PANs Peroxyacyl nitrates. Group of chemicals found in photochemical smog.

Parasite Consumer organism that lives on or in and feeds on a living plant or animal, known as the host, over an extended period of time. The parasite draws nourishment from and gradually weakens its host; it may or may not kill the host. See parasitism.

parasitism Interaction between species in which one organism, called the parasite, preys on another organism, called the host, by living on or in the host. See host, parasite.

parts per billion (ppb)
parts per million (ppm)
parts per trillion (ppt)

Number of parts of a chemical found in 1 billion parts of a particular gas, liquid, or solid.
Number of parts of a chemical found in 1 million parts of a particular gas, liquid, or solid.
Number of parts of a chemical found in 1 trillion parts of a particular gas, liquid,or solid.

passive solar heating system System that captures sunlight directly within a structure and converts it into low temperature heat for space heating or for heating water for domestic use without the use of mechanical devices. Compare active solar heating system.

pasture Managed grassland or enclosed meadow that usually is planted with domesticated grasses or other forage to be grazed by livestock. Compare feedlot, rangeland.

pathogen Organism that produces disease.

per capita GNI Annual gross national income (GNI) of a country divided by its total population. See gross national income. Also per capita GNP.

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

perennial Plant that can live for more than 2 years. Compare annual.

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.

perpetual resource Essentially inexhaustible resource on a human time scale. Solar energy is an example. See renewable resource. Compare nonrenewable resource, renewable resource.

persistence How long a pollutant stays in the air, water, soil, or body. See also inertia.

pest Unwanted organism that directly or indirectly interferes with human activities.

pesticide Any chemical designed to kill or inhibit the growth of an organism that people consider undesirable. See fungicide, herbicide, insecticide.

Petrochemicals Chemicals obtained by refining (distilling) crude oil. They are used as raw materials in manufacturing most industrial chemicals, fertilizers, pesticides, plastics, synthetic fibers, paints, medicines, and many other products.

phosphorus cycle Cyclic movement of phosphorus in different chemical forms from the environment to organisms and then back to the environment.

photochemical smog Complex mixture of air pollutants produced in the lower atmosphere by the reaction of hydrocarbons and nitrogen oxides under the influence of sunlight. Especially harmful components include ozone, peroxyacyl nitrates (PANs), and various aldehydes. Compare industrial smog.

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.

photovoltaic cell (solar cell) Device which converts radiant (solar) energy directly into electrical energy.

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.

pioneer community First integrated set of plants, animals, and decomposers found in an area undergoing primary ecological succession. See immature community, mature community.

pioneer species First hardy species, often microbes, mosses, and lichens that begin colonizing a site as the first stage of ecological succession. See ecological succession, pioneer community.

plaintiff The individual, group of individuals, corporation, or government agency bringing the charges in a lawsuit. Compare defendant.

planetary management worldview Beliefs that (1) we are the planet's most important species; (2) there are always more resources, and they are all for us; (3) all economic growth is good, more economic growth is better, and the potential for economic growth is limitless; and (4) our success depends on how well we can understand, control, and manage the earth's life-support systems for our own benefit. See spaceship-earth worldview. Compare environmental wisdom worldview.

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

plantation agriculture Growing specialized crops such as bananas, coffee, and cacao in tropical developing countries, primarily for sale to developed countries.

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.

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.

point source Single identifiable source that discharges pollutants into the environment. Examples are the (1) smokestack of a power plant or an industrial plant, (2) drainpipe of a meatpacking plant, (3) chimney of a house, or (4) exhaust pipe of an automobile. Compare nonpoint source.

Poison Chemical that in one dose kills exactly 50% of the animals (usually rats and mice) in a test population (usually 60 to 200 animals) within a 14-day period. See median lethal dose.

Politics Process through which individuals and groups try to influence or control government policies and actions that affect the local, state, national, and international communities.

pollutant Particular chemical or form of energy that can adversely affect the health, survival, or activities of humans or other living organisms. See pollution.

Pollution Undesirable change in the physical, chemical, or biological characteristics of air, water, soil, or food that can adversely affect the health, survival, or activities of humans or other living organisms.

pollution cleanup Device or process that removes or reduces the level of a pollutant after it has been produced or has entered the environment. Examples are automobile emission control devices and sewage treatment plants. Compare pollution prevention.

pollution prevention Device or process that (1) prevents a potential pollutant from forming or entering the environment or (2) sharply reduces the amount entering the environment. Also used input pollution control. Compare pollution cleanup.

polychlorinated biphenyls (PCBs) Group of 209 different toxic, oily, synthetic chlorinated hydrocarbon compounds that can be biologically amplified in food chains and webs.

Polyculture Complex form of intercropping in which a large number of different plants maturing at different times are planted together. See also intercropping. Compare monoculture, polyvarietal cultivation.

polyvarietal cultivation Planting a plot of land with several varieties of the same crop. Compare intercropping, monoculture, polyculture.

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

population change Increase or decrease in the size of a population. It is equal to (Births + Immigration) - (Deaths + Emigration).

population dispersion General pattern in which the members of a population are arranged throughout its habitat.

population distribution Variation of population density over a particular geographic area. For example, a country has a high population density in its urban areas and a much lower population density in rural areas.

population dynamics Major abiotic and biotic factors that tend to increase or decrease the population size and age and sex composition of a species.

population size Number of individuals making up a population's gene pool.

population viability analysis (PVA) Use of mathematical models to estimate a population's risk of extinction. See minimum viable population.

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.

poverty Inability to meet basic needs for food, clothing, and shelter.

precautionary principle When there is scientific uncertainty about potentially serious harm from chemicals or technologies, decision makers should act to prevent harm to humans and the environment. See pollution prevention.

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

predation Situation in which an organism of one species (the predator) captures and feeds on parts or all of an organism of another species (the prey).

predator Organism that captures and feeds on parts or all of an organism of another species (the prey).

predator-prey relationship Interaction between two organisms of different species in which one organism, called the predator, captures and feeds on parts or all of another organism, called the prey.

Preservationist Person concerned primarily with setting aside or protecting undisturbed natural areas from harmful human activities. Compare conservation biologist, conservationist, ecologist, environmentalist, environmental scientist, restorationist.

prey Organism that is captured and serves as a source of food for an organism of another species (the predator).

primary consumer Organism that feeds on all or part of plants (herbivore) or on other producers. Compare detritivore, omnivore, secondary consumer.

primary pollutant Chemical that has been added directly to the air by natural events or human activities and occurs in a harmful concentration. Compare secondary pollutant.

primary sewage treatment Mechanical sewage treatment in which large solids are filtered out by screens and suspended solids settle out as sludge in a sedimentation tank. Compare advanced sewage treatment, secondary sewage treatment.

primary succession Ecological succession in a bare area that has never been occupied by a community of organisms. See ecological succession. Compare secondary succession.

prior appropriation Legal principle by which the first user of water from a stream establishes a legal right to continued use of the amount originally withdrawn. Compare riparian rights.

probability Mathematical statement about how likely it is that something will happen.

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. Can also use autotroph. 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 protests produce their own organic nutrients through photosynthesis. Others are decomposers and some feed on bacteria, other protists, or cells of multicellular organisms.

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.

pure command economic system System in which all economic decisions are made by the government or some other central authority. Also centrally planned economy and totally planned economy is used. Compare capitalist market economic system, pure free-market economic system.

pure free-market economic system System in which all economic decisions are made in the market, where buyers and sellers of economic goods interact freely, with no government or other interference. Also pure capitalism. Compare capitalist market economic system, pure command economic system.

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.