Which Way Does a Pond's Energy Go?

During the brief Arctic summer, sunlight reaches shallow ponds almost around the clock. Some of these ponds have no fish, yet many tiny animals live in them. How does energy reach those animals and the birds that visit? A food-web diagram can help us follow the connections.

A sunny starting point. Algae are tiny living things that can use sunlight to make food. They are producers, meaning they make their own food instead of eating other organisms. The food stores energy that algae use to grow. When an animal eats algae, some of that stored energy moves into the animal.

In this example pond, water fleas eat algae. Despite their name, water fleas are tiny swimming animals, not the fleas found on furry pets. Some mosquito larvae also eat algae. Larvae are young insects that have not yet become adults. Diving beetle larvae eat water fleas and mosquito larvae. A visiting shorebird eats these beetle larvae and water fleas.

Reading the arrows. The tricky part is choosing which way an arrow should point. Does it point toward an animal's meal? Not in this diagram. Each arrow points from the food toward the eater. It shows the direction of energy transfer, or movement from one living thing to another.

Diagram: Three connected feeding paths. Read each row from left to right. Repeated names stand for the same group of living things, not a different group. Algae → water fleas → diving beetle larvae → shorebird Algae → mosquito larvae → diving beetle larvae → shorebird Algae → water fleas → shorebird

Caption: This simplified food web shows several routes that energy can take from algae to a shorebird in a fishless Arctic pond.

Look at the first and third rows together. The shorebird can receive energy by eating water fleas directly. It can also receive energy through an extra step, when a diving beetle larva eats water fleas and the bird eats that larva. The repeated names link the rows into a web instead of three separate, unrelated chains.

The diagram also helps us think about a problem. Suppose fewer water fleas are available. Diving beetle larvae still have another food source shown: mosquito larvae. However, the arrows do not tell us whether there are enough mosquito larvae to replace the missing food. This model, or simple representation, shows feeding connections, not the number of animals or how much each one eats.

What the picture leaves out. Sunlight supplies the starting energy, although it is not drawn in the diagram. Not every pond animal or feeding connection appears, either. The caption warns us that the web is simplified. Together, the words and arrows explain how several animals depend on food made by algae, while reminding us that a real pond has more connections than one small diagram can show.