Rabbits introduce and australia over 100 years ago have become series pesta farmers populations increase so drastically that they just place many native species of herbivores say one possible reason why the rabbit population was able to displace the native species

The answer is there are few effective predators of rabbits.

When a new species is introduced into some area, it could very soon increase its population size if there are no natural predators. Predators maintain prey population on some optimal size and that way homeostasis in ecosystem is achieved. Without natural predators, rabbit population will increase its size. With more rabbits in the area, many native species will decrease their population size because they must compete with rabbit population for resources.