An ecosystem is a balanced interaction of plants, animals and the environment in which they live. Look at how the alien pig is part of a chain of events that leads to disaster for native birds. Myrica faya, an alien plant, begins to grow in a native forest. Earthworms are attracted to the roots of the plant.
Pigs turn up the ground in search for the tasty earthworms.
The disturbed ground collects rainwater and puddles form. The puddles provide a comfortable wallow for the pigs. Culex quinquefasciatus, an alien mosquito, is attracted to the standing water in the puddles and wallows. The mosquitoes lay their eggs there and the population grows. Plasmodium relictum is an alien parasite found in many birds, though it has very little affect on introduced birds.