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8.1 POPULATIONS
CLASSROOM CATALYST
OBJECTIVES
• Describe the three main properties of a population.
• Describe exponential population growth.
• Describe how the reproductive behavior of
individuals can affect the growth rate of their
population.
• Explain how population sizes in nature are
regulated.
WHAT IS A POPULATION?
• A population is a group of organisms of the same
species that live in a specific geographical area
and interbreed.
POPULATION DENSITY
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Density is the number of individuals of the same species in that live in a given
unit of area.
Measure of how crowded a population is
Larger organisms
generally have lower
population densities.
Low population density:
More space, resources;
finding mates can be difficult
High population density:
Finding mates is easier; tends to be more competition; more infectious
disease; more vulnerability to predators
Northern pintail ducks
POPULATION DISTRIBUTION
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Dispersion is the pattern of distribution of organisms in a
population. A population’s dispersion may be even/spaced,
clumped, or random.
How organisms are arranged within an area:
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Random distribution:
Organisms arranged in
no particular pattern
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Even/Spaced distribution:
Organisms evenly spaced
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Clumped distribution:
Organisms grouped near resources; most
common distribution in nature
HOW DOES A POPULATION GROW?
• A population gains individuals with each new
offspring or birth and loses them with each death.
• The resulting population change over time can be
represented by the equation below.
• Growth rate is an expression of the increase in the
size of an organism or population over a given
period of time. It is the birth rate minus the death
rate. It can be +, -, or 0.
BIOTIC/REPRODUCTIVE POTENTIAL
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Reproductive potential is the maximum number
of offspring that a given organism can produce.
Many factors influence biotic potential, including
gestation time and generation time.
Organisms with high biotic potential can recover
more quickly from population declines than
organisms with low biotic potential.
POPULATION GROWTH
• Populations have the ability to
grow very quickly, however, if
they have a perfect environment.
• Exponential Growth
• Population growth in which the rate of
growth in each generation is a
multiple of the previous generation
• Conditions are never perfect,
resources are always limited.
• A population can grow
exponentially, but not for long.
CARRYING CAPACITY CONT.
• Carrying Capacity
• The number of individuals that can be supported by an
ecosystem
• S-Shaped Curve : #Births = #Deaths
RESOURCE LIMITS
• A species reaches its carrying capacity when it
consumes a particular natural resource at the same
rate at which the ecosystem produces the
resource.
• That natural resource is then called a limiting
resource.
• The supply of the most severely limited resources
determines the carrying capacity of an
environment for a particular species at a particular
time.
LIMITING FACTORS
• Populations can’t grow forever.
• Forces that slow growth in a populations are called
limiting factors.
• Deaths occur more quickly in a crowded population than in a sparse population.
• A certain proportion of a population may die regardless of the population’s density.
Climate
Human
Disturbance
Natural
Disasters
DensityDependent
Density
Independent
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Predation
Population
Size
Water
Availability
Living
Space
Parasitism
Disease
Food
Competition