keystone species Species that play roles affecting many other organisms in an ecosystem. Compare indicator species, native species, nonnative species.

kilocalorie (kcal) Unit of energy equal to 1,000 calories. See calorie.

kilowatt (kW) Unit of electrical power equal to 1,000 watts. See watt.

kinetic energy Energy that matter has because of its mass and speed or velocity. Compare potential energy.

K-selected species Species that produce a few, often fairly large offspring but invest a great deal of time and energy to ensure that most of those offspring reach reproductive age. Also used K-strategists. Compare r-selected species.

Kwashiorkor Type of malnutrition that occurs in infants and very young children when they are weaned from mother's milk to a starchy diet low in protein. See marasmus, malnutrition.