An ecosystem is defined by how all of the organisms interact with each other and the environment. The best way to explain this is first knowing the terms of mutualism, commenalism, and parasitism. Mutualism is where both organisms need each other to survive. Commenalism is where one organism will benefit, and the other will have no effect positive or negative. Parasitism is where one organism benefits and the other does not. There are important to understand the way that organisms will interact with each other.
In defining how individuals of species will interact this can be explained in many ways. They can be part of their species' population that is broken down into communities. Each population will be in its own niche in the ecosystem. The habitat will depend on what species you are looking at. The food web is the flow of energy throughout the ecosystem- it will show primary producers, primary consumers, secondary consumers, tertiary consumers, and etc.
I hope this helps. If you have any questions, please let me know and I will do my best to answer. :)
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