Food Webs of Reptiles and Amphibians

Creatures in a natural community generally fall into categories that help define their roles in the ecosystem. Depending on what they eat or whether they are eaten, they may be predators or prey, and often are both. Herbivores, or animals that only eat plants, often are relegated to the role of prey. Carnivores, or meat eaters, are predators; omnivores eat both plants and living animals; and scavengers take advantage of decaying plants and animals. Many carnivores, omnivores, and scavengers, in turn, are preyed upon by other carnivores.

Although often unnoticed, herps play a major role as both prey and predators in aquatic and terrestrial ecosystems. In a freshwater pond, frogs and salamanders consume huge quantities of zooplankton, algae, and aquatic insects and are, themselves, eaten by large insects, turtles, fish, snakes, raccoons, and wading birds. On land, rat snakes and black racers are fearsome predators of small rodents in fields and around farms, while garter snakes and brown snakes eat a lot of worms and insects. These snakes, in turn, are preyed upon by hawks, raccoons, foxes, and domestic animals, such as dogs and cats.

Researchers often develop diagrams or models to help them explain the relationships among creatures in a community. A common example is a food chain, which provides a model of the feeding relationships among the many creatures living in an area. In this model, plants are at the beginning and predators are at the end. Indeed, a food chain is a useful way to think about predator-prey relationships and the transfer of food energy in a community. Plants, including microscopic algae, capture light energy from the sun and convert it into living material, or biomass. These plants then provide food and habitat for many smaller invertebrates, which are in turn fed upon by amphibians, reptiles, and other predators.

The concept of a food chain is a bit limited, however, in that it tends to simplify the rather complex associations among organisms. It also does not fully take into account the relationship of organisms to the flow of energy in an ecosystem.

Another way to think of these relationships is as a pyramid, where many small organisms at the bottom serve as food for fewer, and often larger, animals as you climb to the top. Studies of herps from a small South Carolina pond revealed amazingly that thousands of salamanders and frogs can live in the pond in a single year. These medium-sized animals are supported by huge quantities of tiny algae, plankton, and small insects, which form the base of the food pyramid. Higher up the pyramid in this ecosystem are perhaps hundreds of larger herps, fish, birds, and mammals that prey on the medium-sized salamanders and frogs.

Theoretically, the energy in the pond is transferred up the pyramid, from the smaller to larger organisms, in a stepwise manner.The idea of a pyramid sometimes falls short, however, when you consider that some of the largest animals on earth, including elephants, giraffes, moose, and the Galapagos tortoise, eat only plants.

In reality, the way in which food resources are distributed and consumed in an ecosystem is more complex than a food chain or a pyramid. It is better described as a food web, with multiple linkages among the different animals and plants. The feeding habits of painted turtles nicely illustrate why a food web is a useful depletion of predator-prey relationships in pond ecosystems.

Early in life, a painted turtle will spend much of its time as a carnivore, preying on amphibian eggs, tadpoles, snails, and aquatic insects. As they grow, painted turtles shift their diets to include large amounts of vegetation, in some cases becoming predominantly herbivores. Painted turtles also are known to feed on carrion, or dead animals. Thus, a painted turtle fulfills various roles in the food web, including predator, herbivore, and scavenger. The true nature of the complex food web is further revealed when you consider that painted turtles can be an important part of the diet of predators, such as skunks, wading birds, raccoons, foxes, and other herps.

View the lesson plan on how to discover a pond food web.

Excerpted from Hands-On Herpetology.

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