Interactions Among Organisms

Plants and animals living together in an environment are sometimes said to make up a natural community. Often there exists a balance among the living things in a community. Some animals depend on certain plants for food. If there are too many animals, most of these plants will be eaten. The food supply eventually runs short and fewer animals will survive.

Animals depend on their environment for the things they need to live. Most plants depend on animals for their propagation. Some interactions between organisms of certain plant and animal species are very specific. A close interaction between two species over time is called symbiosis. Symbiosis means “living together”. There are several kinds of symbiosis.

The relationship between a bee and a flower is an example of a symbiotic relationship that benefits both organisms. As the bee sips nectar, it transfers pollen from flower to flower. Thus the bee obtains food and the flower is able to reproduce. This symbiotic relationship is called mutualism.

A second type of symbiosis occurs between a shark and a small fish called remora. The remora attaches itself to the shark’s body, gets a free ride, and feed on scraps from the shark’s food. The shark is not affected by the remora. The relationship between the shark and remora is an example of a symbiotic relationship called commensalism in which one organism benefits while the other is not affected.

Another kind of symbiosis is parasitism. An example is when parasitic bacteria feed on their host and cause diseases. A tapeworm living in a mammal’s digestive system weakens its host by absorbing some of the host’s food. So parasitism is a symbiotic relationship that benefits one organism and harms the other.

Predation is another kind of relationship among organism. A spider eating an ant, a frog eating a mosquito, and a hawk eating a rabbit are examples of predation. The spider, frog, and hawk are predators. The ant, mosquito, and rabbit are preys. Predation occurs when one organism feeds on another.

Some species of animals form societies to protect one another from predators. Food, space, and other factors, however, become limited as the animals increase in number. Therefore organisms complete with one another to get the things they need.

Competition between different species occurs when two kinds or organisms try to occupy the same niche.  Two species cannot fill exactly the same niche. Hence the species that is better adapted will succeed. The other species either adapts to a new niche or dies out.

Certain behaviors reduce competition. For example, there is ranking in a population of chimpanzees. The older animals occupy the top level; hence they have the first choice of food and nesting sites. Often only the top-ranking animals reproduce.

Organisms forming a small organizational group always form a higher grouping (population). A population is a relatively permanent association of organisms of the same kind. Individual organisms may multiply and die but collectively, the population persists.

In grassland, there are populations of ragweed, a population of earthworms in a small pot of soil, or a population of birds on trees. The geographical extent of a population many vary from a small space in a plot of soil to a vast space of forest land. Population density many also vary greatly. These variations affect the availability of food and may be another cause of competition among organisms of the same species. For example, bullfrogs feed on insects in a certain pond. But as the population of bullfrogs increases, individual bullfrogs compete for space and food. Thus competition exists among bullfrogs.