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Populations are described by density, spatial distribution, and growth rate.
Populations are groups of the same species in
the same area.
Characteristics of population include:
 Density
 Spatial Distribution
 Growth rate
Population density
 The number of organisms per unit area is
population density.
Spatial distribution
 Dispersion is the pattern of spacing of individuals
of a population within an area.
 Three kinds: clumped, uniform, and random
Limiting factors are biotic or abiotic forces
that keep population from increasing
Limiting factors are either densityindependent or density-dependent.
Density-independent factors
 Any factor in the environment that does not
depend on the number of members in a
population per unit area is a density-independent
 Usually abiotic, and include:
Weather events
Human alteration of the landscape
Air, land, and water pollution
Density-dependent factors
 Any factor in the environment that depends on
the number of members in a population per unit
area is a density-dependent factor.
 Often biotic, and include:
Population growth rate
 The population growth rate (PGR) describes how
fast a given population grows.
▪ Natality: birthrate of a population in a given year.
▪ Mortality: death rate of a population in a given year
▪ Emigration: number of individuals moving away from
the population
▪ Immigration: number of individuals moving into a
Births and immigration
add individuals to
a population.
Deaths and emigration
remove individuals
from a population.
 Exponential growth model
▪ Occurs when growth rate is proportional to population
▪ All populations grow exponentially until they encounter
a limiting factor.
 Logistic growth model:
▪ Occurs when a population’s growth slows or stops
following exponential growth.
▪ A population stops increasing when the number of
births < number of deaths, or when emigration >
 Carrying capacity
▪ Carrying capacity is the maximum number of individuals
of a species (pop size) that an environment can support.
Reproductive patterns
 Species vary in the number of births per
reproduction cycle, age that reproduction begins,
and in the life span.
 Plants and animals are placed into groups based
on these patterns.