Why didn't the brown Treesnake die off after it ate the birds and mammals on Guam?
While the birds and mammals were a major part of the snake's diet when it first entered the scene on Guam, these were not the major food sources of immature snakes. These snakes, as mentioned above, feed primarily on small lizards, which were and still are abundant—so abundant that they constitute enough mass to more than support the massive snake population. Also, because the brown Treesnake is a generalist predator, it was able to adapt to the changing prey base as birds and mammals declined, so that now adult snakes primarily consume lizards as well. The snake population (particularly the adults) does seem to be stressed, as suggested by low fat reserves in captured specimens, but this does not sufficiently limit the population. There is still enough food to support breeding, and every year new young snakes abound.