Well, Terra's my planet. In the first century pre-atomic, some cane farmers on a large island continent introduced a species of toad to try to control the sugar-beetles. That was bad-- the toads were poisonous and the local predators wouldn't eat them, and they turned on local, smaller amphibians and snakes because they were easier than beetles. But there were bullfrogs in the mix, and that was worse; a bullfrog will eat anything it can fit in its mouth. Within a few decades, a couple dozen species of native frog were in danger.

Similar thing happened on Shesha; the rats weren't introduced deliberately. They came in the cargo holds of ships like the props. They decimated the spidermoth population by destroying the nests of the birds the spidermoths preyed on and driving several insect species to the verge of extinction. The companies that had been farming mothsilk failed, catastrophically, and the rats are still there.