Niche overlapThe sharing of niche space by two or more species; similarity of resource
requirements and tolerance of ecological conditions.
Niche widthThe standard deviation of the distribution of resource use. (Cf. niche
breadth.)
Numerical responseA change in the population size of a predator species as a result
of a change in the density of its prey. (Cf. functional response.)
NutrientAny substance required by organisms for normal growth and maintenance.
Nutrient cycleThe path of an element through the ecosystem, including its assim-
ilation by organisms and its regeneration in a reusable inorganic form.OligotrophicPoor in the mineral nutrients required by green plants; applied to an
aquatic habitat with low productivity. (Cf. eutrophic.)
OmnivoreAn organism whose diet is broad, including both plant and animal foods;
specifically, an organism that feeds at more than one trophic level.PandemicA widely distributed epidemic.
PanmixiaRandom mating within a population, a panmictic population.
PanzooticA widely distributed epizootic, often affecting more than one host
species.
ParameterIn statistics, an unknown true characteristic of a statistical population. It
is usually impossible to know the value of a parameter. A statistic estimates a
parameter.
ParasiteAn organism exhibiting a varying but obligatory dependence on another
organism, its host, which is detrimental to the survival and/or fecundityof that host.
(See also macroparasites,microparasites.)
ParasitoidAny of a number of insect species whose larvae live within and consume
their host, usually another insect.
PerennialReferring to an organism that lives for more than one year; lasting
throughout the year.
PhenologyStudy of the periodic (seasonal) phenomena of animal and plant life
and their relations to the weather and climate (e.g. the time of flowering in
plants).
PhenotypeExpression of the characteristics of an organism as determined by the
interaction of its genetic constitution and the environment. (Cf. genotype.)
PhylogenyThe pattern of evolutionary relationships among species or other taxo-
nomic groups.
PleiotropyThe influence of one gene on the expression of more than one trait in
the phenotype.
PoikilothermicReferring to cold-blooded animals, organisms that have no rapidly
operating heat-regulatory mechanism. (Cf. homeothermic.)
PolygenicDetermined by the expression of more than one gene.
PolymorphismThe occurrence together in the same habitat of two or more discon-
tinuous forms of a species in such proportions that the rarest of them cannot merely
be maintained by recurrent mutation or immigration.
PopulationGroup of individuals of a single species in a defined area.
Population growth rate (dN/dt)The rate of growth of a population over a short period
of time, defined by the product of population size, N, and the instantaneous rate
of increase,r. (See logistic equation.)GLOSSARY 397
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