Intrinsic rate of increase r The per

capita rate of increase of a population that has reached a stable age structure, in which fecundity and survivorship are stable and which is not crowded and is not experiencing any competitive or other pressures.

introduced species A species taken by humans to a new location in which it did not previously exist. Such species can have a devastating effect on the native flora and fauna: introduced predators have destroyed flightless ground-nesting birds in New Zealand, and introduced diseases for which there are no natural biological controls can decimate domestic crops and livestock.

intron See exon.

inversion An atmospheric condition in which cold air is trapped by a layer of warm air above it. This may occur, for example, in still conditions when the air at ground level cools rapidly by radiation, becoming cooler than the air above. This can lead to frost pockets occurring in valleys. It also traps pollutants forming smog.

invertebrates Animals without backbones, e.g. insects, snails, starfish, and worms.

in vitro Literally 'in glass'; describing experiments or techniques performed in laboratory apparatus rather than in a living organism. Cell tissue cultures and in vitro fertilization (to produce test-tube babies) are examples. Compare in vivo.

in vivo Literally 'in life'; describing processes that occur or tests made within the living organism. Compare in vitro.

iodine A micronutrient essential in animal diets mainly as a constituent of the thyroid hormones. Iodine is not essential to plant growth although it is accumulated in large amounts by the brown algae.

ion An atom or molecule that carries an electric charge as a result of the loss or gain of electrons.

ion exchange The reversible exchange of ions of similar charge between an insoluble solid and a solution that is in contact with it. Ion exchange is used to purify chemicals, to soften or demineralize water, to separate substances from mixtures in solution, and to lower potassium levels in patients suffering from kidney failure. See also cation exchange capacity.

ionization The formation of ions by the addition or removal of electrons from atoms, molecules, or radicals. It is a major route for the transfer of energy from electromagnetic radiation to matter. See also electromagnetic energy; ionizing radiation.

ionizing radiation Radiation that causes atoms or molecules to lose electrons, so forming ions. There are two main

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