Here are some concepts that backyard naturalists need to know:
The word niche rhymes with "rich," but unlike material riches, all living things have their niches. A niche is the role and position of a species in nature. Someone once joked that an animal's niche is defined by what it eats, and what eats it. Usually niches are thought of more broadly, however. For example, some experts wouldn't be happy with a niche definition not defining the extremes of heat and cold, dryness and wetness, sun and shade, and other climatic factors the particular organism tolerates. They'd also want to know about the organism's reproductive cycle, and the season and time of day when it's active. In short, every aspect of a plant or animal's existence can determine its niche. Another way of looking at it is that a niche is basically an organism's "job" in nature. Actually, the concept of "niche" varies, depending on who you're talking to, and how technical you want to be. It's OK for average people to say "My niche in society is that of being an environmental activist," even though there's no mention of anyone eating anyone else, or of temperature extremes. In the average school context, usually it is OK to say that the niche of the fish swimming above is that of "a freshwater lake, where it mostly eats mosquito larvae, and serves as the main food source of fish larger than it, such as bass."
Habitats are the places plants and animals normally live. The Herring Gull's habitat is along coasts, especially in harbors and around garbage dumps, and also on lakes and rivers. The habitat of beech trees is moist, deciduous forest, and the above fish's habitat is its shallow, freshwater lake. Some important habitats are forests, grasslands, deserts, and marshes. Obviously, the concepts of "niche" and "habitat" sort of overlap, but with "niche" focusing more on the animal's "job," while "habitat" focuses more on what corner of nature the organism occupies.
An ecosystem is these things:
You can see from the above that the boundaries of an ecosystem are not precisely defined. Lakes and their shores constitute ecosystems, as does the entire Eastern Deciduous Forest, which mantles much of Eastern North America and contains many lakes. The earth's largest ecosystem is the outer shell of the whole earth with its oceans and atmosphere. This latter ecosystem is also known by the name of the biosphere.
Thus there are ecosystems within ecosystems within ecosystems...