HeterotrophOrganism that obtains energy and materials by eating other organisms.
(Cf. autotroph.)
HeterozygousHaving two different alleles of a gene, one derived from each parent.
(Cf. homozygous.)
HomeothermicPertaining to warm-blooded animals that regulate their body tem-
perature. (Cf. poikilothermic.)
HomozygousHaving two identical alleles of a gene. (Cf.heterozygous.)
HypothesisUniversal proposition that suggests an explanation for some observed
ecological situation.
HysteresisIn ecosystems, the time lag in the effect of a disturbance differs between
the increase in disturbance and the decrease in disturbance.Immunity(Sometimes confusingly termed “resistance,” or more correctly, specific
resistance.) The ability to combat infection or diseasedue to the presence of anti-
bodiesor activated cells. Essentially it can be divided into three types: (i) acquired
immunityis conferred on an individual after recovery from a disease; (ii) natural
or innate immunityis inherited from parents, or in some cases antibodies may be
passed across the placenta and therefore are present in the blood at birth; (iii) artificial
immunitymay be induced by the injection of either a vaccine, denatured antigens
of a parasite(which induces production of antibodies and thus gives active
artificial immunity), or antiserum which contains antibodies and thus may be used
when the host is already infected. As well as strengthening the host’s resistance,
this also confers passive artificial immunity against any subsequent infection.
InbreedingMating among related individuals.
Inbreeding depressionReduction in fitness caused by inbreeding.
Inclusive fitnessThe sum of fitness of an individual and the fitnesses of its relatives,
the latter weighted according to the coefficient of relationship.
IndigenousA species that occurs in its normal habitats but is not necessarily
confined to that area. (Cf. endemic, exotic.)
Indirect interactionAn interaction between two individuals that involves one or more
intermediary species. (Cf. direct interaction.)
InfectionThe presence of a parasitewithin a host, where it may not cause disease.
Infectious diseaseDiseasecaused by infectionwith a parasitethat can be transmitted
from one individual to another, either directly (e.g. measles) or indirectly, by a
vector(e.g. malaria). Contagious diseaseis a specific subset of infectious disease and
pertains exclusively to those diseases transmitted directly between hosts through
close bodily contact (this includes aerosols).
Infectious periodPeriod during which the infected individual is able to transmit
an infectionto a susceptiblehost or vector. The infectious period may or may not
coincide with the disease.
Instantaneous rate of increase (r)The rate of increase of a population undergoing
exponential growth under a given set of ecological conditions; can be positive,
negative, or zero, and if birth and death rates are constant for sufficient time
will produce a stable age distribution. In the logistic equation, r =rmax(1 −N/K).
(Cf. exponential rate of increase,intrinsic rate of increase.)
IntensityEither (i) the mean number of parasiteswithin infected members of the
host population, or (ii) the mean parasite burden of the entire population. It is394 GLOSSARY
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