Every animal has there own unique way of adapting to the environment, when this happens their off-springs learn how to adapt and so o and so froth,this is natural selcetion.
Along with adaptions animals also have its niche which is what its purpose in the environment is, what it eats, and what eats it.
Some organism's eat the same species and its hard fro them to get their requirements with the competition but mostly that does not happen because the two animals don't have the save niche for example the roadrunner and the elf owl. The elf owl hunts at night and the road runner hunts at day.
Whent 2 organism's do have the same niche one of them will die off becuase of lack of resources.
Parasites (if you din't know) are living organism's that live inside animals or pretty much everything when theirs a cut and they are able to get inside. This affects the animal/person or as we call the host in different ways but etheir way parisites arw Bad