the resources are not in short supply, competition occurs when the organisms seek-
ing that resource harm one another in the process (interference competition).
Competitive exclusion principleThe hypothesis that two or more species cannot
coexist on a single resource that is scarce relative to the demand for it. Also called
Gause’s principle.
Consumer–resource interactionsInteractions in which individuals of one species con-
sume individuals of another. Consumer–resource interactions affect the consumer
positively and the resource negatively.
Contact rateThe average frequency per unit time with which infected individuals
contact, or otherwise put themselves in a position to transmit an infectionto,
susceptible individuals.
Contagious diseaseSee infectious disease.Death rate (dx)The percentage of newborns dying during a specified interval, often
expressed as a function of age (x). (Cf. mortality.)
DemeA local population within which mating occurs among individuals more or
less at random.
Demographic stochasticityRandom variation in birth and death rates in a population.
DemographyThe study of population structure and growth.
DensityNumber of individuals in relation to the space, volume, or other resources
that they need.
Density dependentHaving an influence on individuals in a population that varies
with the density of that population. Often applied to birth and death rates.
Density independentHaving an influence on individuals in a population that does
not vary with the density of that population.
DepensatorySee inverse density dependent.
DeterministicHaving an outcome that is not subject to stochastic (random)
variation.
Deterministic modelMathematical model in which all the relationships are fixed
and the concept of probability does not enter; a given input produces an exact pre-
diction as output. (Cf. stochastic model.)
DetritusFreshly dead or partially decomposed organic matter.
Direct interactionAn interaction between organisms that involves direct physical
contact between the interactors (e.g. predation and herbivory). (Cf. indirect
interaction.)
DiseaseThe debilitating effects of infectionby a parasite; sometimes incorrectly used
to refer to the disease-causing parasite. It is possible for a host to be infected by
a parasite but to show no symptoms of disease.
DispersalMovement of organisms away from their place of birth or from centers of
population density.
DispersionThe spatial pattern of distribution of individuals within populations.
DistributionThe geographic extent of a population or other ecological unit.
DiversityThe number of species in a local area (alpha diversity) or region (gamma
diversity). Also, a measure of the variety of species in a community that takes into
account the relative abundance of each species.
Dominance conditionCommunities or vegetational strata in which one or more species,
by means of their number, coverage, or size, have considerable influence upon or
control of the conditions of existence of associated species.GLOSSARY 391
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