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.