Introduction to Ecology

Ecology, the study of the ways in which living organisms influence and are influenced by the environment. Environment, in this sense, includes not only other living things but also such nonliving, natural constituents as chemical elements, moisture, soil, light, and temperature, and such man-made technological constituents as buildings, highways, machines, fertilizers, and medicines. Ecology is a branch of the science of biology. Specialists in this field are called ecologists.

The word ecology is sometimes used to describe the balance of nature, or the natural ways in which stability is maintained in the interactions of living and nonliving things. For example, pollution is said to disturb the ecology of a stream, meaning that it changes the ways in which fish, insects, algae, and water interact.

One of the main ways in which ecology is applied is through the conservation of natural resources.

Scope and Principles

Ecology can be divided into two main interrelated fields: plant ecology and animal ecology. Human ecology is a part of animal ecology; it is also closely related to sociology because it involves the ability of human beings to adjust to each other culturally as well as biologically.

The term ecology was coined in 1869 by the German biologist Ernst Haeckel, but ecology did not grow as a science until after 1900. At first ecologists studied primarily the natural history of a single species of organism. Now ecologists emphasize the analysis of the patterns formed by the interactions of groups of organisms in a given area.

The type of environment in which an organism lives is called its habitat. A habitat is designated by describing such characteristics as temperature, altitude, amount of light, and geographical features. The relationship of an organism to the rest of its habitat—particularly in regard to its food and enemies—determines its niche, or place within the habitat. Although many different kinds of living things can share the same habitat, a niche is exclusive to each type of organism. Many times, the ecological niche is very particular to a species, lessening competition between two species.

Much ecological work is devoted to the study of communities. A community is the group of organisms that share a particular habitat. A community may be as small as the life in a pond or as large as the living elements of a forest. Many communities are well-defined. Some communities have blurred boundaries and cannot be distinguished from other communities easily.

Certain large communities are called biomes. A biome is distinguished by its types of species and by the temperature and humidity of the area in which it occurs. Biomes include the tropical rain forest, the temperate deciduous forest, the boreal forest, the freshwater marsh, the prairie, the tundra, and the desert. The ocean is sometimes considered as one biome, subdivided into the intertidal, littoral, and deep-sea zones (habitats of bottom-living organisms) and the pelagic zone (habitat of free-floating and free-swimming organisms).

Ecologists use methods and research published in many fields to find out more about the relationship between the organism and its environment. They use data from oceanography and geology to find out more about nonliving components of the system. They can also understand how a change in one nonliving component can affect the environment and thus the organism.

Many ecologists concentrate their studies on the ecosystem—the community together with its nonliving components. (The life zone of the earth, called the biosphere or ecosphere, is made up of all ecosystems.) Systems ecology deals with the complex physical and biological interactions between multiple organisms and their environment. Important to understanding the ecosystem is the study of populations. A population is defined as a geographically localized group of members of the same species. The maximum size of the population that the environment can support is known as the carrying capacity. In practice, populations never reach the carrying capacity due to disadvantageous factors such as predators and diseases. Knowing why a given population increases or decreases in size or remains relatively stable yields information about the effects on the ecosystem of such factors as climate, availability of food, disease, and pollution.

No ecosystem remains static. However, under normal conditions all parts of an ecosystem act and react upon each other through gradual changes until they reach a relative stability. This process is called ecological succession.The stages of succession (called seral stages) progress from a pioneer stagethrough several intermediate stagesto a climax stage that does not change much over centuries.The entire sequence of seral stages for a given region is called a sere.

There are two major types of ecological succession: primary and secondary. In primary succession, the pioneer stage begins when organisms invade an area not previously occupied, such as a sandy area left exposed by a retreating glacier. In secondary succession, the pioneer stage begins when organisms invade an area that was previously occupied, such as an abandoned farm field.

In both types of succession, the intermediate stages occur as the area is gradually changed by successive communities of organisms. For example, the grasses and shrubs present in the early stages eventually die; as they decompose, they build up the thickness and nutrient content of the soil. Eventually the soil becomes thick and rich enough to support trees; small trees are followed by large trees. When a community is established in which each species maintains a relatively stable number of individuals, the ecosystem has reached its climax. Secondary succession achieves climax much more rapidly than primary succession because an area that was previously occupied is richer in nutrients than an area that was not previously occupied.

The stability of an ecosystem depends in large part on the successful interrelationships of organisms in regard to food. Such interrelationships form what is known as a food web. A food web consists of four main parts: nonliving components, producers, consumers, and decomposers.

Nonliving Components

These include such things as oxygen, nitrogen, organic compounds, soil, water, and sunlight, are used in various ways by the living components.


Produces are organisms such as plants and algae, produce their own food from nonliving components through photosynthesis. Plants use the energy of the sun, certain minerals, and water to synthesize food.


Consumers are mostly animals, may be divided into herbivores, which eat producers; carnivores, which eat animals; and omnivores, which eat producers and consumers.


Decomposers are mostly bacteria and fungi that break down dead organisms and return the raw materials to the nonliving parts of the ecosystem. These nutrients are taken up by producers and recycled back into the food web.

A food web is made up of many interrelated food chains. A food chain represents the transfer of food energy from producers of food through a series of consumers of food. Organism feeds on the one under it in the food chain.

Ecological efficiency of an organism is the amount of energy from food that it can turn into chemical energy to be used by various biological and physical processes. Plants can change less than 1 per cent of sun's energy they use into energy that reaches their cells. With each step up the food chain, some amount of energy is lost as heat. This can be depicted as an energy pyramid, where producers are at the base and carnivores are at the top.

The energy pyramid is related to the pyramid of biomass. Parallel to the energy pyramid, the pyramid of biomass has plants at the base and carnivores at the top. This is because the total weight, or biomass, of plants is more than that of herbivores and carnivores.

Some ecosystems can seem deceptively unchanging. This gave rise to the concept of balance of nature. However, recent studies have shown that ecosystems do change, but maintain a balance through subtle and dynamic interactions between various living and nonliving components.

Ecologists also study the natural recycling of the basic elements—elements, such as carbon and nitrogen, on which life on earth depends. Minerals that make up living things need to be recycled for life to continue. Phosphorus, water, carbon, hydrogen, and sulfur follow a cycle in nature, where they are released by dead matter or through various life processes and then used by other organisms in the ecosystem. Disruption in this cycle due to human activities can cause loss of minerals from the cycle.

The Importance of Ecology

Humans are dependent on their environment, as are all other organisms. Any change in the environment even in distant parts of the planet affects living things and their environment elsewhere. All organisms are dependant on each other in many ways. Destruction of one organism in the environment can lead to the destruction of other organisms. The human environment includes the entire earth and may some day include other planets as well. Technological advances have given humans the ability to exert great influence over the environment of all living things. For this reason, it is necessary to have an understanding of ecology in order to survive.

Applied ecology is concerned with the practical applications of the theories of ecology. Among the many applications of ecology are those used in agriculture and medicine. Scientific study of the relations of organisms with their environments helps farmers grow crops in the right soils and climates; provide livestock with suitable food and shelter; eliminate harmful pests; and breed new varieties of plants and animals. Ecological knowledge helps in the fight against disease. For example, knowledge of the malarial mosquito's environmental niche makes it possible to help control malaria by draining the swamps in which the mosquitoes breed.

However, in their efforts to improve the environment humans often make mistakes through lack of ecological understanding. A notable example of an ecological catastrophe caused by seemingly beneficial human intervention in natural processes occurred in Borneo shortly after World War II. A program was undertaken there to control mosquitoes by spraying with DDT. The number of mosquitoes declined drastically, but the roofs of houses began to collapse because they were being eaten by caterpillars. The caterpillars had previously been held under control by certain predatory wasps—which had been killed off by the DDT.

In addition to spraying for mosquitoes, the villagers also sprayed inside their homes to kill flies. Previously, the houseflies had been more or less controlled by lizards called geckos. As the geckos continued eating houseflies, now laden with DDT, the geckos began to die. The dead or dying geckos were eaten by house cats. The cats, which were at the end of the food chain, also began to die from the DDT concentrated in the bodies of the geckos they were eating. So many cats died that rats began invading the houses, eating the villagers' food. The rats multiplied and eventually became potential plague carriers.

Another example of ecological imbalance caused by human manipulation of the natural environment involves the Kaibab Plateau in Arizona, where Indians used to hunt deer for meat and skins. The plateau was designated as the Grand Canyon National Game Preserve in 1906. Public hunting was discontinued and hunters were hired by the government to kill off the deer's natural predators, such as wolves, coyotes, and cougars.

Subsequently the deer population rose from about 4,000 to about 100,000. The vegetation of the plateau that the deer normally fed on was not enough to sustain such a huge deer population. Not only was the plateau stripped of its vegetation, but many deer died of starvation. In order to stabilize the deer population and restore ecological balance, predators were reintroduced and public hunting was reinstated.

Humans have destroyed wildlife habitats in order to build cities, homes, factories, and highways. They have contaminated the environment with such technological products and by-products as pesticides, motor exhaust fumes, industrial wastes, and radioactive fallout. Nonrenewable resources such as fossil fuels are being consumed very fast, and their by-products cause pollution. As a result, the environment has been changed in ways that could eventually make the earth uninhabitable. Because of such problems, increasing attention is being paid to the study of ecology in schools, governments, and by interested groups elsewhere. By understanding ecology, people can take steps to reduce pollution, deforestation, and other negative fallouts of human activity, and the human cost associated with them.