POPULATION BIOLOGY

CHAPTER 5

What affects population size?

•Remember, a population is a group of organisms belonging to a single species that lives in a given area.

•Answer the following questions about what factors can affect population size.

1. Think of a population in your area. How might this population grow in size?

2. How might this same population decrease in size?

How is population size estimated?

•For stationary organisms:–Quadrant method

•For moving organisms:–Mark and Recapture

•For people:–Census

5–1: HOW POPULATIONS GROW

A.Characteristics of Populations

B.Population GrowthC.Exponential GrowthD.Logistic Growth

DEFINITIONS

•Species – groups of organisms that are able to interbreed and produce fertile offspring

•Population – a group of organisms belonging to the same species and living in certain area

Characteristics of Populations

•Geographic distribution•Population Density - the

number of individuals of a population within a given area

•Growth rate

POPULATION GROWTH

•Three factors that affect population size:–Number of births–Number of deaths–Number of individuals that leave (emigration) or enter (immigration) a population

EXPONENTIAL GROWTH

•Exponential Growth – highest rate of reproduction under ideal conditions

POPULATION GROWTH CURVE J SHAPED

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LOGISTIC GROWTH

•Logistic growth – occurs when a population’s growth slows or stops following a period of exponential growth

•Carrying capacity – greatest number of individuals in a population that a given environment can support

POPULATION GROWTH CURVES SHAPED

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Carrying capacity

QUESTIONS1. Imagine a small island that has a

population of five rabbits. How might each of the following factors affect the rabbit population?

a.Climateb.food supplyc. predation

2. Now imagine another small island that has a population of 500 rabbits. How would the same factors affect this population?

3. Which of the factors depend on population size?

4. Which factors do not depend on population size?

5–2: LIMITS TO GROWTH

A. Limiting FactorsB. Density-Dependent Factors

1.Competition2.Predation3.Parasitism and Disease

C. Density-Independent Factors

LIMITING FACTORS

•Limiting factors –causes population growth to decrease–List examples

DENSITY-DEPENDENT LIMITING FACTORS

•Limiting factors that depend on population size–Examples:

•Competition•Disease (contagious)•Predation•Parasitism

A Density-Dependent Limiting Factor

•Competition – competing for resources such as food, space, water, sunlight, mates etc.–Can occur between the same species (intraspecific) or different species (interspecific)

–What may happen due to competition?•Extinction•Adaptation•Emigration

–Ways to avoid intraspecific competition (same species):•Life cycles (frogs and tadpoles do not compete)

•Life span (adults die shortly after young are produced)

•Parental care of young •Dominance and Social Hierarchy (chicken’s pecking order)

•Role separation (bees and ants)

•Behavioral and Physiological Changes (killing young, no mating)

•Emigration•Territoriality (birds’ songs, a buck’s scent)

•Disease (contagious)–The more crowded an area, the quicker disease spreads

•Predation – feeding of one organism on another–Predation may be a limiting factor for the prey population while food availability may be a limiting factor for the predator

–predator-prey relationships show cycles of increases and decreases in both populations in response to one another

–Predators often kill the old, weak, and young

–Predation may keep prey population very near carrying capacity•Example: Moose vs. wolf on an island in Lake Superior

Moose Wolves

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•Parasitism – when organisms live on or in a host; always harmful to host

•The greater the population density, the greater number of parasites are passed from one organism to another

DENSITY-INDEPENDENT LIMITING FACTORS

•Limiting factors that are not related to population density –Examples:

•Temperature (as with ladybugs

POPULATION GROWTH DENSITY-INDEPENDENT LIMITING

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How fast are we growing?• Until about 500 years ago, the

world’s human population remained fairly stable. Then, as advances in medicine, agriculture, and technology occurred, the human population began growing very rapidly. Today, the world’s human population is greater than 6 billion people, and it continues to grow, but at a slower rate.

1. The human population is increasing by about 1.4 percent each year. Assume that the population is 6 billion (6,000,000,000). How large will the population be in one year?

2. If the human population continues to grow at a rate of 1.4 percent per year, the population would double in size (to 12 billion people) in only 51 years! What effect might this increase in population have on the environment and on other people?

5–3: HUMAN POPULATION GROWTH

A. Historical OverviewB. Patterns of Population

Growth1.The

DemographicTransition2.Age Structure

C. Future Population Growth

HUMAN POPULATION• World Population as of 5/23/11 (Estimate) 

6,920,250,312• World Population as of 2050

(projection) 9,404,251,020• U.S. Population as of 5/23/11

(Estimate) 311,408,847One birth every.................................. 7 seconds One death every.................................. 13

secondOne international migrant (net) every............ 30

seconds Net gain of one person every..................... 10

seconds

• U.S. Population for 2050 (projection) 419,854,000

• A World Population Clock• Source for USA and World Pop data: http://www.census.gov/main/www/popclock.html and

www.census.gov/population/www/popclockus.html

Agriculturebegins

Plowingand irrigation

Bubonicplague

IndustrialRevolutionbegins

•World Population reached: • 1 billion in 1804 • 2 billion in 1927, (123 years later) • 3 billion in 1960, (33 years later) • 4 billion in 1974, (14 years later) • 5 billion in 1987, (13 years later)• 6 billion in 1999, (12 years later)• October 12, 1999 was Y6B day (the

day the world population reached 6 billion)

PATTERNS OF POPULATION GROWTH

•Demography – study of human populations

•Since the 1970’s the worldwide population growth rate is decreasing

•However, the population size is still increasing (just at a slightly slower rate)

•Growth rates higher in developing countries–Birthrates higher–More people at reproductive age

•Zero population growth is when birthrate = death rate

•Birthrate, death rate, and age structure affect population growth rate

•Demographic transition – as countries modernize, death rates decrease at first without birthrates decreasing. This causes populations to increase rapidly (USA 1790-1910) Later, birthrates also decrease slowing population growth.

•Age-structure diagrams – show numbers of people in different age groups

Figure 5-13: Age DistributionU.S. Population Rwandan Population

RAPID GROWTH2.1% ANNUALLY

STABLE GROWTH1.7% ANNUALLY

NEGATIVE GROWTH-0.1% ANNUALLY

Male Female

Population of the USA by Age and Sex from 1950-2050 (in

millions)