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, temperatureOr 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 populationCycles 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