environmental stochasticity Random variation in the abiotic environment.
enzootic Describing a disease constrained by geographic boundaries.
Eocene The second oldest epoch of the tertiary period, 55-38 mya. It was characterized by predominance of early hoofed mammals. Many other mammals (e.g. carnivores, bats, and whales) and birds were also present. It was a period of widespread temperate and subtropical forests. The extensive grasslands of the late Tertiary had not yet developed, but large hoofed grazing mammals, such as horses and elephants, were evolving rapidly. See also geological time scale.
eolian Caused or deposited by wind.
EOY See economically optimum yield.
ephemeral A plant that has a very short life cycle, and may complete more than one life cycle within a year. Examples are shepherd's purse (Capsella bursa-pastoris) and certain desert plants that grow, flower, and set seed in brief periods of rain. Compare annual; biennial; perennial.
epidemic A relatively sudden and widespread outbreak of a disease, affecting either a wide area or a large number of people or animals in a small area.
epidemiology The study of the incidence, distribution, and control of diseases affecting large numbers within a population. These include both epidemics of infectious diseases and diseases associated with environmental factors and dietary habits (e.g. lung cancer, some forms of heart disease, etc.).
epifauna Organisms living on the surface of a substrate.
epilimnion In a stratified lake (a lake in which there are distinct layers of different temperature and/or density), the upper layer of warm water. It usually has a fairly uniform temperature, because it undergoes mixing by wind action. See stratification.
epipelagic The oceanic zone extending from the surface to about 200 m, where enough light penetrates to allow photosynthesis.
epiphyte (air plant) Any plant growing upon or attached to another plant or object merely for physical support.
epoch See geological time scale.
Equator An imaginary line at latitude 0° that girdles the Earth and divides it into the northern hemisphere and southern hemisphere.
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