Glossary

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N

n-dimensional hypervolume
see Ecological niche.
'n'-shaped curve
More correctly, 'unimodal': a curve with a single maximum.
Natural selection
The force that causes some individuals in a population to contribute more descendants (and genes) than others to subsequent generations and so leads to changes in the genetic composition of populations over time (evolution). (See also Fitness.)
Necromass
The weight of dead organisms, usually expressed per unit of land or volume of water. The term is sometimes used to include the dead parts of living organisms, e.g. the bark and heartwood of trees, the hair and claws of animals.
Necroparasite
A parasite that kills its host (or a part of it) and continues growth on the dead resource.
Net primary production (NPP)
The total energy accumulated by plants during photosynthesis (gross primary production minus respiration).
Neutral models
Models of communities that retain certain features of their real counterparts but exclude the consequences of biotic interactions; they are used to evaluate whether real communities are structured by biotic forces.
Neutralism
The lack of an interaction between two organisms (or species): neither has any effect on the other.
Niche
The limits, for all important environmental features, within which individuals of a species can survive, grow and reproduce.
Niche complementarity
The tendency for coexisting species which occupy a similar position along one niche dimension, e.g. altitude, to differ along another, e.g. diet.
Niche differentiation
The tendency for coexisting species to differ in their niche requirements.
Niche packing
The tendency for coexisting species between them to fill the available 'space' along important niche dimensions.
Nitrification
The conversion of nitrites to nitrates, usually by microorganisms. The term is commonly used to describe the process of conversion of ammonium ions via nitrites to nitrates.
Nitrogen fixation
The conversion of gaseous nitrogen (N2) into more complex molecules. The process is used industrially to produce nitrogen fertilizers. Biological nitrogen fixation is accomplished by both free-living and symbiotic microorganisms (prokaryotes). The process is more properly called 'dinitrogen fixation'.
Node
The place on a stem where one or more leaves arise.
Non-equilibrium theory
In community ecology, concerned with the transient behavior of a system away from any equilibrium point, it specifically focuses attention on time and variation.
Null hypothesis
The hypothesis that an observed pattern of data and an expected pattern are effectively the same, differing only by chance, not because they are truly different. A statistical significance test is then generally applied to the data to test whether the hypothesis can be rejected. If so, the observed and expected patterns are said to be significantly different. Tests do not establish that the null hypothesis is true. 'Expected' patterns may be derived from theory or from other, related data sets.
Nunatak
Islands standing out in a 'sea of ice' during periods of glaciation and in which species may have persisted.
Nutrient cycling
The transformation of chemical elements from inorganic form in the environment to organic form in organisms and, via decomposition, back to inorganic form.

 
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