@Kio: Yes, it is preventable, but unfortunately we didn't prevent it

So yes, we could of prevented animals from coming into Australia, and yes we have extremely high customs laws now to prevent that sort of thing from happening. HOWEVER, unfortunately, that doesn't stop past mistakes and yes they were mistakes and yes they were preventable but we still did them because we didn't know at the time. So whether or not it's preventable now, the problem is that we didn't prevent it and we had cane toads everywhere. Yes, we did use poison and there was a massive controversy about it affecting our environment, I'm not exactly sure what happened tbh, I do know that our environment wasn't effected that much, if it all, so maybe they didn't use poison after all, I'll have to look it up.
2. Ok so I'm doing some research on this. So, first of all, I dunno exactly where you got the idea that the earth has been cooling over history. According to the data received from
the national climate data center, the earth is in fact warming, regardless of whether human intervention was involved, it is indeed warming in Land surface air temperature, sea surface temperature, Marine Air temperature and Tropospheric temperature, this is in the last 50 years anyway. While it is said that the earth does go in fluctuations of warm and cool, this unprecedented warming isn't in line with the natural cycle of constant warming a cooling, and that's because of an excessive amount of Carbon Dioxide in the recent years. According to
NASA's website our CO2 output presently is more than 400 parts per million, the highest point of CO2 emission in the last 650,000 years has been no more than 300 parts per million. Yes, climate change is natural, it happens a lot, but as I said what we're doing is [speeding up the inevitable], we're not doing anything that's not going to happen anyway, but we're making it happen faster. Just like how medicine delays death, I think we should try delay Global warming.
3. Immigration is heavily controlled in the United States, Australia and even probably Britian. Australia has a very multicultural society, but still many people who come to Australia have to go through a lengthy process in order to come here. A lot of people come here without going through the lengthy process, known as illegal immigrants, and they are more than often taken back or refused access. I'm not saying human migration is the exact same as animal migration, but what I am saying is there are some key differences, like animal migration, human migration is heavily controlled, in order to make sure that a national identity is still kept intact.