An ecosystem is a community of organisms interacting with one another and with their physical surroundings. In a pond, algae, aquatic insects and fish are living parts. Light, temperature and the water itself are nonliving parts that help shape which organisms can survive. A pond can be studied as one ecosystem, but its boundaries are a choice made for the question at hand. Rain runs in, insects fly out, and birds can move between ponds. Those exchanges still matter even when a field sketch ends at the shoreline.

Many ecosystems start with producers that use sunlight to make sugars. In a pond, algae can feed small aquatic grazers, which in turn can be eaten by fish. Draw arrows from the food to the organism that eats it to show the direction of energy transfer. A food chain traces one route. A food web combines many routes, which is usually closer to what happens outdoors. A fish can eat several kinds of prey, and a single grazer may be eaten by several different predators.

When organisms die, decomposers such as fungi and bacteria break down material and help return nutrients to the surroundings. Those nutrients can be taken up again by living organisms. Energy moves through the system and is gradually dissipated as heat, while matter can be reused. To see where to look for these processes, compare leaf litter under a tree with the open ground nearby. Note moisture, shade and what you can see of leaves breaking down. The observations can suggest how local conditions differ, even though they do not measure the decomposition rate directly.