Environmental ScienceChapter 8:

Understanding Populations

Understanding Populations - Big Ideas

• Population sizes change according to changes in the environment.

• The size and growth rate of human population has changed drastically over the last 200 years. Those changes have led to profound changes to almost every place on Earth.

Section 8.1:How Populations Change in Size

GOALS• Describe the 3 main properties of a population• Describe exponential population growth• Relate how the reproductive behavior of

individuals can affect the growth rate of their population

• Explain how nature regulates population size

The Decline of the Passenger Pigeon

• How does a species go from having billions to extinct in just one century…The Passenger Pigeon Story.

• Extinct in the wild since 1900 and last died in the Cincinnati Zoo 1914.

Review: What is a population?

Population: all the members of the same species that live in the same place at the same time

• Field mice living in a corn field, grizzly bears of Yellowstone Park area

Wolves in Yellowstone NP

Population Size

• Why care about the size of the population?

• How can scientists estimate the population (say of Elephant Seals on a beach in CA)?

Population Size• The number of individuals in

a population at a given time

• Sudden and dramatic decreases in population size can indicate an unhealthy population headed toward extinction

• Ecologists often use sampling techniques to estimate population size.

Did You Know? The passenger pigeon was once North America’s most abundant bird. Hunting drove them to extinction in less than 100 years.

Estimating PopulationHow can you estimate the population of trees in this valley?

WHY NOT COUNT THEM ALL?

WHY IS SAMPLING NEEDED?

Mark and Recapture

• Common way of estimating population size

Population Density

What is population density?

What are advantages and disadvantages of high and low density?

Population Density•Measure of how crowded a population is

• Larger organisms generally have lower population densities.

• Low population density: More space, resources; finding mates can be difficult

• High population density: Finding mates is easier; tends to be more competition; more infectious disease; more vulnerability to predators

Population DistributionHow organisms are arranged within an area:

UNIFORM

RANDOM CLUMPED

Occurs where resources needed are found throughout

Wildflowers in a meadow

Occurs when individuals hold territory or compete for spacePlants in a desert

Most Common. Occurs where resources are together

Water hole in desert, humans in cities

Many bird species are territorial. In territorial birds species, what kind of

population structure would you expect?

UNIFORM

Population Distribution

•Random distribution: Organisms arranged in no particular pattern

•Uniform distribution: Organisms evenly spaced

•Clumped distribution: Organisms grouped near resources; most common distribution in nature

What type of distribution?

CLUMPED

WHY?

Population Growth?

• Why are there so few whales, but so many bacteria?

Growth Rate

• Change in the size of a population over a given time period

AND Immigration

MINUS

Emigration

Factors the Determine Population Growth

• A population’s relative birth and death rates (mortality and natality) affect how it grows

• births > deaths = population increase

• deaths > births = population decrease

• Immigration• Emigration

Immigration/Emigration • In addition to births and deaths, population growth

is affected by immigration and emigration—individuals moving into and out of a population.

• Migration, seasonal movement into and out of an area, can temporarily change population size.

Population Calculations

Net population Change = (births + immigration) – (deaths + emigration) There are 300 people living in a village, 50 children are born, 20 people die, 10 immigrate and 2 emigrate. What is the new population?

Calculating Growth Rate

• Determined by the following equation: (birthrate + immigration rate) – (death rate + emigration rate)

• Growing populations have a positive growth rate; shrinking populations have a negative growth rate.

• Usually expressed in terms of individuals per 1000

Current human growth rate 1.25% down from 2.2% in 1963

Every spring, many species of bird travel north to the Arctic to breed, and in the fall travel south again for

the winter. This is an example of _____

MIGRATION

How Populations Grow

• What resources are available to the population?

Patterns of GrowthEXPONENTIAL GROWTH

• Population increases by a fixed percentage every year.

• Normally occurs only when small populations are introduced to an area with ideal environmental conditions

• Rarely lasts long

WHAT SHAPE DOES EXPONENTIAL GROWTH LOOK LIKE?

Starts slowly, then takes off – “J-Shaped Curve”

Why doesn’t exponential growth last long?

Limiting FactorsLimiting Factor Principle:

too much or too little of any abiotic factor can limit or prevent growth of a population, even if all other factors are near or above optimum

• Why are there no citrus fruit growing in NJ?

• Why do trees grow toward the sky?

What Might Be Some Limiting Factors in the Ocean?

• Limiting factors

• may include:

• Salinity

• pH

• Sunlight

• Dissolved oxygen

• Temperature

Carrying CapacityLimiting Factors lead to Carrying Capacity: the largest population an environment can sustain

What is the maximum amount of wolves that can be sustained in Yellowstone?

Carrying Capacity• Limiting resources – used as quickly as provided• Competition within population• Competition for territory

Carrying Capacity

Logistic Growth

Carrying Capacities are NOT fixed

Pop

ula

tion

(T

hou

san

ds)

Black Bear Population in Northern NJ

Exponential Growth – not controlled, occurs when there are no limiting

factors, and little competition

Pop

ula

tion

(T

hou

san

ds)

Pelican Population off the Western Coast of Florida

Logistic Growth – controlled

Population Growth of Wolves and Moose Living in the Isles Royale of Lake Superior

A population of wolves is reintroduced into Yellowstone National Park. For the first

decade, the wolf population grows exponentially. Then, the population growth

slows. The new pattern is known as ___

LOGISTIC GROWTHIn the example above, where wolves are reintroduced to Yellowstone National Park, the number of elk and other prey species within the park are _____Limiting Factors

Biotic Potential•A species’ maximum rate at which its population can grow

•Many factors influence biotic potential, including gestation time and generation time

•Species with high biotic potential can recover more quickly from population declines than species with low biotic potential

Reproductive Potential•The maximum number of offspring that each member of the population can produce in ideal conditions

Biotic Potential

Scorpion Fish• Mature 3-5 years• Release 50,000 to

100,000 eggs• Once fertilized take

12-16 days to hatch• HIGH BIOTIC

POTENTIAL

Orangutans• Females Mature 10

years• Birth to single babies

once every eight years• LOW BIOTIC

POTENTIAL

Population Regulation• Population size may be density dependent or

density independent.

Density Dependent

• Deaths occur more quickly in a crowded population than in a sparse population

• When individuals of a population are densely packed together

• Limited resources, predation and disease result in higher rates of death in dense populations than in sparse populations

Density Independent

• Certain proportion of a population may die regardless of the population’s density

• Affects all populations in a general or uniform way

• Severe weather and natural disasters are often density independent causes of death

Section 8.1 Population Growth Review

• Describe the factors that influence population’s growth rate

• Explain exponential and logistic growth• Explain how limiting factors and biotic

potential affect population growth

• Crash Course - Ecology