Living in a hostile environment
An introduction of animal species such as, rats, dogs, insects or any other types of species, non-native to a particular environment can carry many diseases. Even mankind are carriers of diseases that will plague an environment bringing great harm to the surrounding area.
Many species cannot adapt to a hostile diseased infested living environment, some start simply disappearing an will go unnoticed an die off.
Populations of organisms can even become jeopardized in danger of extinction, due to changing environmental conditions or predation parameters.