Population Growth and
Regulation
Environmental resistance limits
population growth
How is Population Growth Regulated?
• In 1859 Charles Darwin wrote, “There is no
exception to the rule that every organic being
increases at so high a rate that, if not
destroyed, the Earth would soon be covered
by the progeny of a single pair.”
• But in nature, exponential growth occurs only
under special circumstances and for a limited
time.
Boom and bust cycles
Many short lived, rapidly reproducing species- algae to insects- have seasonal
Population cycles that are linked to predictable changes in rainfall, temperature
Or nutrient availability.
Contd..
• In temperate climates, insect populations
grow rapidly during the spring and summer,
then crash with the killing hard frosts of
winter.
Lemming Population Cycles
Read page number 801with your partner and discuss the pattern of population
Cycles of Lemming and share it with the other group.
Boom-and bust pattern in Lemming
Population cycles
•
•
•
•
•
Lemming populations, grow until the rodents overgraze their fragile arctic tundra
ecosystem.
Lack of food, increasing population of predators, and overpopulation may all
contribute to sudden mortality.
Many deaths occur as waves of lemmings emigrate from regions of high
population density. During these movements they are easy targets of predators,
many drown, but cannot make it all the way across.
Reduced lemming population contributes to a decline in predator numbers and a
recovery of the plant community on which the lemmings normally feed.
This results in the next round of exponential growth of the lemming population.
Exponential growth in white-footed
mouse
• When oak trees produce large acorn crops
exponential growth occurs in this organism.
• Exponential growth also occurs when
individuals invade a new habitat with
favorable conditions and little competition.
Abandoned farm field is an ideal habitat for weedy annual plants and
perennial grasses whose populations may increase exponentially at
first.
Introduction of exotic species
•
•
•
•
•
Also results in exponential growth.
Introduction of gypsy moth in oak forest.
Defoliation takes place
Affects the bark of the tree
Larvae of this moth seen everywhere.
S-Curve
The S-curve of population growth
S-curve contd..
• As individuals join a population, competition for
resources intensifies.
• As plants invade an abandoned farm field and their
populations grow, competitions for space, water,
sunlight, and soil nutrients increases until further
expansion is impossible.
• As crowding increases, __________ and _________
spread more readily.
• Why do you think this happens?
• May be lack of food
• Stress caused by adverse social interactions that may
occur under crowded conditions.
So what happens after a period of
exponential growth?
• Crowded animals may emigrate to establish new
populations
• Die at a younger age.
• Not reproduce at all.
• As a result growth rate declines and the population
tends to stabilize at or below the maximum number
the environment can sustain.
• For a long time the growth rate fluctuates around zero.
• This type of population growth is typical of long-lived
organisms colonizing a new area and is represented
graphically by an S-shaped growth curve.
Carrying capacity
• carrying capacity is the maximum population size that an ecosystem can
sustain. It is determined by the continuous availability of two types of
resources.
• Renewable resources which include food and inorganic nutrients, water,
and light.
• The second type of resource is nonrenewable – space
• if a population exceeds the carrying capacity of it environment, excess
demands on resources may damage the ecosystems. This damage can
reduce the carrying capacity and thus the ability of the ecosystem to
sustain the population.
• Population either declines until the ecosystem recovers or the
population is permanently reduced.
• For example overgrazing by cattle on dry western grasslands has reduced
the grass cover and allowed sagebrush (which cattle will not eat) to
thrive. Once established, sagebrush replaces edible grasses and reduces
the land’s carrying capacity for cattle.
Factors of environmental resistance
• Density-independent factors
• These factors limit population size regardless of the
population density (number of individuals per given
area)
• The most important natural density-independent
factors are climate and weather. Hurricanes, drought,
floods, and fire can have profound effects on local
populations, regardless of population density.
• Many insects and annual plant populations are limited
in size by the number of individuals that can be
produced before the first hard freeze.
How do organisms overcome
environmental resistance?
• Many mammals, for example, develop thick coats and store
fat for the winter.
• Some hibernate
• Birds migrate to long distances to find food and a
hospitable climate.
• Trees and bushes survive the rigors of winter by entering a
period of dormancy, dropping their leaves and drastically
slowing their metabolic activities.
• Human activities can also limit the growth of natural
populations
• Pesticides and pollution can cause drastic declines in
Density plays an important role in the
spread of diseases.
• Density-dependent factors
• These factors increase in effectiveness as the
population density increases
Role of predators and prey in
controlling populations
• Read page number 805 and 806 and note
down the role of parasites and predators in
controlling population.
• How do they control population?
• Do they coevolve with the prey?
• Disucuss among your group members.
Role of competition for resource in
controlling population