An ecosystem is all the organisms living in an area with their non-living environment. There are a lot of organisms that live in an ecosystem. If you count all of the species you will have a community. If you chose one species and count all of that one species, you will find that species population. Population is the number of a species that live in an ecosystem.

Each population has a niche. A niche is how a organism lives. Be careful not to confuse niche with habitat. A habitat is where a organism lives. Populations have limiting factors. Limiting factors are some things that may restrict the number of individuals in an population. Some limiting factors are predators, lack of resources, and competition. Competition is when two of the same or different species fight over food, water, and living space.
 
The interaction between organisms is called symbiotic relationship or symbiosis. There are three types of symbiotic relationships. They are commensalism, parasitism, and mutualism. Commensalism is when one organism benefits and the other in not effected. Parasitism is were one organism benefits and the other is adversely affected. Mutualism occurs when both organisms benefits.