Population Biology

Objectives:•Explain how populations grow•Identify Factors that inhibit the growth of a populations.•Summarize forces behind and issues human population growth.

Population Growth

An increase in the size of a population over time.

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

PopulationDensitythe number of individuals per unit of

area

DistributionHow individuals in a population are spaced out

across the range of the population.• Randomly• Uniformly• Clumped

GROWTH RATEDifference between birthrate and death rate.

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

Populations DO NOT grow indefinitely! (Level-off)

Limiting factors•Food•Space

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.

Carrying Capacity

Number of organisms that an environment can support.

Reproductive pattern is the most important factor that determines population growth

Limiting Factors

Factor that controls growth of a population.

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.

Limits to Population Growth

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)

Predator-Prey Relationships

Human Population Growth

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

Patterns of Human Population Growth

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