02 Glossary

Glossary	Ch01 Ch02 Ch03 Ch04 Ch05 Ch06 Ch07 Ch08 Ch09 Ch10 Ch11 Ch12 Ch13 Ch14
Chapter 2
abiotic	Nonliving. Compare biotic.
absolute humidity	Amount of water vapor found in a certain mass of air (usually expressed as grams of water per kilogram of air). Compare relative humidity.
acclimation	Adjustment to slowly changing new conditions. Compare threshold effect.
accuracy	Extent to which a measurement agrees with the accepted or correct value for that quantity, based on careful measurements by many people. Compare precision.
acid	See acid solution.
acid deposition	The falling of acids and acid-forming compounds from the atmosphere to the earth's surface. Acid deposition is commonly known as acid rain, a term that refers only to wet deposition of droplets of acids and acid-forming compounds.
acid rain	See acid deposition.
acid solution	Any water solution that has more hydrogen ions (H⁺) than hydroxide ions (OH^-); any water solution with a pH less than 7. Compare basic solution, neutral solution.
aerobic respiration	Complex process that occurs in the cells of most living organisms, in which nutrient organic molecules such as glucose (C₆H₁₂O₆) combine with oxygen (O₂) and produce carbon dioxide (CO₂), water (H₂O), and energy. Compare photosynthesis.
alpha particle	Positively charged matter, consisting of two neutrons and two protons, that is emitted as a form of radioactivity from the nuclei of some radioisotopes. See also beta particle, gamma rays.
anaerobic respiration	Form of cellular respiration in which some decomposers get the energy they need through the breakdown of glucose (or other nutrients) in the absence of oxygen. Compare aerobic respiration.
aquatic	Pertaining to water. Compare terrestrial.
atmosphere	Whole mass of air surrounding the earth. See stratosphere, troposphere.
atom	Minute unit made of subatomic particles that is the basic building block of all chemical elements and thus all matter; the smallest unit of an element that can exist and still have the unique characteristics of that element. Compare ion, molecule.
atomic number	Number of protons in the nucleus of an atom. Compare mass number.
autotroph	See producer.
bacteria	Prokaryotic, one-celled organisms. Some transmit diseases. Most act as decomposers and get the nutrients they need by breaking down complex organic compounds in the tissues of living or dead organisms into simpler inorganic nutrient compounds.
benthos	Bottom-dwelling organisms. Compare decomposer, nekton, plankton.
beta particle	Swiftly moving electron emitted by the nucleus of a radioactive isotope. See also alpha particle, gamma rays.
biodegradable	Capable of being broken down by decomposers.
biodiversity	Variety of different species (species diversity), genetic variability among individuals within each species (genetic diversity), variety of ecosystems (ecological diversity), and functions such as energy flow and matter cycling needed for the survival of species and biological communities (functional diversity).
biogeochemical cycle	Natural processes that recycle nutrients in various chemical forms from the nonliving environment to living organisms and then back to the nonliving environment. Examples are the carbon, oxygen, nitrogen, phosphorus, sulfur, and hydrologic cycles.
bioinformatics	Applied science of managing, analyzing, and communicating biological information.
biological community	See community.
biological diversity	See biodiversity.
biomass	Organic matter produced by plants and other photosynthetic producers; total dry weight of all living organisms that can be supported at each trophic level in a food chain or web; dry weight of all organic matter in plants and animals in an ecosystem; plant materials and animal wastes used as fuel.
biosphere	Zone of earth where life is found. It consists of parts of the atmosphere (the troposphere), hydrosphere (mostly surface water and groundwater), and lithosphere (mostly soil and surface rocks and sediments on the bottoms of oceans and other bodies of water) where life is found. Sometimes called the ecosphere.
biotic	Living organisms. Compare abiotic.
calorie	Unit of energy; amount of energy needed to raise the temperature of 1 gram of water 1 degree C (unit on Celsius temperature scale). See also kilocalorie.
carbon cycle	Cyclic movement of carbon in different chemical forms from the environment to organisms and then back to the environment.
carnivore	Animal that feeds on other animals. Compare herbivore, omnivore.
cell	Smallest living unit of an organism. Each cell is encased in an outer membrane or wall and contains genetic material (DNA) and other parts to perform its life function. Organisms such as bacteria consist of only one cell, but most of the organisms we are familiar with contain many cells. See eukaryotic cell, prokaryotic cell.
chemical	One of the millions of different elements and compounds found naturally and synthesized by humans. See compound, element.
chemical change	Interaction between chemicals in which there is a change in the chemical composition of the elements or compounds involved. Compare nuclear change, physical change.
chemical evolution	Formation of the earth and its early crust and atmosphere, evolution of the biological molecules necessary for life, and evolution of systems of chemical reactions needed to produce the first living cells. These processes are believed to have occurred about 1 billion years before biological evolution. Compare biological evolution.
chemical formula	Shorthand way to show the number of atoms (or ions) in the basic structural unit of a compound. Examples are H₂O, NaCl, and C₆H₁₂O₆.
chemical reaction	See chemical change.
chemosynthesis	Process in which certain organisms (mostly specialized bacteria) extract inorganic compounds from their environment and convert them into organic nutrient compounds without the presence of sunlight. Compare photosynthesis.
chlorinated hydrocarbon	Organic compound made up of atoms of carbon, hydrogen, and chlorine. Examples are DDT and PCBs.
chromosome	Grouping of various genes and associated proteins in plant and animal cells that carry certain types of genetic information. See genes.
closed system	System in which energy but not matter is exchanged between the system and its environment. Compare open system.
community	Populations of all species living and interacting in an area at a particular time.
compound	Combination of atoms, or oppositely charged ions, of two or more different elements held together by attractive forces called chemical bonds. Compare element.
consensus science	Scientific data, models, theories, and laws that are widely accepted by scientists considered experts in the area of study. These results of science are very reliable. Compare frontier science.
consumer	Organism that cannot synthesize the organic nutrients it needs and gets its organic nutrients by feeding on the tissues of producers or of other consumers; generally divided into primary consumers (herbivores), secondary consumers (carnivores), tertiary (higher-level) consumers, omnivores, and detritivores (decomposers and detritus feeders). In economics, one who uses economic goods.
convergent plate boundary	Area where earth's lithospheric plates are pushed together. See subduction zone. Compare divergent plate boundary, transform fault.
core	Inner zone of the earth. It consists of a solid inner core and a liquid outer core. Compare crust, mantle.
crust	Solid outer zone of the earth. It consists of oceanic crust and continental crust. Compare core, mantle.
cyanobacteria	Single-celled, prokaryotic, microscopic organisms. Before being reclassified as monera, they were called blue-green algae.
decomposer	Organism that digests parts of dead organisms and cast-off fragments and wastes of living organisms by breaking down the complex organic molecules in those materials into simpler inorganic compounds and then absorbing the soluble nutrients. Producers return most of these chemicals to the soil and water for reuse. Decomposers consist of various bacteria and fungi. Compare consumer, detritivore, producer.
deductive reasoning	Using logic to arrive at a specific conclusion based on a generalization or premise. It goes from the general to the specific. Compare inductive reasoning.
detritivore	Consumer organism that feeds on detritus, parts of dead organisms, and cast-off fragments and wastes of living organisms. The two principal types are detritus feeders and decomposers.
detritus	Parts of dead organisms and cast-off fragments and wastes of living organisms.
detritus feeder	Organism that extracts nutrients from fragments of dead organisms and their cast-off parts and organic wastes. Examples are earthworms, termites, and crabs. Compare decomposer.
deuterium (D; hydrogen-2)	Isotope of the element hydrogen, with a nucleus containing one proton and one neutron and a mass number of 2.
dissolved oxygen (DO) content	Amount of oxygen gas (O₂) dissolved in a given volume of water at a particular temperature and pressure, often expressed as a concentration in parts of oxygen per million parts of water. See biological oxygen demand.
divergent plate boundary	Area where earth's lithospheric plates move apart in opposite directions. Compare convergent plate boundary, transform fault.
DNA (deoxyribonucleic acid)	Large molecules in the cells of organisms that carry genetic information in living organisms.
earthquake	Shaking of the ground resulting from the fracturing and displacement of rock, which produces a fault, or from subsequent movement along the fault.
ecological diversity	Variety of forests, deserts, grasslands, oceans, streams, lakes, and other biological communities interacting with one another and with their nonliving environment. See biodiversity. Compare functional diversity, genetic diversity, species diversity.
ecological efficiency	Percentage of energy transferred from one trophic level to another in a food chain or web.
ecology	Study of the interactions of living organisms with one another and with their nonliving environment of matter and energy; study of the structure and functions of nature.
economic system	Method that a group of people uses to choose (1) what goods and services to produce, (2) how to produce them, (3) how much to produce, and (4) how to distribute them to people. See capitalist market economic system, pure command economic system, pure free-market economic system.
ecosphere	See biosphere.
ecosystem	Community of different species interacting with one another and with the chemical and physical factors making up its nonliving environment.
ecosystem services	Natural services or natural capital that support life on the earth and are essential to the quality of human life and the functioning of the world's economies. See natural resources.
electromagnetic radiation	Forms of kinetic energy traveling as electromagnetic waves. Examples are radio waves, TV waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X rays, and gamma rays. Compare ionizing radiation, nonionizing radiation.
electron (e)	Tiny particle moving around outside the nucleus of an atom. Each electron has one unit of negative charge and almost no mass. Compare neutron, proton.
element	Chemical, such as hydrogen (H), iron (Fe), sodium (Na), carbon (C), nitrogen (N), or oxygen (O), whose distinctly different atoms serve as the basic building blocks of all matter. There are 92 naturally occurring elements. Another 23 have been made in laboratories. Two or more elements combine to form compounds that make up most of the world's matter. Compare compound.
energy	Capacity to do work by performing mechanical, physical, chemical, or electrical tasks or to cause a heat transfer between two objects at different temperatures.
energy efficiency	Percentage of the total energy input that does useful work and is not converted into low-quality, usually useless heat in an energy conversion system or process. See energy quality, net energy. Compare material efficiency.
energy productivity	See energy efficiency.
energy quality	Ability of a form of energy to do useful work. High-temperature heat and the chemical energy in fossil fuels and nuclear fuels are concentrated high-quality energy. Low-quality energy such as low-temperature heat is dispersed or diluted and cannot do much useful work. See high-quality energy, low-quality energy.
erosion	Process or group of processes by which loose or consolidated earth materials are dissolved, loosened, or worn away and removed from one place and deposited in another. See weathering.
eukaryotic cell	Cell containing a nucleus, a region of genetic material surrounded by a membrane. Membranes also enclose several of the other internal parts found in a eukaryotic cell. Compare prokaryotic cell.
evaporation	Conversion of a liquid into a gas.
experiment	Procedure a scientist uses to study some phenomenon under known conditions. Scientists conduct some experiments in the laboratory and others in nature. The resulting scientific data or facts must be verified or confirmed by repeated observations and measurements, ideally by several different investigators.
feedback loop	Circuit of sensing, evaluating, and reacting to changes in environmental conditions as a result of information fed back into a system; it occurs when one change leads to some other change, which eventually reinforces or slows the original change. See negative feedback loop, positive feedback loop.
fermentation	See anaerobic respiration.
first law of energy	See first law of thermodynamics.
first law of thermodynamics	In any physical or chemical change, no detectable amount of energy is created or destroyed, but in these processes energy can be changed from one form to another; you cannot get more energy out of something than you put in; in terms of energy quantity, you cannot get something for nothing (there is no free lunch). This law does not apply to nuclear changes, in which energy can be produced from small amounts of matter. See also second law of thermodynamics.
flows	See throughputs.
food chain	Series of organisms in which each eats or decomposes the preceding one. Compare food web.
food web	Complex network of many interconnected food chains and feeding relationships. Compare food chain.
fossil fuel	Products of partial or complete decomposition of plants and animals that occur as crude oil, coal, natural gas, or heavy oils as a result of exposure to heat and pressure in the earth's crust over millions of years. See coal, crude oil, natural gas.
fossils	Skeletons, bones, shells, body parts, leaves, seeds, or impressions of such items that provide recognizable evidence of organisms that lived long ago.
frontier science	Preliminary scientific data, hypotheses, and models that have not been widely tested and accepted. Compare consensus science.
functional diversity	Biological and chemical processes or functions such as energy flow and matter cycling needed for the survival of species and biological communities. See biodiversity, ecological diversity, genetic diversity, species diversity.
Gaia hypothesis	Hypothesis that the Earth is alive and can be considered a system that operates and changes by feedback of information between its living and nonliving components.
gamma rays	Form of ionizing electromagnetic radiation with a high energy content emitted by some radioisotopes. They readily penetrate body tissues. See also alpha particle, beta particle.
genes	Coded units of information about specific traits that are passed on from parents to offspring during reproduction. They consist of segments of DNA molecules found in chromosomes.
genetic diversity	Variability in the genetic makeup among individuals within a single species. See biodiversity. Compare ecological diversity, functional diversity, species diversity.
geology	Study of the earth's dynamic history. Geologists study and analyze rocks and the features and processes of the earth's interior and surface.
gross primary productivity (GPP)	The rate at which an ecosystem's producers capture and store a given amount of chemical energy as biomass in a given length of time. Compare net primary productivity.
habitat	Place or type of place where an organism or population of organisms lives. Compare ecological niche.
half-life	Time needed for one-half of the nuclei in a radioisotope to emit its radiation. Each radioisotope has a characteristic half-life, which may range from a few millionths of a second to several billion years. See radioisotope.
heat	Total kinetic energy of all the randomly moving atoms, ions, or molecules within a given substance, excluding the overall motion of the whole object. Heat always flows spontaneously from a hot sample of matter to a colder sample of matter. This is one way to state the second law of thermodynamics. Compare temperature.
herbivore	Plant-eating organism. Examples are deer, sheep, grasshoppers, and zooplankton. Compare carnivore, omnivore.
heterotroph	See consumer.
high-quality energy	Energy that is concentrated and has great ability to perform useful work. Examples are high-temperature heat and the energy in electricity, coal, oil, gasoline, sunlight, and nuclei of uranium-235. Compare low-quality energy.
high-quality matter	Matter that is concentrated and contains a high concentration of a useful resource. Compare low-quality matter.
homeostasis	Maintenance of favorable internal conditions in a system despite fluctuations in external conditions. See constancy, inertia, resilience.
hydrocarbon	Organic compound of hydrogen and carbon atoms. The simplest hydrocarbon is methane (CH₄), the major component of natural gas.
hydrologic cycle	Biogeochemical cycle that collects, purifies, and distributes the earth's fixed supply of water from the environment to living organisms and then back to the environment.
hydrosphere	The earth's (1) liquid water (oceans, lakes, other bodies of surface water, and underground water), (2) frozen water (polar ice caps, floating ice caps, and ice in soil, known as permafrost), and (3) small amounts of water vapor in the atmosphere. See also hydrologic cycle.
igneous rock	Rock formed when molten rock material (magma) wells up from the earth's interior, cools, and solidifies into rock masses. See rock cycle. Compare metamorphic rock, sedimentary rock.
inductive reasoning	Using observations and facts to arrive at generalizations or hypotheses. It goes from the specific to the general and is widely used in science. Compare deductive reasoning.
inorganic compounds	All compounds not classified as organic compounds. See organic compounds.
input	Matter, energy, or information entering a system. Compare output, throughput.
ion	Atom or group of atoms with one or more positive (1) or negative (2) electrical charges. Compare atom, molecule.
ionizing radiation	Fast-moving alpha or beta particles or high-energy radiation (gamma rays) emitted by radioisotopes. They have enough energy to dislodge one or more electrons from atoms they hit, forming charged ions in tissue that can react with and damage living tissue. Compare nonionizing radiation.
isotopes	Two or more forms of a chemical element that have the same number of protons but different mass numbers because they have different numbers of neutrons in their nuclei.
kinetic energy	Energy that matter has because of its mass and speed or velocity. Compare potential energy.
law of conservation of energy	See first law of thermodynamics.
law of conservation of matter	In any physical or chemical change, matter is neither created nor destroyed but merely changed from one form to another; in physical and chemical changes, existing atoms are rearranged into different spatial patterns (physical changes) or different combinations (chemical changes).
law of tolerance	Existence, abundance, and distribution of a species in an ecosystem are determined by whether the levels of one or more physical or chemical factors fall within the range tolerated by the species. See threshold effect.
limiting factor	Single factor that limits the growth, abundance, or distribution of the population of a species in an ecosystem. See limiting factor principle.
limiting factor principle	Too much or too little of any abiotic factor can limit or prevent growth of a population of a species in an ecosystem, even if all other factors are at or near the optimum range of tolerance for the species. See range of tolerance.
lithosphere	Outer shell of the earth, composed of the crust and the rigid, outermost part of the mantle outside the asthenosphere; material found in earth's plates. See crust, mantle.
low-quality energy	Energy that is dispersed and has little ability to do useful work. An example is low-temperature heat. Compare high-quality energy.
low-quality matter	Matter that is dilute or dispersed or contains a low concentration of a useful resource. Compare high-quality matter.
magma	Molten rock below the earth's surface.
mantle	Zone of the earth's interior between its core and its crust. See lithosphere. Compare core, crust* .*
mass	Amount of material in an object.
mass number	Sum of the number of neutrons (n) and the number of protons (p) in the nucleus of an atom. It gives the approximate mass of that atom. Compare atomic number.
material efficiency	Total amount of material needed to produce each unit of goods or services. Also called resource productivity. Compare energy efficiency.
matter	Anything that has mass (the amount of material in an object) and takes up space. On the earth, where gravity is present, we weigh an object to determine its mass.
matter quality	Measure of how useful a matter resource is, based on its availability and concentration. See high-quality matter, low-quality matter.
metabolism	Ability of a living cell or organism to capture and transform matter and energy from its environment to supply its needs for survival, growth, and reproduction.
metamorphic rock	Rock produced when a preexisting rock is subjected to high temperatures (which may cause it to melt partially), high pressures, chemically active fluids, or a combination of these agents. See rock cycle. Compare igneous rock, sedimentary rock.
microorganisms	Organisms such as bacteria that are so small they can be seen only by using a microscope.
mixture	Combination of one or more elements and compounds.
model	Approximate representation or simulation of a system being studied.
molecule	Combination of two or more atoms of the same chemical element (such as O₂) or different chemical elements (such as H₂O) held together by chemical bonds. Compare atom, ion.
monera	See bacteria, cyanobacteria.
natural greenhouse effect	Heat buildup in the troposphere because of the presence of certain gases, called greenhouse gases. Without this effect, the earth would be nearly as cold as Mars, and life as we know it could not exist. Compare global warming.
natural ionizing radiation	Ionizing radiation in the environment from natural sources. See ionizing radiation.
natural law	See scientific law.
natural radioactive decay	Nuclear change in which unstable nuclei of atoms spontaneously shoot out particles (usually alpha or beta particles) or energy (gamma rays) at a fixed rate.
negative feedback loop	Situation in which a change in a certain direction provides information that causes a system to change less in that direction. Compare positive feedback loop.
net primary productivity (NPP)	Rate at which all the plants in an ecosystem produce net useful chemical energy; equal to the difference between the rate at which the plants in an ecosystem produce useful chemical energy (gross primary productivity) and the rate at which they use some of that energy through cellular respiration. Compare gross primary productivity.
neutral solution	Water solution containing an equal number of hydrogen ions (H⁺) and hydroxide ions (OH^-); water solution with a pH of 7. Compare acid solution, basic solution.
neutron (n)	Elementary particle in the nuclei of all atoms (except hydrogen-1). It has a relative mass of 1 and no electric charge. Compare electron, proton.
nitrogen cycle	Cyclic movement of nitrogen in different chemical forms from the environment to organisms and then back to the environment.
nitrogen fixation	Conversion of atmospheric nitrogen gas into forms useful to plants by lightning, bacteria, and cyanobacteria; part of the nitrogen cycle.
nucleus	Extremely tiny center of an atom, making up most of the atom's mass. It contains one or more positively charged protons and one or more neutrons with no electrical charge (except for a hydrogen-1 atom, which has one proton and no neutrons in its nucleus).
nutrient cycle	See biogeochemical cycle.
omnivore	Animal that can use both plants and other animals as food sources. Examples are pigs, rats, cockroaches, and people. Compare carnivore, herbivore.
open system	System, such as a living organism, in which both matter and energy are exchanged between the system and the environment. Compare closed system.
organic compounds	Compounds containing carbon atoms combined with each other and with atoms of one or more other elements such as hydrogen, oxygen, nitrogen, sulfur, phosphorus, chlorine, and fluorine. All other compounds are called inorganic compounds.
organism	Any form of life.
output	Matter, energy, or information leaving a system. Compare input, throughput.
percolation	Passage of a liquid through the spaces of a porous material such as soil.
permafrost	Perennially frozen layer of the soil that forms when the water there freezes. It is found in arctic tundra.
permeability	Degree to which underground rock and soil pores are interconnected and thus a measure of the degree to which water can flow freely from one pore to another. Compare porosity.
phosphorus cycle	Cyclic movement of phosphorus in different chemical forms from the environment to organisms and then back to the environment.
photosynthesis	Complex process that takes place in cells of green plants. Radiant energy from the sun is used to combine carbon dioxide (CO₂) and water (H₂O) to produce oxygen (O₂) and carbohydrates (such as glucose, C₆H₁₂O₆) and other nutrient molecules. Compare aerobic respiration, chemosynthesis.
physical change	Process that alters one or more physical properties of an element or a compound without altering its chemical composition. Examples are changing the size and shape of a sample of matter (crushing ice and cutting aluminum foil) and changing a sample of matter from one physical state to another (boiling and freezing water). Compare chemical change, nuclear change.
phytoplankton	Small, drifting plants, mostly algae and bacteria, found in aquatic ecosystems. Compare plankton, zooplankton.
plasma	An ionized gas consisting of electrically conductive ions and electrons. It is known as a fourth state of matter.
plate tectonics	Theory of geophysical processes that explains the movements of lithospheric plates and the processes that occur at their boundaries. See lithosphere, tectonic plates.
plates	See tectonic plates.
population	Group of individual organisms of the same species living in a particular area.
porosity	Percentage of space in rock or soil occupied by voids, whether the voids are isolated or connected. Compare permeability.
positive feedback loop	Situation in which a change in a certain direction provides information that causes a system to change further in the same direction. Compare negative feedback loop.
potential energy	Energy stored in an object because of its position or the position of its parts. Compare kinetic energy.
precision	Measure of reproducibility, or how closely a series of measurements of the same quantity agree with one another. Compare accuracy.
primary consumer	Organism that feeds on all or part of plants (herbivore) or on other producers. Compare detritivore, omnivore, secondary consumer.
primary productivity	See gross primary productivity, net primary productivity.
producer	Organism that uses solar energy (green plant) or chemical energy (some bacteria) to manufacture the organic compounds it needs as nutrients from simple inorganic compounds obtained from its environment. Compare consumer, decomposer.
proton (p)	Positively charged particle in the nuclei of all atoms. Each proton has a relative mass of 1 and a single positive charge. Compare electron, neutron.
pyramid of biomass	Diagram representing the biomass, or total dry weight of all living organisms, that can be supported at each trophic level in a food chain or food web. See pyramid of energy flow, pyramid of numbers.
pyramid of energy flow	Diagram representing the flow of energy through each trophic level in a food chain or food web. With each energy transfer, only a small part (typically 10%) of the usable energy entering one trophic level is transferred to the organisms at the next trophic level. Compare pyramid of biomass, pyramid of numbers.
pyramid of numbers	Diagram representing the number of organisms of a particular type that can be supported at each trophic level from a given input of solar energy at the producer trophic level in a food chain or food web. Compare pyramid of biomass, pyramid of energy flow.
radiation	Fast-moving particles (particulate radiation) or waves of energy (electromagnetic radiation). See alpha particle, beta particle, gamma rays.
range of tolerance	Range of chemical and physical conditions that must be maintained for populations of a particular species to stay alive and grow, develop, and function normally. See law of tolerance.
resource productivity	See material efficiency.
respiration	See aerobic respiration.
rock	Any material that makes up a large, natural, continuous part of earth's crust. See igneous rock, metamorphic rock, mineral, sedimentary rock.
rock cycle	Largest and slowest of the earth's cycles, consisting of geologic, physical, and chemical processes that form and modify rocks and soil in the earth's crust over millions of years.
salinity	Amount of various salts dissolved in a given volume of water.
scavenger	Organism that feeds on dead organisms that were killed by other organisms or died naturally. Examples are vultures, flies, and crows. Compare detritivore.
science	Attempts to discover order in nature and use that knowledge to make predictions about what should happen in nature. See consensus science, frontier science, scientific data, scientific hypothesis, scientific law, scientific methods, scientific model, scientific theory.
scientific data	Facts obtained by making observations and measurements. Compare scientific hypothesis, scientific law, scientific methods, scientific model, scientific theory.
scientific hypothesis	Educated guess that attempts to explain a scientific law or certain scientific observations. Compare scientific data, scientific law, scientific methods, scientific model, scientific theory.
scientific law	Description of what scientists find happening in nature repeatedly in the same way, without known exception. See first law of thermodynamics, law of conservation of matter, second law of thermodynamics. Compare scientific data, scientific hypothesis, scientific methods, scientific model, scientific theory.
scientific methods	Ways that scientists gather data and formulate and test scientific hypotheses, models, theories, and laws. See scientific data, scientific hypothesis, scientific law, scientific model, scientific theory.
scientific model	Simulation of complex processes and systems. Many are mathematical models that are run and tested using computers.
scientific theory	Well-tested and widely accepted scientific hypothesis. Compare scientific data, scientific hypothesis, scientific law, scientific methods, scientific model.
second law of thermodynamics	In any conversion of heat energy to useful work, some of the initial energy input is always degraded to a lower-quality, more dispersed, less useful energy, usually low-temperature heat that flows into the environment; you cannot break even in terms of energy quality. See first law of thermodynamics.
secondary consumer	Organism that feeds only on primary consumers. Compare detritivore, omnivore, primary consumer.
sedimentary rock	Rock that forms from the accumulated products of erosion and in some cases from the compacted shells, skeletons, and other remains of dead organisms. See rock cycle. Compare igneous rock, metamorphic rock.
species	Group of organisms that resemble one another in appearance, behavior, chemical makeup and processes, and genetic structure. Organisms that reproduce sexually are classified as members of the same species only if they can actually or potentially interbreed with one another and produce fertile offspring.
species diversity	Number of different species and their relative abundances in a given area. See biodiversity. Compare ecological diversity, genetic diversity.
stability	Ability of a living system to withstand or recover from externally imposed changes or stresses. See constancy, inertia, resilience.
storage area	Place within a system where energy, matter, or information can accumulate for various lengths of time before being released. Compare input, output, throughput.
stratosphere	Second layer of the atmosphere, extending about 17-48 kilometers (11-30 miles) above the earth's surface. It contains small amounts of gaseous ozone (O₃), which filters out about 95% of the incoming harmful ultraviolet (UV) radiation emitted by the sun. Compare troposphere.
subatomic particles	Extremely small particles--electrons, protons, and neutrons--that make up the internal structure of atoms.
subduction zone	Area in which oceanic lithosphere is carried downward (subducted) under the island arc or continent at a convergent plate boundary. A trench ordinarily forms at the boundary between the two converging plates. See convergent plate boundary.
subsidence	Slow or rapid sinking of part of the earth's crust that is not slope related.
sulfur cycle	Cyclic movement of sulfur in different chemical forms from the environment to organisms and then back to the environment.
surface runoff	Water flowing off the land into bodies of surface water. See reliable runoff.
synergistic interaction	Interaction of two or more factors or processes so the combined effect is greater than the sum of their separate effects.
synergy	See synergistic interaction.
system	Set of components that function and interact in some regular and theoretically predictable manner.
tectonic plates	Various-sized areas of the earth's lithosphere that move slowly around the mantle's flowing asthenosphere. Most earthquakes and volcanoes occur around the boundaries of these plates. See lithosphere, plate tectonics.
temperature	Measure of the average speed of motion of the atoms, ions, or molecules in a substance or combination of substances at a given moment. Compare heat.
terrestrial	Pertaining to land. Compare aquatic.
tertiary (higher-level) consumers	Animals that feed on animal-eating animals. They feed at high trophic levels in food chains and webs. Examples are hawks, lions, bass, and sharks. Compare detritivore, primary consumer, secondary consumer.
threshold effect	Harmful or fatal effect of a small change in environmental conditions that exceeds the limit of tolerance of an organism or population of a species. See law of tolerance.
throughput	Rate of flow of matter, energy, or information through a system. Compare input, output.
time delay	Time lag between the input of a stimulus into a system and the response to the stimulus.
tolerance limits	Minimum and maximum limits for physical conditions (such as temperature) and concentrations of chemical substances beyond which no members of a particular species can survive. See law of tolerance.
transform fault	Area where the earth's lithospheric plates move in opposite but parallel directions along a fracture (fault) in the lithosphere. Compare convergent plate boundary, divergent plate boundary.
transpiration	Process in which water (1) is absorbed by the root systems of plants, (2) moves up through the plants, (3) passes through pores (stomata) in their leaves or other parts, and (4) evaporates into the atmosphere as water vapor.
trophic level	All organisms that are the same number of energy transfers away from the original source of energy (for example, sunlight) that enters an ecosystem. For example, all producers belong to the first trophic level, and all herbivores belong to the second trophic level in a food chain or a food web.
troposphere	Innermost layer of the atmosphere. It contains about 75% of the mass of earth's air and extends about 17 kilometers (11 miles) above sea level. Compare stratosphere.
volcano	Vent or fissure in the earth's surface through which magma, liquid lava, and gases are released into the environment.
water cycle	See hydrologic cycle.
weathering	Physical and chemical processes in which solid rock exposed at earth's surface is changed to separate solid particles and dissolved material, which can then be moved to another place as sediment. See erosion.