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The Ecosystem : An Inter-acting Community

Competition between organisms

Green plants cannot survive for very long without sunlight and they become unhealthy if they do not obtain minerals and water. When plants are growing close together, in a forest for example, the tallest plants will receive most sunlight. The smaller plants will receive less sunlight because the taller plants shade them from the sun's rays.

Meudon Forest, France © Shirley Burchill

Ecologists say that all of the plants in the forest are in competition with each other; they are competing for the sunlight. In a dense forest, many seedlings which germinated in the spring may not survive the winter because they did not receive enough sunlight to make and store food.

Plants do not only compete for sunlight. In desert ecosystems, where water is scarce, the cactus plants do not grow very close together. Their roots, however, radiate out from the cactus plant, just under the surface of the soil. The roots of a cactus need to absorb as much water as possible when it rains. In this ecosystem it is the roots of the cactus plants which compete for space below the ground.

Cacti in the Sonoran Desert, Arizona  © Shirley Burchill

To avoid competition with their offspring, plants use animals, wind and other mechanisms to disperse their seeds or spores away from the parent plants. Even so, plants normally produce a large number of seeds or spores to make certain that at least a few of them will germinate and grow into mature plants.

Seeds contain food stores. Chemicals such as starch and protein will provide the energy for the seed to germinate and produce green leaves. These green leaves will then make their own food by photosynthesizing.

Herbivores which eat the same part of a plant, seed-eating birds for example, are in competition with each other. If there are too many primary consumers they will eat too many plants. If the number of plants is reduced this will eventually mean that some animals will starve. To avoid competing with each other, herbivores specialize by eating different plants or different parts of the same plants.

An ecosystem depends on a balance, sometimes called the balance of nature, which only provides for a certain number of organisms to survive within it.

Although the number of animals decreases along the food chain, the size of the carnivores usually increases. Carnivores generally need to be larger and stronger than their prey if they are to catch and kill it. The alternative to increased size is co-operation with others. A pack of hyenas can attack and kill a large herbivore. A single hyena could not tackle such a large prey on its own.

It is interesting that herbivores come in all shapes and sizes, from the elephant to the aphid.

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