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.

immature community Community at an early stage of ecological succession. It usually has a low number of species and ecological niches and cannot capture and use energy and cycle critical nutrients as efficiently as more complex, mature communities. Compare mature community.

immigration Migration of people into a country or area to take up permanent residence.

indicator species Species that serve as early warnings that a community or ecosystem is being degraded. Compare keystone species, native  species, nonnative species.

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.

industrial revolution Use of new sources of energy from fossil fuels and later from nuclear fuels, and use of new technologies, to grow food and manufacture products. Compare agricultural revolution, environmental revolution, hunter-gatherers, information and globalization revolution.

industrial smog Type of air pollution consisting mostly of a mixture of sulfur dioxide, suspended droplets of sulfuric acid formed from some of the sulfur dioxide, and a variety of suspended solid particles. Compare photochemical smog.

industrialized agriculture Using large inputs of energy from fossil fuels (especially oil and natural gas), water, fertilizer, and pesticides to produce large quantities of crops and livestock for domestic and foreign sale. Also used hig-input agriculture. Compare subsistence farming.

inertia Ability of a living system to resist being disturbed or altered. Compare constancy, resilience.

infant mortality rate Number of babies out of every 1,000 born each year that die before their first birthday.

infiltration Downward movement of water through soil.

inland wetland Land away from the coast, such as a swamp, marsh, or bog, that is covered all or part of the time with fresh water. Compare coastal wetland.

inorganic compounds All compounds not classified as organic compounds. See organic compounds.

input Matter, energy, or information entering a system. Compare output, throughput.

insecticide Chemical that kills insects.

instrumental value Value of an organism, species, ecosystem, or the earth's biodiversity based on its usefulness to us. Compare intrinsic value.

integrated pest management (IPM) Combined use of biological, chemical, and cultivation methods in proper sequence and timing to keep the size of a pest population below the size that causes economically unacceptable loss of a crop or livestock animal.

Intercropping Growing two or more different crops at the same time on a plot. For example, a carbohydrate-rich grain that depletes soil nitrogen and a protein-rich legume that adds nitrogen to the soil may be intercropped. Compare monoculture, polyculture, polyvarietal cultivation.

interference competition Situation in which one species limits access of another species to a resource, regardless of whether the resource is abundant or scarce. See exploitation competition, interspecific competition.

internal cost Direct cost paid by the producer and the buyer of an economic good. Compare external benefit, external cost, full cost.

interplanting Simultaneously growing a variety of crops on the same plot. See agroforestry, intercropping, polyculture, polyvarietal cultivation.

interspecific competition Members of two or more species trying to use the same limited resources in an ecosystem. See competition, competitive exclusion principle, intraspecific competition.

intertidal zone Area of shoreline between low and high tides.

intraspecific competition Two or more organisms of a single species trying to use the same limited resources in an ecosystem. See competition, interspecific competition.

intrinsic rate of increase (r) Rate at which a population could grow if it had unlimited resources. Compare environmental resistance.

intrinsic value Value of an organism, species, ecosystem, or the earth's biodiversity based on its existence, regardless of whether it has any usefulness to us. Also used inherent value. Compare instrumental value.

invertebrates Animals that have no backbones. Compare vertebrates.

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.