A population is all the individual organisms found in a given habitat, of one species. So you could talk about a population of wolves in the woods. If you want to talk about the wolves and rabbits in the woods, then you’d be referring to a community.
A community is made up of the various populations in a habitat. So the summation of all the living things in a given area is called a community. What then is an ecosystem?
An ecosystem comprises the community of living organisms in a habitat, together with all the non-living components such as water, soil, temperature, etc. called abiotic factors.
As such, the biotic component of the ecosystem consists of the ecological community.
Not all organisms feed the same. Some make their own food and energy, such as photosynthesising plants. These are autotrophs meaning they make their own nutrients, either through light energy or chemical energy.
Some bacteria living in extreme conditions such as deep sea vents rely on chemical reactions to derive energy. Although still autotrophic, these are chemoautotrophic while the photosynthesising organisms are photoautotrophic. Both are under the autotrophic umbrella.
Organisms that cannot make their own organic molecules using simpler molecules found in their environment, rely on feeding on those that can. These are heterotrophs and include humans. Since heterotrophs lack any ability to convert light energy or the electron donating potential from certain chemicals into reactions to produce the carbohydrates, lipids and proteins needed for life, they extract them from primary producers such as plants, or other heterotrophs at higher trophic levels.
Some heterotrophs secrete enzymes into the environment to break down waste materials from other organisms, then absorb the nutrients. This happens in soil bacteria and fungi and is termed saprotrophic nutrition.
A community includes organisms that are autotrophic and heterotrophic, interacting in intricate ways and depending on each other, as well as competing within the same species or niche, as the case may be.
Energy flow is exchanged between organisms in the form of food, from those that produce it to those that consume it. Nutrients as well as gases are exchanged, for example oxygen. Oxygen is a by-product of photosynthesis, and also serves the crucial function of aerobic respiration in many different species. This underlines the interdependence of species in a community, and indeed at the global scale.