Ecology is the study of organisms interacting with one another and their surroundings.
The following are some key terms on ecology
1) habitat: the place where an organism lives. eg:coral reefs is a habitat of clown fish

http://en.wikipedia.org/wiki/Clownfish
2) population: A group of organisms of the same living in a habitat. Eg: the redshanks that lives in mangrove swamps consist of a population.
www.ecologyasia.com/html-loc/sungei-buloh.htm

3) Community: All the populations of organisms which live and interact with one another in a habitat.
4) Ecosystem: A community and its abiotic environment together makes up an ecosystem

www.scienceclarified.com/Di-El/Ecosystem.html
They are all related as populations of organisms live and interact with one another make up a community.
populations→communities→ecosystems.