A visit to a pond is always good entertainment, with a cast of characters that reads like a fairy tale. Damselflies flit about on gossamer wings, giant water bugs lurk under the surface, dragonflies patrol the shoreline, and frogs perch on lily pads. Peering into the pond, one sees a drama unfold where the players face challenges of finding food, getting air, moving about, and evading predators. The many organisms in a pond ecosystem are dependent upon each other and the aquatic environment to meet their needs for survival.
Small and shallow, ponds are quiet bodies of fresh water, with water-loving plants growing along the shoreline and sometimes out into the middle. With little wave action, ponds often have muddy bottoms, as fine silt particles settle out of the still water. A pond offers many possible living arrangements. Insects like water striders occupy the surface of a pond where the water tension provides a kind of skin on which they can skate without falling through. Others, like mosquito larvae, hang beneath the surface and feed on particles they strain from the water. Larger fish and turtles live in the open water of a pond, as do numerous tiny floating plants and algae and animals like water fleas and copepods. Organic debris collects on the bottom of ponds, providing food for detritus-feeders and hiding places for crayfish and predatory insects.
Paralleling the shoreline of a pond is a region called the littoral zone where the water is shallow enough for the light to penetrate to the bottom and rooted aquatic plants can grow. Emergent plants like cattails, sedges, and beautiful blue flag irises grow closest to the shore, with leaves and flowers above the water, stems and roots underwater. In somewhat deeper water lies the floating-leaf zone. Here we find diminutive duckweed leaves with tiny, dangling roots that get nutrients directly from the water, and lily pads with long stems connecting to their roots in the pond bottom. Beyond this zone is the region of submergent plants, like elodea, with leaves that grow beneath the surface, catching sunlight that filters through the water.
In ponds, as in other ecosystems, plants are primary producers, capturing the sun’s energy to make sugars and starches for plant growth. Phytoplankton, microscopic floating organisms that also perform photosynthesis, are even more important as producers than the plants we can see and are so numerous they turn the water green. These are eaten by hordes of microscopic floating animals or zooplankton, which in turn are food for numerous insects and fish. At the top of the food chain, otters, kingfishers, and herons visit ponds to feed on fish, frogs, and crayfish.
The edges of a pond often yield the most exciting discoveries, especially for children who can easily reach into the shallow water with nets and sieves. Here in the littoral zone a great variety of small but fascinating creatures make their homes, some clinging to plant stems, some on the surface, some on the bottom, and some swimming freely. The plants and animals in a pond are connected in a complex food web in which the lines often blur between herbivore, carnivore, omnivore, and detritivore as many animals feed opportunistically on anything – plant or animal, alive or dead – smaller than themselves. All pond creatures must obtain food, hide from predators, and navigate through the water. Sitting quietly by a pond, you can watch these activities firsthand.
On the surface, gregarious whirligig beetles spin and twirl about like bumper cars, patrolling the pond in schools, though occasionally one may shoot off on its own, so fast it leaves a wake. Whirligigs have oval, stream-lined bodies covered with a waxy water-repellent coating. Their small antennae, positioned at water level, help them sense vibrations of struggling prey caught in the water’s surface film. Their eyes are divided into two parts, one for seeing above and the other below the water, helpful when looking out for food or predators. Whirligigs feed on live or dead insects or discarded exoskeletons, making them both predators and scavengers.
Beneath the surface, numerous insects may be found on submerged plants, many of these the larval forms of terrestrial insects. Most people are familiar with the slender damselflies that flit gracefully around the edges of ponds. Their immature form is less well-known as it is completely aquatic. Damselfly larvae are called nymphs because they have no pupal stage but change gradually into adults as they go through a series of molts. Their greenish color makes for good camouflage as they stalk their prey and hide from predators on the underwater stems of plants.
The muddy bottom of a pond is a good place to scout for dragonfly nymphs where they lie in ambush for tadpoles, minnows, and other insects. An extendible, scoop-like lower jaw with grabbing spines lets them nab unsuspecting prey. When ready to become adults, dragonfly and damselfly nymphs climb out of the water on the stems of emergent plants. Their skin splits open down the back and the adult emerges, dries its wings, and flies away to hunt flying insects and find a mate. It can take up to two years for damselfly nymphs and six years for dragonfly nymphs to mature, though their adult lives may last only a summer.
A number of pond inhabitants swim freely between the surface and bottom in the littoral zone. Tadpoles propel themselves with waving tails, while minnows use fins. Water boatmen row about like little boats with oar-like legs. Backswimmers have long paddles too, but they swim upside down! Mosquito pupae, aptly named “tumblers,” somersault crazily through the water when disturbed and then float like bubbles up to the surface. Caddisfly larvae carry their twiggy cases on their backs as they crawl along plant stems, while scuds, tiny shrimp-like crustaceans, motor along on their sides. With paddles, oars, tails, and fins, these creatures are well adapted for moving around in the water.
Animals need food, water, and shelter – all easily found in a pond – but they also need air. In large lakes waves mix air into the water, but in small ponds most of the dissolved oxygen comes from photosynthesis. During the daytime, plants and phytoplankton take in carbon dioxide and release oxygen, while animals take in oxygen and release carbon dioxide. The decay of organic material at the bottom of ponds releases carbon dioxide as well, and so these gases pass back and forth among the pond dwellers and their environment in a continual cycle.
Getting oxygen from the water presents a special challenge for pond creatures. Water scorpions and giant water bugs have breathing tubes like snorkels on the ends of their abdomens, which they poke up through the water surface. Diving beetles capture a bubble of air at the surface and carry it underwater on the tip of the abdomen for an air supply. Whirligig beetles hold air bubbles under their front wings, like scuba tanks. Gills are common among immature insects. Damselfly nymphs have three leaf-shaped gills on the ends of their abdomens, and mayflies have gills on their sides. Dragonfly nymphs pump water into their abdomens where internal gills extract the oxygen. Expelling the water causes them to shoot forward, a kind of jet propulsion unique in the insect world.
A visit to a pond stirs the imagination as we glimpse its many inhabitants and consider the complex connections between these organisms and their aquatic environment. A rich and diverse ecosystem, the pond is home to an enchanting array of living things.
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Holland, Mary. Naturally Curious. North Pomfret, Vermont: Trafalgar Square Books, 2010.
Reid, George K. and Herbert S. Zim. Pond Life. New York: Golden Press, 1987.
Voshell, J. Reese. A Guide to Common Freshwater Invertebrates of North America. Blacksburg, VA: McDonald and Woodward, 2002.