Glossary for Environmental Science

Environmental Science Main Page

The Dictionary of Ecology and Environmental Science
Henry W. Art, General Editor
Henry Holt and Company, Inc.
Copyright © 1993

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Abiotic:

Non-living; usually applied to the physical characteristics of biological systems, such as moisture, nutrients, soils, solar radiation, etc.

Abrasion:

1) General: grinding or wearing of rock material by impact of objects in transport 2) Eolian: wind-accomplished erosion of rock or soil masses by impact of saltating grains.

Absolute age:

A determination of geochronology based on a fixed unit of measure, of age in years

Absolute humidity:

The total moisture contained in a given amount of air, measured in units of mass per unit volume (ex: grams of water vapor per cubic meter of air)

Acetic acid:

CH3COCH3, a clear liquid with a strong odor and flavor that is the prime ingredient (after water) of vinegar. Acetic acid is a weak acid and an important solvent used to manufacture organic chemicals and plastics

Acid:

1) A substance that forms hydrogen ions upon dissolving in water and neutralizes bases to form a salt plus water. Acids are characterized by their tendency to lose a proton (or to gain an electron pair) in reactions. Acids turn litmus paper red and can be recognized by their usually sour taste (as in vinegar, acetic acid) and low pH. 2) Acidic, having properties that resemble acids, such as a sour taste.

Acidic:

Having properties like those of acids, such as a high concentration of hydrogen ions and therefore a low pH value; sour in taste or corrosive in nature. See alkaline, basic

Acid precipitation:

Rain, sleet, mist, snow, or other precipitation whose pH is significantly lower than neutral (neutral pH is 7; normal precipitation has a pH of about 5.6 because dissolved carbon dioxide from the atmosphere produces carbonic acid). Normal precipitation becomes acidic (lower in pH) when water vapor interacts with oxides of sulfur and nitrogen in the atmosphere.

Acid rain:

The common name for acid precipitation, especially in liquid form. See acid precipitation

Aeration:

Process of mixing with air or oxygen. Aeration is the basis for secondary wastewater treatment: Air is bubbled through the water to increase amount of dissolved oxygen. More oxygen promotes the action of aerobic bacteria, which break down organic wastes and thereby purify the water.

Aerosol:

A suspension of extremely small particles or tiny droplets of liquids in air or in a gas. Gases under pressure are used to dispense a variety of substances as aerosols, from spray paints and insecticides to asthma medications. The use of chloroflourocarbons (CFCs) as propellants in aerosol cans has been linked to the destruction of the earth’s protective ozone layer. See ozone

AIDS:

Acronym for Acquired Immuno-Deficiency Syndrome, a fatal human disease in which the body can no longer defend itself against infections. Victims of AIDS often die from any of a number of opportunistic diseases, including tuberculosis and pneumonia. AIDS is caused by a retrovirus, human immunodeficiency virus (HIV), which is spread by exchange of body fluids. See HIV

Algae:

A general category of photosynthesizing unicellular and multicellular organisms, primarily aquatic. Some are prokaryotes (cyanobacteria, formerly known as blue-green algae) but most belong to the kingdom Protista, eukaryotes that are neither plants, animals, nor fungi. Algae (singular: alga) include many unicellular planktonic organisms, diatoms, and multicellular seaweeds.

Alkaline:

1) The opposite of acid, also called basic; having a high pH value and thus a low concentration of hydrogen ions and a high concentration of hydroxide ions. 2) Containing alkalies, as in alkaline soil or an alkaline chemical reaction

Atmosphere:

The gas layer surrounding a planet. Excluding the water vapor, the earth’s atmosphere consists (by volume) of 78 percent nitrogen, 21 percent oxygen, 0.9 percent argon, 0.035 percent carbon dioxide (this level is rising), and minute quantities of other gases (including neon, krypton, helium, and air pollutants). Water vapor may constitute up to 3 percent, depending on the relative humidity and temperature of the atmosphere. See Stratosphere, Troposphere

Autotroph:

An organism that can manufacture its own food from inorganic compounds through photosynthesis or chemosynthesis. Autotrophs include green plants, algae, some Protista, and some bacteria. Also called primary producer. Some autotrophic bacteria use inorganic compounds, such as hydrogen sulfide and ammonium, as their energy source in chemosynthesis. See producer, consumer, decomposer

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Basic:

Alkaline; demonstrating the qualities of a base. See acidic

Biochemical Oxygen Demand:

(BOD) An indication of the extent to which water is polluted by sewage or other organic waste. It is a measure of the dissolved oxygen consumed by microorganisms as they break down the organic matter in a sample of water; the greater the quantity of organic matter, the greater the oxygen required for its decomposition. It is expressed in parts per million of dissolved oxygen consumed. Also called biological oxygen demand.

Biodegradable:

Capable of being decomposed into natural substances (such as carbon dioxide and water) by biological processes, especially bacterial action. See nonbiodegradable

Biomass:

1) The total amount of all the biological material, the combined mass of all the animals and plants living in a specific area, or of a given population. Usually, expressed as oven-dry weight per area (grams per square meter, kilograms per hectare, or pounds per acre). 2) Organic matter used as fuel (biomass fuel).

Birth rate:

The ratio of the number of live births to a total population over a given period of time, usually one year. For humans, the rate is expressed as births per thousand persons per year. See death rate

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Carbon cycle:

The cyclic pathway through which carbon circulates throughout nature. Plants through photosynthesis fix carbon dioxide (CO2) from the air, producing carbohydrates and other organic compounds. Animals and plants metabolize these carbohydrates for energy, releasing (CO2) into the air. After the death of organisms, their carbon may be released as (CO2) back into the atmosphere through decomposition. If decomposition is slower than the fixation of (CO2), carbon may accumulate.

Carnivores:

Organisms that eat live animals, especially a member of the Carnivora order of mammals with characteristic large, sharp teeth. Cats, dogs, lions are carnivores. See herbivore, omnivore, frugivore

Catalyst:

Any substance that causes or speeds up a chemical reaction without itself undergoing a permanent chemical change. Enzymes are proteins produced by the body that act as catalysts in many physiological processes, such as digestion.

Climax:

Final phase of succession in an ecosystem in which populations of animals and plants remain in a relatively self-perpetuating state.

Closed system:

A system in which there is an exchange of energy with the environment, but no exchange of matter.

Co-dominant:

Two or more species that jointly are the most prevalent or significant species within a plant community. See subdominant

Community:

All the groups of organisms living together in the same area, usually interacting or depending on each other for existence. Also called biological community.

Concentration:

The amount of one substance dissolved or mixed in a specified quantity of a solution or mixture. Also, a process used to increase the concentration of a substance, such as evaporation of water from a solution.

Conservation:

Management of natural resources to provide maximum benefit over a sustained period of time. Conservation includes preservation and forms of wise use, including reducing waste, balanced multiple use, and recycling.

Consumer:

An organism that ingests other organisms, either living or dead. All animals and most microorganisms are consumers (herbivores, carnivores, parasites, and detritivores). Fungi and a few plants are consumers, including saprophytes (Indian pipes) and carnivorous plants (pitcher plant, Venus flytrap). See producer, decomposer

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Death rate:

The ratio of the number of deaths to the total number of individuals in a specific population during a particular time period, usually one year. For humans, the rate is expressed as deaths per thousand persons per year. See birth rate

Decay:

1) Decomposition of dead organic matter, usually by bacterial or fungal activity. 2) Short for radioactive decay. See radioactive decay

Decompose:

To break down; to rot or decay, especially through microbial action.

Decomposer:

An organism that derives its nourishment from dead organic matter (animal and plant bodies), breaking down the complex molecules into simpler organic molecules. Decomposers include earthworms, mushrooms, and other fungi, and bacteria. Also called microconsumer, detritivore, saprobe, or reducer. See consumer, producer

Degradable:

Substance that can be broken down into smaller, less complex molecules. See biodegradable

Density:

1) The mass per unit of volume of a substance, usually stated in grams per cubic centimeter (or pounds per cubic foot). Density gives an indication of how closely packed the molecules of a substance are; for example, liquid water has a density of 1 gram per cubic centimeter, while liquid mercury has a density of 13.59 grams per cubic centimeter. 2) The size of a population of a given species in a given area.

Detritus:

Waste material. Detritus is usually organic material, such as dead or partially decayed plants and animals, or excrement. Detritus can also be small particles of minerals from weathered rock, such as sand and silt.

Digestion:

The process in which food is broken down into compounds that cells can absorb, store, or oxidize and use as nourishment. 2) The use of anaerobic bacteria to produce biogas (methane) from refuse such as manure 3) The use of bacteria (usually aerobic) to break down raw sewage into simpler compounds, used in secondary wastewater treatment.

Direct solar energy:

Usable energy obtained directly from sunlight, as through solar cells, solar collectors, and passive solar structures. See indirect solar energy

Dissolved oxygen:

(DO) Amount of oxygen gas contained in water or sewage, usually given in parts per million at a specified temperature and atmospheric pressure. It is a measure of the ability of water to support aquatic organisms. Water with very low dissolved oxygen content (less than 5 ppm), which is usually caused by too much or improperly treated organic wastes, does not support fish and similar organisms.

Dominance:

1) The condition of a species contributing a major portion of the biomass or numbers within a community, and thereby influencing the abundance of other species in the community. 2) The ability of one gene to prevail over another different gene in causing a particular trait (phenotype). 3) Condition in which behavior establishes more important social positions for some individual within a population (creating a dominance hierarchy).

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Ecology:

The branch of biology that studies the relationships among living organisms and between organisms and their environments. It is derived from the Greek OIKOS, meaning house, therefore it is literally the study of the house.

Economic efficiency:

Benefits of an economic transaction exceed costs.

Ecosystem:

A functioning unit of nature that combines biotic communities and the abiotic environments with which they interact. Ecosystems vary greatly in size and characteristics. Also called biogeocoenosis.

Emigration:

The movement of populations of a species out of a particular area or habitat. Includes the departure of humans from one country in order to relocate in a new country. See immigration, migration

Emission:

1) Waste discharged into the environment by industrial or other human processes. Usually used to refer to gaseous discharges, but can also refer to liquid discharge or radioactivity. 2) The giving off of electrons, electromagnetic energy, or radioactivity.

Environment:

The whole sum of the surrounding external conditions within which an organism, a community, or an object exists. Environment is not an exclusive term; organisms can be and usually part of another organism’s environment.

Epidemic:

Spreading rapidly and affecting a large portion of a population at one time. Also, an outbreak of disease with such extensive characteristics.

Equilibrium:

A state of balance in a system, where opposing factors cancel each other out (or are entirely absent). In thermal equilibrium, heat gain balances any heat loss occurring between an object and its surroundings. A body at rest or moving at constant velocity also demonstrates a balance of opposing forces (or an absence of any forces).

Equilibrium Constant, Keq:

The equilibrium constant is a measure of the extent of the reaction between the two chemical species. The value characterizes the point of stability of a chemical reaction at a given temperature. At this point of stability or equilibrium, the system has no tendency to change because the forward rate of the reaction is exactly balanced by the reverse rate of reaction.

Erosion:

The physical removal of rock or soil particles by a transport agent such as running water, wind, glacial ice, and gravity.

Eutrophication:

The process by which a body of water acquires a high concentration of nutrients, especially phosphates and nitrates. These typically promote excessive growths of algae. As the algae die and decompose, high levels of organic matter and the decomposing organisms deplete the water of available oxygen, causing the death of other organisms, such as fish. Eutrophication is a natural, slow-aging process for a water body, but human activity greatly speeds up the process.

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Flux:

1) A continuous flow, as in a stream, or succession of changes, as in a state of flux. 2) Rate of flow, as of a liquid or other fluid or radiation, across a given area.

Forage:

Food for animals, especially that obtained by grazing or browsing. Also, to look for food.

Fossil fuels:

Any deposit of fossil organic material that is combustible enough to be used as a fuel. Coal, oil and natural gas are fossil fuels.

Frugivore:

An animal that feeds on fruit, especially those primates that cannot digest the cellulose in leaves and therefore restrict their plant diet to fruit. See herbivore, omnivore, carnivore

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Gestation:

Pregnancy in mammals; the development of offspring in the uterus from conception to birth.

Grassland:

A biome comprised mainly of indigenous grasses or grasslike plants, the predominant vegetation type in semi-arid temperate and tropical latitudes. Grassland is used to refer to semi-natural meadows and cultivated farm meadows as well as to natural prairies.

Grazer:

An herbivore that eats non-woody plant matter.

Grazing:

The act of animals feeding on fresh grass and herbaceous plants.

Growth Rate:

1) The speed at which a plant, animal, or colony of microorganisms increases in size. 2) The increase (or decrease) of a population over a period of time. In humans, the rate of change is calculated from the number of births minus the number of deaths per thousand in a population, with net migration added; it is expressed as percentage

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Half-life:

A property used to characterize radioactive substances. The half-life is the period of time required for half of a quantity of a radioactive nuclide to undergo decay. Half-life is independent of conditions such as temperature and chemical form, and specific to each nuclide. See radioactive decay

Herbivore:

Any of a large group of animals whose diet consists of plants. Granivores and frugivores are also herbivorous animals. See carnivore, omnivore

HIV:

Abbreviation for human immunodeficiency virus, the retrovirus that causes AIDS. See AIDS

Humus:

An organic soil material so thoroughly decayed that the identity of the biologic source cannot be recognized.

Hydrocarbons:

Any compound containing only carbon and hydrogen. Hydrocarbons form a large group of organic chemicals that includes most petroleum and coal tar products such as benzene, propane, and propylene.

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Immigration:

The movement of an organism, population, or species into a new area. In population dynamics models, immigration is a factor contributing to population growth. See emigration, migration

Indirect solar energy:

Energy derived from sources that are at least one step removed from the energy of sunlight; this includes most traditional energy sources. Wood is a form of indirect solar energy; trees capture the sun’s energy through photosynthesis, producing carbon compounds that release energy when burned. These carbon compounds can, over millions of years, become coal or oil. Hydroelectricity, wind all biomass energy, and clean thermal energy conversion are examples of indirect solar energy. See direct solar energy

Infection:

Condition of disease in animals, humans, or plants from invasion by pathogenic organisms such as bacteria, protozoa, viruses, fungi, or animal parasites. Infection can spread by contact with diseased organisms, airborne microbes, and animal carriers such as mosquitoes.

Inorganic:

1) Not coming from animal or vegetable sources, as in inorganic fertilizers. 2) Describing compounds that either contain no carbon or contain only carbon bound to elements other than hydrogen.

International Biological Programme:

(IBP) A worldwide study of biological productivity of terrestrial, freshwater and marine communities, conservation, and human adaptability carried out from 1964 to 1974 by the nongovernmental International Council of Scientific Unions, with participation by over 40 countries.

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Kinetic energy:

The energy by virtue of mass in motion. The kinetic energy of a non-relativistic moving object equals half the product of the object’s mass and the square of its velocity. See potential energy

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Metabolism:

All of the chemical and physical activities that sustain an organism. Metabolism involves the breakdown of organic compounds to create energy, which animals use to grow, repair tissues, provide heat, and engage in physical activity. Plant metabolism involves the creation as well as utilization of organic compounds. In some organisms, metabolism includes metamorphosis.

Microbe:

A microorganism, an organism that can only be seen with the aid of a microscope, especially a bacterium producing disease or fermentation.

Migration:

1) The seasonal movement of animal populations from one region to another and back again, often covering tremendous distances. 2) The daily vertical movements of some freshwater plankton several meters downward during the day and back to the surface at night. 3) Movement of individuals into or out of local populations (includes emigration and immigration). See immigration, emigration

Mortality Rate:

The number of deaths per 100,000 people in a population over a year.

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Natural Gas:

A naturally occurring gaseous hydrocarbon that is principally composed of methane. Natural gas is valued as an efficient and clean-burning fuel.

Net Primary Production:

(NPP) The total amount of energy produced and stored through photosynthesis in a specific community or ecological system (gross primary production), minus the quantity consumed during respiration by the photosynthetic organisms. NPP is expressed as oven dry weight (grams) of tissues/area or energy content of tissues (Kcal)/area. Also called apparent photosynthesis, or net assimilation, and often used interchangeably with net primary productivity (although that is technically the productivity per year.)

Nitrogen Oxides:

(NOx), compounds formed by the oxidation of nitrogen, including nitrogen dioxide and nitric oxide. These compounds are air pollutants because they undergo photochemical reactions to form smog and ozone; they are also believed to contribute to the depletion of the earth’s protective ozone layer. See photochemical smog, smog, ozone, (SOx)

Nonbiodegradable:

Describing a substance that is not broken down by natural processes and so remains in its original form for long periods of time. Many plastics and some pesticides are non biodegradable. See biodegradable

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Octane:

A hydrocarbon containing eight carbon atoms (C8H18). Octane is a liquid and is found in petroleum; it exists as 18 different isomers. The term octane number is used informally for rating gasoline.

Omnivore:

An animal that feeds in more than one trophic level, for example, potentially eating animal, plant, and fungal material. See carnivore, herbivore

Organic Matter:

Material derived from decaying organic molecules of natural organisms (the remains of plants and of animals). Organic matter is essential for healthy soil. It may be partially recognizable, as in rough compost or leaves on their way to becoming leaf mold; when fully broken down, organic matter in soils is called humus. See humus

Organisms:

Any unicellular or multicellular living body whose different components work together as a whole to carry out life processes. Animals, plants, fungi, and microbes are all organisms.

Overgrazing:

To allow too many animals to graze in one area, causing a gradual change in the vegetation to less desirable species, or causing increased erosion so that portions of the pasture or the rangeland become bare. Although overgrazing may provide short-term economic gain, in the long term it produces land that supports fewer animals or even destroys the land for grazing, greatly reducing the potential for economic gain.

Oxidation:

Any chemical reaction that involves atoms or molecules losing electrons. The oxidation of one compound is always coupled with the reduction of another compound, and so the process is sometimes called oxidation-reduction. The term originally described a reaction in which oxygen combined chemically with another substance, but now includes many reaction that do not involve oxygen, such as dehydrogenation. See reduction

Oxidizing agent:

Any substance that is capable of removing electrons from another substance. An oxidizing agent carries out the process of oxidation and is itself reduced. (i.e. it gains electrons.) See oxidation, reduction

Ozone:

(O3), a compound that is formed when oxygen gas is exposed to ultraviolet radiation. In the outer atmosphere (stratosphere) ozone acts to shield the earth from excessive radiation. In the lower atmosphere (troposphere), however, it forms from combustion gases and is a major air pollutant contributing to photochemical smog. Ozone has commercial uses as a bleaching agent and water purifier. See photochemical smog

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Passerines:

A common name for the numerous and widespread perching birds belonging to the order Passeriformes.

Peroxyacetyl nitrates:

A group of air-polluting chemical compounds found in photochemical smog; they are produced by the action of sunlight on oxides of nitrogen, hydrocarbons, and ozone. Peroxyacetyl nitrates are usually known by their acronym, PAN. PAN irritate the eyes, nose, and throat; they are toxic to plants. Sometimes called peroxyacylnitrates.

Petroleum:

Crude mixtures of hydrocarbon oils, occurring naturally in the earth’s crust as a result of long-term geological action on decaying organic matter from eons ago. Unrefined petroleum often contains minerals such as sulfur and vanadium in addition to hydrocarbons. When coexisting natural gas and water are removed from petroleum, it becomes crude oil and is used for the production of many fuels and chemicals.

pH:

A measure of the relative concentration of hydrogen ions in a solution; this vlue indicates the acidity or alkalinity of the solution. It is calculated as the negative logarithm to the as ten of the hydrogen ion concentration in moles per liter. A pH value of 7 indicates a neutral solution; pH values greater than 7 are basic, and those below 7 are acidic. Vinegar has a pH of 3; ocean water has a pH value of approximately 8.

Phosphate:

1) The phosphate ion PO4-3. 2) A salt (or ester) of phosphoric acid (H3PO4). Phosphates are essential nutrients for plant and animal metabolism; some forms of rock and decaying organic matter provide sources. Because phosphorous is often the limiting nutrient in an ecosystem, when large quantities of phosphates are released into water bodies, they cause a boom in algae populations that deplete the available oxygen supply and contribute to eutrophication. See superphosphate, eutrophication

Phosphorus:

(P) R2C(O)-O-CH(CH2OP)-( CH2OC(=O)R1), A nonmetallic element with atomic weight 30.97 and atomic number 15. It exists in a number of different forms; one form of pure phosphorus is poisonous and burns skin on contact. Phosphorus is an essential nutrient for plants and animals; it is part of DNA molecules. It is used in matches to make them flare up quickly with the heat of friction.

Photochemical smog:

Air pollution in the form of a brown haze often seen over Los Angeles, CA, and other cities. It occurs on sunny days in areas with large volumes of automobile traffic. Photochemical smog is produced when sunlight acts on nitrogen oxides, (NOx), ozone, and hydrocarbons in a series of complex reactions. Photochemical smog is a respiratory irritant and can kill or alter plant tissues. See smog

Photooxidation:

Short for photochemical oxidation, chemical reactions that change (oxidize) compounds using energy from sunlight and some oxidant, such as oxygen (O2) or ozone (O3). Photooxidation produces photochemical smog. See photochemical smog

Photosynthesis:

The series of chemical reactions by which plant cells transform light energy into chemical energy through producing simple sugars (or other energy compounds) and oxygen from carbon dioxide and water. It is measured with a variety of units, including milligrams (mg) of carbon dioxide (CO2) per gram of leaf tissue per hour, mg(CO2)/mg chlorophyll/hour, mg(CO2)/square decimeter of leaf surface/hour, and mg(CO2)/ square meter aquatic or marine surface/day or hour.

Phytoplankton:

Small (usually microscopic) aquatic plants; those plankton capable of photosynthesis, such as unicellular algae.

Plateau:

A relatively flat and broad upland region that is considerably higher than adjacent regions and is bounded on at least one side by a scarp.

Pollutant:

A substance that is irritating, harmful, or toxic to plant or animal life; something that causes pollution.

Pollution:

An undesirable change in the environment, usually the introduction of abnormally high concentrations of hazardous or detrimental substances, heat or noise. Pollution usually refers to the results of human activity, but volcano eruptions and contamination of a water body by dead animals or by animal excrement are also pollution.

Population:

1) Organisms of the same species that inhabit a specific area 2) The total count of individuals within such a group

Population dynamics:

The study of fluctuations in population levels and the major biotic and environmental factors that cause these fluctuations

Potential energy:

Energy stored in a substance or a body because of its position (or the position of its component parts) or state, rather than its current motion. Water pumped up into a storage tank has potential energy because when it is allowed to fall, gravity enables the water to accomplish work. Gravitational energy, electrical energy, nuclear energy, and chemical energy are all forms of potential energy. See kinetic energy

Prairie:

A form of grassland vegetation typical of semiarid temperate latitudes in continental interiors, as in the United States. Prairies are typically large, flat areas covered with a mixture of grass species and a few herbs.

Predation:

An interaction between two organisms of different species in which one (usually larger) individual or species hunts, kills, and eats another individual or species.

Predator:

An animal that hunts and kills other animals for its food.

Primary succession:

The sequential development of communities on an area that was not previously occupied by organisms, such as the surface of a recent volcano or deglaciated land. See secondary succession

Producer:

Another term for autotroph. (Also called primary producer). See autotroph, consumer, decomposer

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Radical:

A group of atoms or a single atom with an unpaired electron and no charge. In general, radicals are quite reactive and as a consequence have short lifetimes. Radical species result from homolytic bond cleavage reactions such as the decomposition of CFCs when exposed to ultraviolet light: F2ClC-Cl+UV light à F2ClC · +Cl· Radicals have been implicated in the ozone destruction process.

Radioactive decay:

The process of causing radioactivity; the spontaneous or induced disintegration of a substance in which ionizing radiation such as alpha particles, beta particles, or gamma rays is given off. Radioactive substances have characteristic rates of radioactive decay called half-lives.

Reducing agent:

A chemical substance that is capable of donating electrons to another substance. A reducing agent carries out the process of reduction and is itself oxidized (i.e. it loses electrons). See oxidation, reduction

Reduction:

The opposite of oxidation; a chemical reaction in which a compound undergoes a decrease in oxidation state by gaining electrons. Reduction of one compound is always accompanied by oxidation of another compound. Many reduction reactions involve removing oxygen atoms from a compound or liberating metals from their compounds. See oxidation

Reservoir:

1) An artificial body of surface water that is retained by a dam. 2) A natural underground rock formation that retains water, oil, or natural gas.

Residence time:

The average length of time a molecule of a substance such as a nutrient takes to pass through a specific compartment of a model (such as a watershed) or ecosystem. For example, the residence time of water in a lake is calculated by dividing the volume of the lake by the mean annual water flux into the lake.

Respiration:

The process in which a living organism or cell utilizes oxygen to convert food compounds into carbon dioxide and water, releasing energy.

Retrovirus:

Any of several viruses that contain RNA and which are able to manufacture DNA copies of their RNA and to insert these into the chromosomes of their human or animal hosts. AIDS is cause by a retrovirus, and these microorganisms often produce cancerous tumors. See AIDS, HIV

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Secondary succession:

Ecological succession in an area where a climax terrestrial community (or other vegetation) previously existed and was removed, without removing or completely destroying the underlying soil. Secondary succession occurs after timber harvesting and abandonment of agricultural lands. See primary succession

Sediment:

A general term for any unconsolidated particulate material that has been deposited by an agent of transport, such as water, ice, or wind.

Sewage:

Aqueous discharge from municipal or industrial sanitary collection systems, especially as pertains to human fecal waste.

Sex Ratio:

The proportion of males and females within a specific population; it is usually expressed as the number of males per 100 females.

Smog:

Photochemical smog: The air pollution resulting when hydrocarbon and nitrogen oxides (NOx) are exposed to sunlight, causing them to undergo a photochemical reaction to produce much more harmful chemicals. Often used in a more general sense to refer to any visible air pollution, especially at levels high enough to reduce visibility and to cause irritation to eyes, lungs, etc. The word was coined in 1905 as a combination of smoke and fog. See photochemical smog

Species:

A naturally occurring population or a group of potentially interbreeding populations that is reproductively isolated (i.e. cannot exchange genetic material) from other such populations or groups. This definition does not apply to asexually reproducing forms such as many types of Monera or Protista, etc.

Soil:

1) A combination of mineral and organic matter with water and air above the bedrock surface. 2) The upper part of the regolith that supports life. 3) Earth material modified by physical, chemical and biological processes such that it supports rooted plant life.

Solar energy:

Any form of energy derived from the energy coming from the sun, excluding naturally modified plant and animal matter, such as coal, natural gas, or oil. Solar energy can be direct or indirect, and includes production of heat from solar collectors or passive solar design, production of electricity from photovoltaic cells, and indirect forms of solar energy such as wind power, burning of methanol, and heating with wood. See direct solar energy, indirect solar energy

(SOx):

A symbol for the group of sulfur oxide air pollutants, such as sulfur dioxide and sulfur trioxide. The full list is (SO2), (SO3), (S2O7)-4, (SO4)-2. They produce acid rain when they interact with water vapor in the atmosphere. Compare (NOx).

Steady-state:

1) A condition of equilibrium in which opposing forces balance so that they cancel each other out, as when inputs to a system balance outputs, or death rate balances birth rate to maintain a stable population size.

Stratosphere:

The region of the earth’s atmosphere between the troposphere and the mesosphere. It is characterized by horizontal winds and little temperature change as altitude increases. It extends from roughly 20 to 50 km above the earth’s surface. See troposhphere

Subdominant:

An important species within a community, but one that is less prevalent, smaller or of less prevalent, smaller, or of less importance than the dominant species. See dominance

Succession:

The gradual change between types of transient communities or ecosystems that involves changes in the plant and animal species composition. Given a sufficient period of time, and a lack of repeated interruptions, succession eventually leads to a steady-state community, a climax community. See climax, primary succession, secondary succession

Sulfuric Acid:

(SO3) A secondary air pollutant produced from sulfur dioxide (from burning sulfur-containing fuels) by the action of sunlight. Sulfur trioxide forms sulfuric acid when it dissolves in water or water vapor, producing acid rain.

Superphosphate:

A principal form of phosphorous used in plant fertilizers. It consists of rock phosphate that has been treated with sulfuric acid to make it more soluble and thus faster acting. Superphosphate contains a higher percentage of (P2O5), the form of phosphorous most easily assimilated by plants. Although superphosphates give fast, impressive results, they are more easily leached away than the slower-acting, untreated rock phosphate. They can thus more easily contribute to eutrophication.

Survival Ratio/Rate:

See survivorship

Survivorship:

1) The proportion of a given population that reaches a particular age. 2) (1x) The number of individuals in a particular age group counted at the beginning of a given period tat are still alive at the end of the period.

Susceptibility:

The degree to which an organism is prone to infection by a particular disease, or is sensitive to a particular drug or poison.

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Temperate deciduous forest:

The major biome found in regions with moderate temperatures, distinct seasons, and abundant rainfall (30-60 inches). It generally corresponds to some of the most densely populated regions of the globe and thus is often greatly modified. The forest has well-developed shrub and herb layers. In North America, this biome contains several important, distinct types of vegetation, including beech-maple and maple-basswoods of north-central states, oak-hickory forests in western and southern regions, oak (originally oak-chestnut) and mixed hardwood forests of the Appalachians, and the southern pine disclimax forest.

Temperate rainforest:

The conifer-dominated vegetation found along the coast from Alaska to central California, areas with high humidity (frequent fog), abundant rainfall (30 to 150 inches), and moderate temperatures restricted to a fairly small range. It has more understory than the northern boreal forests and includes the redwood forest of California. Also called moist coniferous forest.

Terrestrial:

Of or relating to the land rater than the water; the opposite of aquatic. Terrestrial organisms live or grow on the land.

Toxicity:

1) The potency of a poisonous substance, the degree to which it is harmful to organisms. 2) The amount of poison found in a substance or produced by an organism.

Toxic Threshold:

The level at which a substance causes poisonous effects or death.

Toxins:

Poisons (usually proteins) produced by living organisms, especially those capable of stimulating the production of antibodies. Botulism and tetanus are both caused by bacterially produced toxins.

Tragedy of the Commons:

Depletion or degredation of a resource to which people have free and unmanaged access, as in the depletion of a commercially desirable species of fish in the open sea beyond areas controlled by coastal countries. The concept was popularized by Garrett Hardin in the late 1960s in an essay by the same name.

Tropical rainforest:

A forest biome found in the tropics near the equator, in climates with continually warm to hot weather and very heavy rainfall. Tropical rainforests are green all year long, characterized by high species diversity, soils that are usually old and nutrient-poor, stratification into distinct layers of trees and shrubs, and many epiphytes and vines. The largest tropical rainforest is in the Amazon region, which covers a large portion of South America.

Troposphere:

The lowest layer of the earth’s atmosphere, extending from the earth’s surface up to about 7 to 17 km (5 to 10 mi). Within the troposphere, temperature usually decreases at a regular rate with increasing altitude. Weather is confined to this turbulent layer of the atmosphere. See Stratosphere

Turbulence:

Fluid flow characterized by constant, apparently random, small-scale changes in speed and direction. Also called turbulent flow.

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Vegetation:

All of the plants growing in and characterizing a specific area or region; the combination of different plant communities found there.

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Wet Deposition:

Particulate air pollutants from the atmosphere carried to the earth’s surface by some form of precipitation. A rainout (or washout) is a form of wet deposition.

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Zooplankton:

Animal species of plankton in freshwater or marine environments; plankton that do not produce their own energy but feed on phytoplankton (plant plankton) or smaller animal plankton. They are barely able to swim, if at all, and so are carried by water currents. Rotifers and microcrustaceans are zooplankton.
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