important to distinguish between these two usages, because unless prevalenceis
100%, the latter will be smaller than the former.
InteractionSee direct interaction,indirect interaction.
Intermediate hostA host organism which acquires an infectious agent and in which
an obligatory period of development or multiplication/replication occurs before the
agent becomes infectious to another host. For example, tabanid flies are vectors
(only) of surra (Trypanosoma evansi). Their habit of interrupted feeding allows the
direct mechanical transmission of the trypanosome from one host to another on
the flies’ mouthparts. Tabanids are both vectors and intermediate hosts of the nema-
tode (Pelecitus roemeri) that act in a similar fashion to mosquitos playing the role
of intermediate hosts and vectors of malaria. Snails are the intermediate hosts of
schistosomes but they are not vectors; the infectious stage of the parasite escapes
from the snail and finds and penetrates the definitive host.
Interspecific competitionCompetition between members of different species.
Intrinsic rate of increase (rmax)The rate of increase of a population undergoing expon-
ential growth under optimum ecological conditions; the maximum instantaneous
rate that a species is capable of. It is a characteristic of a species. (Cf. exponential
rate of increase, instantaneous rate of increase.)
Inverse density dependentA rate (births or deaths) that decreases proportionately
as population size or density increases. Also called depensatory.
IrruptionA sudden expansion of the range of a species that may or may not be accom-
panied by an increase in population. (Cf. eruption.)
IsoclineOn a population graph, a line designating combinations of competing
populations, or predator and prey populations, for which the growth rate of one
of the populations is zero.Keystone speciesA species whose functional role in a community is disproportionately
greater than that predicted by its abundance; usually whose removal has strong
effects on community diversity and composition. Keystone species are often top
predators.LandscapeA large-scale ecosystem.
Leslie matrixA matrix of values of age-specific fecundity and survivorship used to
project the size and age structure of a population through time; a population matrix.
Life historyThe set of adaptations of an organism that influence the life-table
values of age-specific survival and fecundity, such as reproductive rate or age at
first reproduction.
Life tableA summary by age of the survivorship and fecundity of individuals in a
population.
LimitationA process that determines the size of the equilibrium population.
Limiting factorAny factor that causes population limitation.
Limiting resourceA resource that is scarce relative to the demand for it.
Local stabilityAbility to withstand perturbations of a small magnitude and not be
affected. (Cf. fragility,global stability,robust.)
Logistic equationModel of population growth rate(dN/dt) described by the two con-
stants rmaxand K, the carrying capacity. Thus, dN/dt =rmaxN(1 −N/K). It produces
a symmetrical S-shaped curve with Kthe upper asymptote.GLOSSARY 395
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