Ecosystems: Overview


An ecosystem is made up of the complex interactions of a community of organisms of different species with one another and with their abiotic (nonliving) environment.

A biological community consists of a mixture of populations of individual species; a population consists of potentially interbreeding members of a species. Individual organisms interact with members of their own species as well as with other species. An ecosystem is formed by this web of interactions among species, along with the physical, chemical, and climatic conditions of the area.

Abiotic and Biotic Interactions

Abiotic environmental conditions include temperature, water availability, soil nutrient content, and many other factors that depend on the climate, soil, and geology of an area. Living organisms can alter their environment to some degree. A canopy formed by large forest trees, for example, will change the light, temperature, and moisture available to herbaceous plants growing near the forest floor.

The environmental conditions in a particular area can also be affected by the conditions of neighboring areas. The disturbance of a stream bank can lead to erosion, which will affect aquatic habitat for a considerable distance downstream. It can be difficult to anticipate the wide-ranging affects of ecosystem disturbance.

Species and individuals within an ecosystem may interact directly with one another through the exchange of energy and material. Predators, for example, obtain their energy and meet their nutritional needs through consumption of prey species. Organisms also interact indirectly through modification of their surrounding environment.

Earthworms physically modify soil structure, affecting aeration and the transport ofwater through the soil. These alterations of the physical environment, in turn, affect plant root growth and development as well as the ability of plants to secure nutrients.

Ecosystems are not closed systems: Energy and material are transferred to and from neighboring systems. The flow of energy or material among the components of an ecosystem, and exchanges with neighboring ecosystems, are governed by functions of the abiotic and biotic ecosystem components.

These ecological processes operate simultaneously on many different temporal and spatial scales. At the same time that a microorganism is consuming a fallen leaf, the process of soil formation is occurring through chemical and physical weathering of parent material; plants are competing with one another for light, water, and nutrients.

Boundaries and Temporal Scales


Because of the exchange of energy and material, it is not possible to draw clear boundaries around an ecosystem. A watershed, for example, is formed by topographic conditions that create physical barriers guiding the gravitational flow of water, yet wind carries seeds and pollen over these barriers, and animals can move from watershed to watershed.

The strength of the interactions among neighboring systems is the basis on which humans delineate ecosystem boundaries. In truth, all ecosystems around the world interact with one another to some degree.

Ecological processes operate on many different time scales as well. Some operate over such long time scales that they are almost imperceptible to human observation. The process of soil formation occurs over many human life spans.

Other processes operate over extremely short time intervals. The reproduction of soil bacteria, the response of leaves to changing temperature over the length of a day, and the time required for chemical reactions in the soil are all very short when compared to a human life span.

Ecosystem Disturbances

Ecosystems are subject to disturbance, or perturbation, when one or more ecosystem processes are interrupted. Disturbance is a natural ecological process, and the character of many ecosystems is shaped by natural disturbance patterns.

Ecosystem Disturbances
Ecosystem Disturbances
There are species that require disturbance in order to regenerate themselves. These species may be present in great abundance following a disturbance. Their abundance then decreases over time, and if there is no disturbance to renew the population, they will eventually die out and no longer be present in the ecosystem.

The successful reproduction of many prairie species may be dependent on periodic fires. Suppression of fire as a means of protecting an ecosystem may lead to the local extinction of small plants, which depend on periodic fires to increase light availability by removing larger grasses and providing nutrients to the soil. The formation of sandbars in streams may be controlled by periodic flood events that remove great amounts of sediment from streambanks.

Protection of existing ecosystems can depend on the protection or simulation of natural disturbances. This is even true of old-growth forests; the natural disturbance interval due to fire or windstorm may be centuries, and yet interruption of the natural disturbance pattern may lead to shifts in species composition or productivity.

Increasing the frequency of disturbance can also affect ecosystem structure and function. Repeated vegetation removal will favor species that take advantage of early-successional conditions at the expense of species that are more adapted to late-successional conditions.

In order to ensure continued functioning of ecosystem processes and the survival of all species, it is necessary to have a mix of systems in early-successional and late-successional stages in a landscape.

Human resource utilization must be managed within this context in order to ensure the long-term sustainability of all ecosystem components and to reduce the chances of extinction of some species due to human alteration of natural disturbance intervals.

Ecosystem Stability

Ecosystem Stability
Ecosystem Stability
A system is stable, or in equilibrium, if it can return to its previous condition at some time after disturbance. The length of time required to return to the original condition is the recovery time.

The reestablishment of a forest following harvesting and the renewed production of forage following grazing both depend on the inherent stability of the affected ecosystem. The stability of an ecosystem is dependent on its components and their interrelationships.

Disturbancemay primarily affect one component of an ecosystem, as with salmon fishing in the Pacific Ocean. The ability of the entire ecosystem to adjust to this disturbance depends on the complexities of the interrelationships between the salmon, their predators and prey, and their competitors.

The length of the recovery time varies with the type of system, the natural disturbance interval, and the severity of the disturbance. A system is usually stable only within some bounds.

If disturbed beyond these recovery limits the system may not return to its previous state but may settle into a new equilibrium. There are examples in the Mediterranean region of systems that were overgrazed in ancient times that have never returned to their previous species composition and productivity.

Forest managers, farmers, fishermen, and others must understand the natural resiliency of the systems within which they work and stay within the bounds of stability in order to ensure sustainable resource utilization into the future.

Matter and Energy Cycles

Ecological processes work through the cycling of matter and energy within the system. Nutrient cycling consists of the uptake of nutrients from the soil and the transfer of these nutrients through plants, herbivores, and predators until their eventual return to the soil to begin the cycle anew. Interruption of these cycles can have far-reaching consequences in the survival of different ecosystem components.

These cycles also govern the transport of toxic substances within a system. It took many years before it was realized that persistent pesticides such as DDT would eventually be concentrated in top predators, such as raptors.

The decline in populations of birds of prey because of reproductive failure caused by DDT was a consequence of the transport of the chemical through ecosystem food webs. Likewise, radionucleides from the 1986 explosion at the Chernobyl nuclear reactor have become concentrated in certain components of the ecosystems where they were deposited.

This is particularly true of fungi, which take radionucleides and heavymetals from their food sources but do not shed the substances. Humans eating mushrooms from these forests can receive larger than expected doses of radiation, because the concentration in the fungi is much greater than in the surrounding system.

A basic understanding of ecosystem properties and processes is critical in designing management methods to allow continued human utilization of systems while sustaining ecosystem structure and function.

With increasing human population and advancing living standards, more and more natural ecosystems are being pushed near their limits of stability. It is therefore critical for humans to understand how ecosystems are structured and function in order to ensure their sustainability in the face of continued, and often increasing, utilization.