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Ecology, Ecosystems and the Environment
(From The Sciences, 6th ed., by Trefil and Hazen)

Student Activity: Click Here.

Characteristics of Ecosystems

Ecosystems are richly varied. They occur on virtually every body of water and parcel of land on Earth, from the deepest ocean trench to the highest mountain range to the soil in your backyard. Yet, in spite of this diversity, all ecosystems share a few basic characteristics. As you read about these characteristics, think about how they apply to an ecosystem near your home.

1. Every Ecosystem Consists of Both Living and Nonliving Parts
Nonliving or abiotic parts form the chemical and physical environment of the ecosystem —the water, soil, atmosphere, and so forth. Local climate, including average temperature, rainfall, winds, and Sun exposure, are important physical properties of land ecosystems, whereas water temperature, pressure, salinity, and acidity help to characterize ecosystems in oceans, lakes, and other bodies of water.


Living organisms form the biotic part of an ecosystem; they form an ecological community, which may be defined as all the individuals in an area that interact with each other to maintain life. In a forest ecosystem, for example, an ecological community will include trees, shrubs, insects, birds, snakes, and squirrels, as well as fungi, bacteria, and a host of other microscopic organisms in the soil.

2. Energy Flows through Ecosystems
The most important interactions of organisms in an ecological community are by way of a food chain or food web, which indicates who feeds on whom. Each species in a food web obtains energy and chemicals from other organisms; in turn, each species provides energy and chemicals for other organisms. Insects eat plants, birds eat insects, bacteria and fungi in the soil decompose birds and other organisms when they die, and plants obtain vital nutrients from the soil. Food webs for ecological communities may be extremely complex.


The flow of energy between trophic levels is an important unifying characteristic of all ecosystems. The first trophic level of photosynthetic plants, which use only the Sun’s energy, provides energy for herbivores in the second trophic level. Herbivores, in turn, pass some of their energy to carnivores of the third trophic level and so on. Decomposers, including bacteria and fungi, obtain energy from all other trophic levels. In each energy transfer from one trophic level to another, most of the available energy cannot be recovered in a useful form; it eventually radiates into space as waste heat. In fact, only about 10% of the energy available at one trophic level normally finds its way to the next. Thus, as energy flows through an ecosystem, it must be replaced continuously.

3. Matter Is Recycled by Ecosystems
Atoms continuously cycle from one part of Earth to another. Perhaps the easiest way to understand the cycling of atoms through Earth’s biosphere is to follow the carbon cycle. This cycle can be illustrated by looking at the possible path of a single atom of carbon that leaves your lungs the next time you breathe out a molecule of carbon dioxide. This carbon atom enters the atmosphere, where many different things can happen to it. It can, for example, be taken up by a plant during photosynthesis and then be incorporated into the tissues of a tree or a blade of grass. The plant can then be eaten so that the carbon atom becomes part of the tissue of an herbivore. Alternatively, the carbon can simply return to the atmosphere if the plant dies and rots without being eaten.


If the carbon atom is taken into the tissue of an herbivore, then it may show up on your dinner plate one day and be taken into your body as part of some food you eat. It might even be incorporated into your own body to stay there until you die, or to move through the chemical cycles. In either case, the carbon atom, in time, will enter the atmosphere again.


Another possible track for a carbon atom is described here. It can enter the ocean by being added to a mollusk shell or the skeleton of a microscopic organism. Upon the death of the organism, these hard parts sink to the ocean bottom, where, in the form of calcium carbonate, they are turned into limestone. In this case, the carbon atom can remain locked up for hundreds of millions of years until the limestone is weathered and the carbon is released into the atmosphere.
A single atom of carbon, in other words, may have gone through many different chemical reactions during the 4.5-billion-year life of the planet and will continue to do so as long as Earth has living things on it. The one thing it will not do, however, is leave the planet. A similar story can be told for an atom of nitrogen or phosphorus or any other chemical element.

4. Every Organism Occupies an Ecological Niche
The ecological niche, a central concept in ecology, refers to a particular mode of survival —a particular way of obtaining matter and energy—within an ecosystem. In a forest ecosystem there may be a niche that can be filled by one or more kinds of warm-blooded, insect-eating, nocturnal animals—bats, for example. Mushrooms growing in shaded wooded areas may fill another niche. Each plant or animal in an ecosystem fills an ecological niche, and different organisms compete for dominance in their preferred ecological niche.

5. Stable Ecosystems Achieve a Balance among Their Populations
This balance, called homeostasis, reflects the fact that matter and energy are limited resources that must be shared among all individuals of an ecosystem. An ecosystem in homeostasis will exhibit some variations in population sizes, as food supplies and other factors vary from season to season and year to year. But the overall distribution of species is usually relatively constant. A one-acre sunny meadow, for example, will boast a large but limited amount of grasses, flowering plants, and other vegetation. Year in and year out, those plants support a limited population of insects, which in turn will feed perhaps a few dozen birds.

6. Ecosystems Are Not Permanent, but Change over Time
While ecosystems may appear to be stable, in fact we know that they change over time. On the longest timescales, the effects of plate tectonics will change the climate in a given area, converting a desert into a fertile plain, for example. On shorter timescales, the advance and retreat of glaciers can have a similar effect, as can changes in patterns of precipitation. Even on very short timescales, the introduction of a new species, by humans or by other natural processes, can profoundly change the pattern of life in a given area. As the science of ecology progresses, understanding and predicting these sorts of changes is becoming a major research goal.

The Law of Unintended Consequences

The complex interweaving of living things in their environment leads to a central insight in the science of ecology, an idea called the law of unintended consequences.  It is virtually impossible to change one aspect of a complex system without affecting other parts of the system, often in as-yet unpredictable ways.


Whenever we alter something in an ecosystem other changes will follow, and we have to consider what those changes might be. Examples of this “law” appear in the news almost daily: building levees on the Mississippi River has caused unintended intensification of flooding; extracting petroleum and water from underground reservoirs has caused unintended land subsidence; building jetties into the ocean has resulted in unintended erosion of beaches. Each of these systems is interdependent, so the whole responds to every stimulus.


As often happens when scientists and engineers encounter complex systems for the first time, a good deal of observation and trial and error has to take place before an understanding of the system begins to emerge.  Unfortunately, during that period of study serious mistakes can be made (see the discussion of Lake Victoria in the following section). Eventually, however, people learn how to proceed and begin to undertake large-scale projects with some confidence. At the moment, for example, the largest reconstruction project ever attempted is being undertaken in the Everglades of South Florida. The Everglades Restoration Plan is designed to restore the Everglades by changing the flow of water in the entire southern part of the state. As the plan proceeds, you can be sure that everyone—engineers and environmentalists alike—will have the law of unintended consequences firmly in mind.

Environmental Issues


There is a growing concern for environmental sustainability due to the growing human population (now 7 billion), increasing demand for resources, pollution, and climate change. Major threats to water quantity, quality, sustainability, and a high demand for major crops, such as corn, wheat, and cotton have placed a strain on the environment. Continual loss of habitat continues to threaten biodiversity, and increased extinction rates alienate us from finding future medicinal answers to growing incidence in local diseases. Ecosystems are affected directly and indirectly by point and non-point source pollution.