h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

In their native habitat, mongooses may have natural predators to keep their populations in check. When you bring them to places like the Hawaiian or Caribbean islands (usually to keep the rat populations under control), you unleash something terrible. Human intervention is needed to keep mongooses from running rampant, and too little such intervention is currently happening.
http://www.tsusinvasives.org/home/database/herpestes-javanicus

But it's kind of reassuring that you would feature an animal that your readers don't have to worry about losing the battle for survival.

Well you have in the USA things like weasels and martens and skunks that fill the same ecological niche that mongooses do in Africa. Where they're an established part of the ecology, they cause no problems. All the birds are already adapted with various behaviours and tricks to avoid them, and yes, there are bigger predators who eat them and keep them under control. The problem is with those islands that have previously been free from similar predators and so the birds have evolved no defenses against them.

Very interesting painting and words....

Good explanation, Willem.

Thanks Paulh and Phred Firecloud!