Essentials Of Ecology

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Abaptation

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The process by which the present match between organisms and their environment, and the constraints on this match, have been determined by evolutionary forces acting on ancestors. The prefix 'ab-' emphasizes that the heritable characteristics of an organism are consequences of the past and not anticipation of the present or future.

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Abiotic

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Non-living; usually applied to the physical and chemical aspects of an organism's environment.

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Abundance

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The number of organisms in a population, combining 'intensity' (density within inhabited areas) and 'prevalence' (number and size of inhabited areas).

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Acclimation

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The habituation of an organism's physiological response to environmental conditions (usually applied to laboratory environments).

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Acclimatization

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The habituation of an organism's physiological response to environmental conditions (usually applied to natural environments).

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Acid rain'

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Rain with a very low pH (often below 4.0) resulting from emissions to the atmosphere of oxides of sulfur and nitrogen.

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Adaptation

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A confusing word used to mean quite different things. (i) Characteristics of organisms evolved as a consequence of natural selection in its evolutionary past and which result in a close match with features of the environment and/or constrain the organism to life in a narrow range of environments. The prefix 'ad-' is unfortunate as it implies that the process anticipates the present or the future (see Abaptation). (ii) Changes in the form or behavior of an organism during its life as a response to environmental stimuli, e.g. the formation of sun and shade leaves on the same tree and the acquisition of cold tolerance as a result of prior experience of low temperatures. (iii) Changes in the excitability of a sense organ as a result of continuous stimulation.

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Adenosine triphosphate (ATP)

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Molecule composed of adenine, ribose and three phosphate groups bound by high-energy linkages and associated with energy transfer in living cells.

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Adiabatic expansion

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Expansion taking place without heat entering or leaving the system.

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Adventitious roots

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Roots that arise in 'abnormal' positions, e.g. from a stem or leaf. The contrast is with the primary roots that develop from the axis of a seedling and roots that arise from other roots.

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Aerobic decomposition

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The process of breakdown of organic molecules to simple inorganic constituents when oxygen is in free supply.

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Aesthetic injury level

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The level of pest abundance above which aesthetic or sociological considerations suggest control measures should be taken against the pest.

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Aestivation

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A state of dormancy during the summer or dry season. (The word is also used by botanists to describe the arrangement of the parts in a flower bud.)

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Aggregated distribution

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The distribution of organisms in which individuals are closer together than they would be if they were randomly or evenly distributed.

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Aggregation of risk

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A pattern in which prey vary, from prey patch to prey patch, more than would be expected by chance alone in their risk of being attacked by a predator (especially applied to hosts varying in their risk of being attacked by parasitoids).

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Aggregative response

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The response of a predator through which it spends more time either in habitat patches with higher densities of prey, leading to higher densities of predators in patches with higher densities of prey, or in habitat patches with lower densities of prey, leading to higher densities of predators in patches with lower densities of prey. (See also Aggregation of risk.)

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Alcohol dehydrogenase (ADH)

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An enzyme catalysing the conversion of alcohols into aldehydes and ketones, and the reverse.

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Allelochemical

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A substance produced by one organism that is toxic or inhibitory to the growth of another.

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Allochthonous material

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Organic matter entering a stream, lake or ocean but derived from an adjacent terrestrial system.

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Allogenic succession

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A temporal succession of species at a location that is driven by external influences which alter conditions.

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Allometry

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The study of the changing proportions of the parts of an organism as size changes, either in individual growth (ontogenetic) or comparing related organisms of different sizes (phylogenetic).

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Allopatry

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Occurring in different places; usually refers to geographical separation of species.