The World in a Pond

For our purposes, an ecosystem includes an energy source (the sun) and all the living (plants, animals, decomposers) and nonliving (air, soil, water, etc.) components which occupy an area and interact so the unit is self-perpetuating. An ecosystem might be a lake or stream and its watershed. An ecosystem is composed of a variety of habitats (areas which supply the survival needs of an organism, living thing). Following is a brief overview of some relationships which occur in an ecosystem.



All ecosystems must have a source of energy (usually the sun) because all organism functions such as growth and reproduction require energy. Energy moves through the ecosystem by a series of events that link organisms together.

Plants have a green chemical called chlorophyll. Plants use chlorophyll to capture energy from the sun (solar energy) to split carbon dioxide atoms and then combine the carbon atoms with oxygen and hydrogen (photosynthesis) to make sugars (food). Solar energy is transformed into chemical energy stored in the bonds that hold the atoms of the sugar molecules. Oxygen also is released. Plants are essential to all ecosystems because they produce oxygen and food needed by all other living things. Animals that eat plants (herbivores) use them to make animal parts or burn them to produce energy for their cell functions. Any compounds not used immediately are combined and stored as fats. Tissues of animals eaten by other animals (predators/ carnivores) are broken down and re-combined into new parts for that animal, and so on. Thus, all animals depend on plants for food.

Food Chains, Webs, and Pyramids

Energy flows from the sun (solar energy) to plants through photosynthesis (where the energy is transformed to chemical energy) through a series of animals being eaten by other animals. This is called a food chain. A simple food chain may start with microscopic green algae. A mayfly naiad (immature mayfly) might feed on these tiny green algae and in turn be eaten by a trout. The trout might ultimately become a meal for a great blue heron or a person.

Because organisms may have more than one food source, they are involved in a number of food chains. These networks of simple food chains overlap forming a food web. The diagram below of a food web in a pond shows how complex food chains can become. Arrows point from the food item to the animal(s) which eat it. People who eat fish are also part of this food web.

food webs


A food “pyramid” can be used to depict the progressively smaller numbers of organisms at successive levels. Plants are at the base of the pyramid with a few “top carnivores” at the peak. As energy moves from one trophic level to the next through organisms, energy is lost as heat to maintain cellular functions of each organism. The sloping sides of the food pyramid show less energy is available at each level. An ecosystem can support only so many organisms, as many as can be supported by the energy available. This is called the carrying capacity of the ecosystem.

Nutrient Cycling

All living organisms are made up of tiny units called cells and all require certain substances such a carbon, nitrogen, and oxygen to make cell parts. They also need oxygen for respiration. Like energy, these substances are passed through the food chain continuously by a process called nutrient cycling.

Carbon Cycle

The carbon cycle is the movement of carbon through the earth’s ecosystem.

Ecosystem Health

All components of an ecosystem (plants, animals, rocks, dirt, water, etc.) are connected to each other. If a species is removed from the ecosystem, it affects the animals that eat it as well as the plants or animals it eats. Biodiversity (the numbers of species of plants, animals, and microorganisms) is an indication of the health of ecosystems.


Activities listed below are from the Aquatic WILD guide and relevant to Iowa. Activities with supplemental information are linked below. Use the supplemental information in conjunction with the Aquatic WILD activity.

Demonstration Models including Enviroscape, Stream Table, and Groundwater Models may be available near you in Iowa. Check the My County Parks website for your nearest county conservation board naturalist to see if they have materials to borrow!