off the loss of a litter or two, native fauna are not so numerous, and this artificially inflated predation rate is wiping them out. Rabbits have essentially set predators on their marsupial competitors.

So, yeah. Australia is essentially a slow country full of slow creatures used to just holding on, and rabbits are fast creatures from a fast country used to constant resources. Settlers gave them those resources, and they responded accordingly.