Feral cats can jump as high as six feet. Juvenile mice can fit through a hole smaller than an aspirin tablet. Stoats move so quickly they can scale an electric fence between high voltage pulses and land on the other side unscathed. We have learned these things by trying to fence these animals out.
We are fencing them out to atone for our mistakes. Human beings have rearranged the matrix of life on Earth, sometimes on purpose and sometimes by accident. In the nineteenth century, the British wanted to blow bugles and hunt foxes in Australia just like they did on their Yorkshire estates, so they shipped some over. The foxes still live there in vast numbers, devouring numbats and wallabies.