Animals get their energy from food. Herbivores, like deer and hare, feed on plants. Carnivores, like lions and wolves, eat meat. Omnivores, which include pigs, bears, and humans, eat both plants and animals. In an ecosystem, all the organisms that depend on one another in order to eat form a food chain. Plants are at the bottom of this chain. They get their energy from the sun, which allows them to manufacture the substances they need for their development. Most animals depend directly or indirectly on plants. In this way, even carnivores that feed on herbivores depend on the plants that feed their prey.
Some organisms benefit from other species, without necessarily eating them. These special relationships have different names, depending on the type of association. Symbiosis is the association of two organisms of different species that mutually benefit from living together and cannot survive without each other. For example, coral is associated with algae, called zooxanthellae. It is a relationship that is vital to both. Mutualism is a relationship of mutual aid between two organisms of different species. In this way, the sea anemone and the clownfish protect each other, but their association is not vital. Commensalism is an association where one species benefits from another, without harming it or being beneficial to it. For example, the remora is a fish that attaches itself to another organism, such as a shark, and travels with it without disturbing it. Finally, parasitism is a harmful association, where one species lives off another, using that species’ resources for its own benefit. Certain flatworms, called tapeworms, parasitize the intestine of mammals.
Formed from the association of an alga and a fungus, lichens live in symbiosis. The alga manufactures the organic matter needed by both partners, while the fungus supplies them with water and mineral elements.
The dodder, a parasitic plant, has no leaves and is incapable of photosynthesis. Unlike other plants, it cannot use the sun’s energy in order to develop. It must live wound around the stalk of another plant, out of which it pumps organic matter using its suckers.
The remora, a fish of the tropical seas, attaches itself to the belly of a shark with an organ that acts like a suction cup. In this way, it travels long distances, benefiting from the protection of the shark and collecting its food scraps without harming it. This is called commensalism.