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.

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.

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.

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.

animals Eukaryotic, multicelled organisms usually capable of mobility and unable to produce their own food (heterotrophs), such as sponges, jellyfishes, arthropods (insects, shrimp, lobsters), mollusks (snails, clams, oysters,  octopuses), fish, amphibians (frogs, toads, salamanders), reptiles (turtles, lizards, alligators, crocodiles, snakes), birds, and mammals (kangaroos, bats, cats, rabbits, elephants, whales, porpoises, monkeys, apes, humans). See carnivores, herbivores, omnivores.

asexual reproduction Reproduction in which a mother cell divides to produce two identical daughter cells that are clones of the mother cell. This type of reproduction is common in single-celled organisms. Compare sexual reproduction.

atmosphere Whole mass of air surrounding the earth. See stratosphere, troposphere.

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.

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.

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.

biome Terrestrial regions inhabited by certain types of life, especially vegetation. Examples are various types of deserts, grasslands, and forests.

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.

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.

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.

climate Physical properties of the troposphere of an area based on analysis of its weather records over a long period (at least 30 years). The two main factors determining an area's climate are temperature, with its seasonal variations, and the amount and distribution of precipitation. Compare weather.

community Populations of all species living and interacting in an area at a particular time.

condensation nuclei Tiny particles on which droplets of water vapor can collect.

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.

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.

detritivore Consumer organism that feeds on detritus, parts of dead organisms, and castoff 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.

dew point Temperature at which condensation occurs for a given amount of water vapor.

dissolved oxygen Amount of oxygen gas (O2) dissolved in a given volume of water at a particular

(DO) content temperature and pressure, often expressed as a concentration in parts of oxygen per million parts of water. See biological oxygen demand.

DNA (deoxyribonucleic acid) Large molecules in the cells of organisms that carry genetic information in living organisms.

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.

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.

ecotone Transitional zone in which one type of ecosystem tends to merge with another ecosystem. See edge effect.

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.

fermentation See anaerobic respiration.

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.

freshwater life zones Aquatic systems where water with a dissolved salt concentration of less than 1% by volume accumulates on or flows through the surfaces of terrestrial biomes. Examples are (1) standing (lentic) bodies of fresh water such as lakes, ponds, and inland wetlands and (2) flowing (lotic) systems such as streams and rivers. Compare biome.

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.

fungi Eukaryotic, mostly multicellular organisms such as mushrooms, molds, and yeasts. As decomposers, they get the nutrients they need by secreting enzymes that speed up the breakdown of organic matter in the tissue of other living or dead organisms. Then they absorb the resulting nutrients.

genetic diversity Variability in the genetic makeup among individuals within a single species. See biodiversity. Compare ecological diversity, functional diversity, species diversity.

global warming Warming of the earth's atmosphere because of increases in the concentrations of one or more greenhouse gases primarily as a result of human activities. See greenhouse effect, greenhouse gases.

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.

herbivore Plant-eating organism. Examples are deer, sheep, grasshoppers, and zooplankton. Compare carnivore, omnivore.

heterotroph See consumer.

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.

infiltration Downward movement of water through soil.

invertebrates Animals that have no backbones. Compare vertebrates.

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.

microorganisms Organisms such as bacteria that are so small they can be seen only by using a microscope.

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.

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.

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.

nutrient Any food, element, or compound an organism must take in to live, grow, or reproduce.

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.

organism Any form of life.

percolation Passage of a liquid through the spaces of a porous material such as soil.

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.

phytoplankton Small, drifting plants, mostly algae and bacteria, found in aquatic ecosystems. Compare plankton, zooplankton.

plankton Small plant organisms (phytoplankton) and animal organisms (zooplankton) that float in aquatic ecosystems.

plants (plantae) Eukaryotic, mostly multicellular organisms such as algae (red, blue, and green), mosses, ferns, flowers, cacti, grasses, beans, wheat, rice, and trees. These organisms use photosynthesis to produce organic nutrients for themselves and for other organisms feeding on them. Water and other inorganic nutrients are obtained from the soil for terrestrial plants and from the water for aquatic plants.

population Group of individual organisms of the same species living in a particular area.

precipitation Water in the form of rain, sleet, hail, and snow that falls from the atmosphere onto the land and bodies of water.

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.

prokaryotic cell Cell that does not have a distinct nucleus. Other internal parts are also not enclosed by membranes. Compare eukaryotic cell.

protists (protista) Eukaryotic, mostly single-celled organisms such as diatoms, amoebas, some algae (golden brown and yellow-green), protozoans, and slime molds. Some protists produce their own organic nutrients through photosynthesis. Others are decomposers and some feed on bacteria, other protists, or cells of multicellular organisms.

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.

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.

relative humidity Amount of water vapor in a certain mass of air, expressed as a percentage of the maximum amount it could hold at that temperature. Compare absolute humidity.

respiration See aerobic respiration.

runoff Fresh water from precipitation and melting ice that flows on the earth's surface into nearby streams, lakes, wetlands, and reservoirs. See reliable runoff, surface runoff, surface water. Compare groundwater.

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.

secondary consumer Organism that feeds only on primary consumers. Compare detritivore, omnivore, primary consumer.

sexual reproduction Reproduction in organisms that produce offspring by combining sex cells, or gametes (such as ovum and sperm), from both parents. This produces offspring that have combinations of traits from their parents. Compare asexual reproduction.

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.

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.

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.

tertiary (higherlevel) 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.

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.

vertebrates Animals that have backbones. Compare invertebrates.

water cycle See hydrologic cycle.

zooplankton Animal plankton. Small floating herbivores that feed on plant plankton (phytoplankton). Compare phytoplankton.