Ecology can cover many aspects, you can look at individuals and species, but what happens when you’re looking at more than just one organism?
You can study populations, organisms of the same species living together. Up from this is community ecology, which is the interactions of different species in an environment, whether that is predation, co-operation or competition. Ecosystems include the community of an area along with all of its non-living aspects such as soil, materials, and the flow of energy. Bigger than ecosystems are biomes, such as grassland, they exist in different places, and differ from each other slightly, but bring about similar evolutionary changes. The largest form of ecology is to study the biosphere, which is essentially everything.
From cells to the biosphere ecology is essentially the ground rules for living on earth.
From Crash Course: Biology on YouTube presented by Hank Green.