Ecology is the branch of biology that deals with the relations of organisms to one another and to their physical surroundings. Ecosystem is a biological community interacting organisms and their physical environment.

A Limiting factor is a something that keeps down a population of organisms. Here is an example: If there were the same amount of prey and predators the prey can't over populate because the predators keep the population under control.	spotted turtle Food Web
Not all relationships among organisms involve food. Many organisms live together and share resources in other ways. Any close relationship between species is called symbiosis. Here is an example of one: Mutualism is where one organism benefits the other organism, like the clown fish and sea anemone.	It's important to have many species instead of having a few. It is easier to hunt in packs because you can take down big game like moose and buffalo. It's safer to stay in groups because if a wolf ran into a pack of coyotes  the coyotes can scare it away.