Exotic species have had a tremendous effect on Australia as it is, in ecological terms, an island, not a continent, and islands experience an unparalleled rate of extinction. In the 1970s, the population the western brush wallaby began to decline as the population of the red fox dramatically increased. The red foxes particularly targeted the juvenile wallabies as soon as they left their mother's pouch. According to a survey taken in 1970 in the Jarrah Forests of the Darling Range, there were 10 individuals per 100 square kilometres; another survey was taken in 1990 and the population had declined to 1 per 100 square kilometres. The population of the western grey kangaroo, which only differs from the western brush wallaby in its large size, was also monitored during this time; the kangaroos' population remained constant during the 20-year period.