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Population Biology
Objectives:•Explain how populations grow•Identify Factors that inhibit the growth of a populations.•Summarize forces behind and issues human population growth.
Researchers Study Populations• Geographic range• Density and distribution • Growth rate• Age Structure
METHODS USED TO INVESTIGATE POPULATION GROWTH IN ORGANISMS:•Lab-Place microorganisms, such as bacteria or yeast cells, into tube or bottle or nutrient solution and observe how rapidly the population grows.• Field-Introduce a plant or animal species into a new environment that contains abundant resources and then observe the population growth of that species.
Geographic Range• Area inhabited by a population• Can be huge, depending on species– Bacteria on rotting pumpkin (smaller than cubic
meter) to cod in Atlantic Ocean (Canada to North Carolina
DistributionHow individuals in a population are spaced out
across the range of the population.• Randomly• Uniformly• Clumped
Calculating Growth Rate—use “doubling time”
•Time needed for any population to double its size. •Doubling Time (years) = 70/ Annual Percent Growth Rate
Age Structure: number of males and females of each age a population contains-can help predict growing rapidly, slowly or not at all.
• Differences in environmental conditions and past history may cause populations to differ in their age distributions.
• The future growth of a population depends on its current age distribution.
Factors that can affect population size
• Birthrate• Death rate• Rate at which
individuals enter or leave the population
Birthrate and Death rate
• Birthrate higher than death rate-populations grow
• Death rate higher than birthrate-populations likely to shrink
• Birthrate = death rate – population stays same size
MOBILITY
Immigration: Movement of individuals into a populationEmigration: Movement of people between countries
No effect on world population, but does affect national population.Migration: Movement in or out of a population
EXPONENTIAL GROWTH
•Not Linear (straight line)•J-shaped –Initially growth is slow, then rapid growth due to total number of reproducing organisms (exponential growth)•As population gets larger, it grows faster•Unlimited resources
Life-history PatternsRapid Life-history patterns•Unpredictable and change rapidly•Typically small body size, mature rapidly, reproduce early and have short life span.•Populations increase rapidly and decline rapidly in unfavorable conditions.•e.g. mosquitoes, bacteria
Long Life-history patterns•Reproduce and mature slowly.•Maintain population size near carrying capacities.•e.g. elephants, humans, bears, whales, cacti, bristlecone pine
Organisms in new environments
• Move in new place, populations grows exponentially over time
• E.g. exotic species; European gypsy moth
Logistical Growth
When a population’s growth slows and then stops, following a period of
exponential growth. (s curve)
Logistical Growth (s curve)
• Phase 1-Exponential Growth– Population grows exponentially (J shaped curve)– Resources are unlimited– Growth and reproduction rapid– Population size and rate of growth are increasing
more and more rapidly
Logistical Growth
• Phase 2-Growth Slows Down– Population growth slows down– Does not mean population size decreases– Rate of growth slows, population increasing more
slowly
Logistical Growth
• Phase 3-Growth stops– Some point rate of growth drops to zero– Levels off– Under same conditions-population will remain
near or at this size indefinately.
Types of limiting factors
• Competition• Predation• Parasitism and disease• Unusual weather• Natural disaster
Density-Dependent Limiting Factors
Operate strongly only when population density reaches certain levels.
Density Independent FactorsFactors that affect all populations in
similar ways, regardless of population size and density.
Acting separately or together, limiting factors determine the
carrying capacity of an environment for a species
Organisms Interactions Limit Population Size
• Predation-Prey-cycles of increases and decreases over time
• Competition for Resources—Food, water and territory (Demand exceeds supply, populations decrease)—Density Dependent
• Effects of Crowding and Stress (aggression, decrease in parental care, decreased fertility and decreased resistance to disease)
Historical Overview
• Human populations tend to increase• Rate of that increase has changed over time:• Then, Now and Future– Early human existence– Exponential Human Population Growth– Predictions of Malthus– World Population Growth Slows
Human Population Growth
– For most of human existence growth was slow• Limiting factors kept death rates high; Harsh life; food
hard to fine; predation and disease; many children did not survive to adulthood.
Human Population Growth– Exponential human population growth• Civilization advanced, life easier; human population
growth began to grow more rapidly• 1800 – industrial revolution• Improvement in nutrition, sanitation, medicine, and
healthcare reduced death rates.
Historical Overview Continued
• Predictions of Malthus– English Economist Thomas Malthus suggested that
only war, famine and disease could limit human popultion growth.• Regulated by competition (war)• Limited resources (famine)• Parasitism (disease)• Other density-dependent factors
World Population Growth Slows• Exponential growth continued until the second half
of the 20th century.• Human Growth rate reached its peak b/w 1962-
1963, and then began to drop.• Size of global human population growing; rate
slowing down.– 1804 (1 billion) to 1927 (2 billion)- 123 years to double– 33 years to double to 3 billion– Now takes longer for global human population to grow
by 1 billion
DEMOGRAPHIC TRENDSDemography- Study of human population growth characteristics.Demographers study growth rate, birthrates, death rates, age structure, and geographic distribution. Help predict why some countries have high growth rates and others grow more slowly.
Humans reduce environmental effects by eliminating competing organisms, increasing food production, and controlling disease organisms.
Demographic Transition
• Dramatic change from high birthrates and death rates to low birthrates and death rates.
• US, Japan and European completed• 3 Stages:– Stage 1- Birthrate and death rate = high– Stage 2 – Birthrates high, death rates low– Stage 3- Birthrates and death rates = low
Age Structure and Population Growth
• Compares age structures in different countries
• Us-nearly equal numbers of people in each age group-predicts slow, steady growth
• Guatemala – more young children and teens –predicts rapid population growth
Future Population Growth
• To predict future– Age structure for a country– Effects of disease on death rates
• Current models- 9 billion by 2050• Data suggests-global human population will
grow more slowly than it has been growting