Limits to GrowthLimits to Growth

5-25-2

Limiting FactorsLimiting Factors

• Remember, primary productivity of an ecosystem can be reduced by limiting nutrients

• In the context of populations, limiting factors can cause population size to decrease

• A resource base that is limited can also affect the long-term survival of a species

• Ex: pandas and bamboo

Density-Dependant FactorsDensity-Dependant Factors

• Limiting factors that depend on population size

• Density-dependent factors become limiting only when population density reaches a certain level

• Do not affect small, scattered populations

• Examples of density-dependent limiting factors:– Competition– Predation– Parasitism– Disease

CompetitionCompetition

• Occurs when populations become crowded

• All organisms in a population need sunlight, food, water, space, etc.

• Competition among members of the same species is density dependent

• The more members in the population, the faster the resources are used up

• Competition also happens between members of different species– Major force behind evolutionary change

• When 2 species compete for same resource, they are under pressure to change– Over time, evolve to occupy diff. niches

PredationPredation

• Populations in nature are controlled by predation– Known as predator-prey relationship

• Ex: – sea otters and urchins– Sea otters and whales

Isle RoyaleIsle Royale• Well known example of predator-prey

relationship is the moose/wolf populations on Isle Royale in Lake Superior– When moose numbers are high, wolf #’s increase

– As wolves feed on moose, moose #’s decrease, causing a decrease in wolves

– With fewer wolves, moose #’s can increase again

Parasitism & DiseaseParasitism & Disease

• Parasites can also limit population growth

• Parasites are similar to predators – weaken and kill hosts

• Large populations can be under stress making them more susceptible to diseases

• More members = easier to pass “things” around

Density-Independent FactorsDensity-Independent Factors

• Factors that affect populations regardless of their size

• Examples: – Unusual weather– Season cycles– Human activities (damming rivers, clear

cutting)

• Populations usually respond to such factors with a crash in population size

• After crash, numbers may increase quickly, or stay low for awhile

• Storms and hurricanes can wipe out populations of insects

• Extreme cold or hot weather can also take a toll on populations

• Droughts can affect entire populations of vegetation, which can affect populations of consumers

• Environments are always changing

• Most populations can adapt to a certain amount of change (grow or shrink)

• Major upsets in ecosystems can lead to long-term decline in certain populations

• Human activities have caused some of these upsets

5-3 Human Population Growth5-3 Human Population Growth

• Like the populations of many other living organisms, the size of the human population tends to increase with time.

• For most of human existence, the population grew slowly

• Until fairly recently, only half the children in the world survived to adulthood.

• About 500 years ago, the human population began growing more rapidly.

• Agriculture and industry made life easier and safer.

• The world's food supply became more reliable, and essential goods could be shipped around the globe.

• Improved sanitation, medicine, and health care dramatically reduced the death rate and increased longevity.

• With these advances, the human population experienced exponential growth, as shown in the figure at right.

Patterns of Population GrowthPatterns of Population Growth

• English economist Thomas Malthus observed that human populations were growing rapidly.

• Malthus predicted that such growth would not continue indefinitely.

• Instead, according to Malthus, war, famine, and disease would limit human population growth.

• Scientists have identified a variety of other social and economic factors that can affect human populations.

• The scientific study of human populations is called demography

•   Birthrates, death rates, and the age structure of a population help predict why some countries have high growth rates while other countries grow more slowly.

Demographic TransitionDemographic Transition

• Over the past century, population growth in the United States, Japan, and much of Europe has slowed dramatically.

• Demographers have developed a hypothesis to explain this shift.

• According to this hypothesis, these countries have completed the demographic transition, a dramatic change in birth and death rates.

Age Structure DiagramAge Structure Diagram

• Demographers can predict future growth using models called age-structure diagrams, or population profiles.

• Age-structure diagrams show the population of a country broken down by gender and age group.

• Each bar in the age-structure diagram represents individuals within a 5-year group. Percentages of males are to the left of the center line and females to the right in each group.

Future Population GrowthFuture Population Growth

• To predict how the world's human population will grow, demographers must consider many factors including:– the age structure of each country – prevalence of life-threatening diseases