The Venezuela's Eco Portal to Eco-Tourism & Ecology

International Biosphere

Biosphere, relatively thin life-supporting stratum of the Earth's surface, extending from a few kilometres into the atmosphere to the deep-sea vents of the oceans. The biosphere is a global ecosystem composed of living organisms (biota) and the abiotic (nonliving) factors from which they derive energy and nutrients.

The biosphere can be broken down into segments of abiotic and biotic components, called ecosystems. Oceans, lakes, and wetlands are aquatic ecosystems, while forests, deserts, and tundras are terrestrial ecosystems. Through these systems, energy flows and chemicals essential to life are cycled. The biosphere itself can be studied as a worldwide ecosystem through which the interconnectedness of all life and life-supporting systems on the Earth can be understood.

Organisms in the biosphere are classified into trophic levels, or feeding relationships, that constitute the food chain. Primary producers, or autotrophs, are those organisms that convert energy from the Sun (photoautotrophs) or from inorganic substances (chemoautotrophs) to produce organic compounds. Green plants make up the largest group of primary producers. The next trophic level is that of primary consumers, or herbivores (organisms that eat plants or algae). Secondary consumers are carnivores that feed on herbivores, while tertiary consumers feed on secondary consumers, and so on. Detritivores, or decomposers, are consumers that feed on organic detritus.

The process of energy flow occurs across the trophic levels. Energy enters the system through solar radiation, which primary producers convert to chemical energy (organic compounds) by the process of photosynthesis. Of the light energy that strikes the Earth, only about 1 percent is used in photosynthesis. Energy flows unidirectionally through the food chain and is dissipated at each successive stage; roughly 10 percent of energy is transferred from one trophic level to the next.

Unlike energy, which flows into and out of the system, chemicals are recycled in the biosphere. Elements essential to life such as carbon, nitrogen, phosphorus, and sulfur are drawn from the geosphere, or nonliving world, which consists of the atmosphere (air), the hydrosphere (water), and the lithosphere (rocks and soil of the terrestrial surface). Once taken up by organisms, the elements cycle between biotic and abiotic states according to their biogeochemical cycles. The cycling of water is also necessary to the maintenance of life.

The biotic portion of ecosystems can be broken down into communities--i.e., assemblages of populations of different species that live in proximity and may interact with one another. Populations, still smaller organizational units, are groups of individuals of the same species located in a particular geographic area. Environmental conditions such as temperature, water availability, light, and periodic disturbances affect the distribution of organisms, and interactions between the species themselves further influence the composition of the community. Interspecific interactions include competition, antagonism, and predation.

Interactions with the biotic and abiotic components of their ecosystems have shaped the distribution and evolution of species, resulting in a diverse array of organisms. These organisms contribute to the steady-state environment of each ecosystem and, thus, to the maintenance of biospheric processes. Disturbances, both natural and man-made, to even seemingly small parts of the system may have significant and far-reaching effects.

ecosystem, the complex of living organisms, their physical environment, and all their interrelationships in a particular unit of space.

The principles underlying the study of ecosystems are based on the view that all the elements of a life-supporting environment of any size, whether natural or man-made, are parts of an integral network in which each element interacts directly or indirectly with all others and affects the function of the whole. All ecosystems are contained within the largest of them, the ecosphere, which encompasses the entire physical Earth (geosphere) and all of its biological components (biosphere).

An ecosystem can be categorized into its abiotic constituents, including minerals, climate, soil, water, sunlight, and all other nonliving elements, and its biotic constituents, consisting of all its living members. Linking these constituents together are two major forces: the flow of energy through the ecosystem, and the cycling of nutrients within the ecosystem.

The fundamental source of energy in almost all ecosystems is radiant energy from the sun. The energy of sunlight is used by the ecosystem's autotrophic, or self-sustaining, organisms. Consisting largely of green vegetation, these organisms are capable of photosynthesis--i.e., they can use the energy of sunlight to convert carbon dioxide and water into simple, energy-rich carbohydrates. The autotrophs use the energy stored within the simple carbohydrates to produce the more complex organic compounds, such as proteins, lipids, and starches, that maintain the organisms' life processes. The autotrophic segment of the ecosystem is commonly referred to as the producer level.

Organic matter generated by autotrophs directly or indirectly sustains heterotrophic organisms. Heterotrophs are the consumers of the ecosystem; they cannot make their own food. They use, rearrange, and ultimately decompose the complex organic materials built up by the autotrophs. All animals and fungi are heterotrophs, as are most bacteria and many other microorganisms.

Together, the autotrophs and heterotrophs form various trophic (feeding) levels in the ecosystem: the producer level, composed of those organisms that make their own food; the primary-consumer level, composed of those organisms that feed on producers; the secondary-consumer level, composed of those organisms that feed on primary consumers; and so on. The movement of organic matter and energy from the producer level through various consumer levels makes up a food chain. For example, a typical food chain in a grassland might be grass (producer) mouse (primary consumer) snake (secondary consumer) hawk (tertiary consumer). Actually, in many cases the food chains of the ecosystem overlap and interconnect, forming what ecologists call a food web. The final link in all food chains is made up of decomposers, those heterotrophs that break down dead organisms and organic wastes. A food chain in which the primary consumer feeds on living plants is called a grazing pathway; that in which the primary consumer feeds on dead plant matter is known as a detritus pathway. Both pathways are important in accounting for the energy budget of the ecosystem.

As energy moves through the ecosystem, much of it is lost at each trophic level. For example, only about 10 percent of the energy stored in grass is incorporated into the body of a mouse that eats the grass. The remaining 90 percent is stored in compounds that cannot be broken down by the mouse or is lost as heat during the mouse's metabolic processes. Energy losses of similar magnitude occur at every level of the food chain; consequently, few food chains extend beyond five members (from producer through decomposer), because the energy available at higher trophic levels is too small to support further consumers.

The flow of energy through the ecosystem drives the movement of nutrients within the ecosystem. Nutrients are chemical elements and compounds necessary to living organisms. Unlike energy, which is continuously lost from the ecosystem, nutrients are cycled through the ecosystem, oscillating between the biotic and abiotic components in what are called biogeochemical cycles. Major biogeochemical cycles include the water cycle, carbon cycle, oxygen cycle, nitrogen cycle, phosphorus cycle, sulfur cycle, and calcium cycle. Decomposers play a key role in many of these cycles, returning nutrients to the soil, water, or air, where they can again be used by the biotic constituents of the ecosystem.

The orderly replacement of one ecosystem by another is a process known as ecosystem development, or ecological succession. Succession occurs when a sterile area, such as barren rock or a lava flow, is first colonized by living things or when an existing ecosystem is disrupted, as when a forest is destroyed by a fire. The succession of ecosystems generally occurs in two phases. The early, or growth, phase is characterized by ecosystems that have few species and short food chains. These ecosystems are relatively unstable but highly productive, in the sense that they build up organic matter faster than they break it down. The ecosystems of the later, or mature, phase are more complex, more diversified, and more stable. The final, or climax, ecosystem is characterized by a great diversity of species, complex food webs, and high stability. The major energy flow has shifted from production to maintenance.

Human interference in the development of ecosystems is widespread. Farming, for example, is the deliberate maintenance of an immature ecosystem--one that is highly productive but relatively unstable. Sound management of ecosystems for optimal food production should seek a compromise between the characteristics of young and mature ecosystems, and should consider factors that affect the interaction of natural cycles. Short-term production can be maximized by adding energy to the ecosystem in the form of cultivation and fertilization. Such efforts, however, can hinder efficient energy use in the long run by producing an imbalance of nutrients, an increase in pollutants, or a heightened susceptibility to plant diseases as a consequence of intensive inbreeding of crops.

Although an awareness of the interdependence between human society and its environment was already prominent in ancient philosophy and religion, the formulation of the basic principles of systems ecology as a scientific discipline began in the late 19th century. During the second half of the 20th century, the study of ecosystems has become increasingly sophisticated and is now instrumental in the assessment and control of the effects of agricultural development and industrialization on the environment. On farms, for instance, it has shown that optimal long-term production of pasturage requires a moderate grazing schedule in order to ensure a steady renewal of the moisture and nutrient content of the soil and has emphasized the need for multiple-use strategies in the cultivation of arable lands. Systems ecology has been concerned with the consequences of accumulated insecticides and has provided a way of monitoring the climatic effects of atmospheric dust and carbon dioxide released by the burning of fossil fuels (e.g., coal, oil, and natural gas). It has helped to determine regional population capacities and has furthered the development of recycling techniques that may become essential in humanity's future interaction with the environment.