Download Population Ecology

Population Ecology
Chapter 45
Impacts, Issues
The Numbers Game
 Once, thousands of people lived on Easter
Island, clearing forests and planting crops –
when resources ran out, there was no escape
45.1 Population Demographics
 Each population has characteristic demographics
• Statistics that describe population size, density,
distribution, and age structure, that are shaped by
ecological factors, and may shift over time
 Population size
• The number of individuals in a population
Population Demographics
 Age structure is the number of individuals in
each of several age categories
• Sometimes grouped as pre-reproductive,
reproductive, and post-reproductive
 Pre-reproductive and reproductive groups make
up the population’s reproductive base
Population Demographics
 Population density is the number of individuals
in a specified portion of a habitat
• Refers to how many individuals are in an area,
not how they are dispersed
 Population distribution is the pattern in which
individuals are dispersed in their habitat
• Clumped, nearly uniform, or random
Three Patterns of Population Distribution
45.2 Elusive Heads to Count
 Ecologists use capture-recapture methods to
estimate population sizes of animals that move
• Marked animals are released; the proportion
recaptured represents proportions in the population
Counting Stationary Organisms
 To estimate population size of organisms that
don’t move, an area may be divided into equal
areas (quadrats) and a random sample counted
45.1-45.2 Key Concepts
The Vital Statistics
 Ecologists explain population growth in terms of
population size, density, distribution, and
number of individuals in different age categories
 Field studies allow ecologists to estimate
population size and density
45.3 Population Size
and Exponential Growth
 Populations are dynamic units, continually
adding and losing individuals
• Populations increase by births and immigration
(arrival of individuals from other populations)
• Populations decrease by deaths and emigration
(individuals that take up residence elsewhere)
Migration
 Migration
• A recurring round-trip between regions, usually in
response to environmental factors
• May occur daily or seasonally
• Changes in population size average out over time
Changes in Population Size
 Per capita growth rate, r
• A measure of the growth rate of a population
per capita growth rate (r) =
per capita birth rate (b) – per capita death rate (d)
 Zero population growth
• Number of births balances number of deaths
• Population size remains stable
Population Growth (G)
 Population growth (G) for a given interval is
calculated based on per capita growth rate (r)
and the number of individuals (N)
G=rxN
Exponential Growth
 Exponential growth
• Population size increases by the same proportion
in every successive time interval as long as r
remains constant and greater than zero
• Though r is constant, the population grows faster
and faster, with a characteristic doubling time
• The plot of population size against time results in
a characteristic J-shaped curve
Exponential Growth in Mice
Animation: Exponential growth
Exponential Growth in Bacteria:
Variations in r
 Size of r affects the speed of exponential growth
Biotic Potential
 Biotic potential
• The maximum possible per capita growth rate for
a species under optimal conditions
 All species have a characteristic biotic potential
• Bacteria: 100% every half hour
• Humans: 2% to 5% per year
45.3 Key Concepts
Exponential Rates of Growth
 A population’s size and reproductive base
influence its rate of growth
 When the population is increasing at a rate
proportional to its size, it is undergoing
exponential growth
45.4 Limits on Population Growth
 Natural populations seldom grow unchecked;
competition and crowding can slow growth
 Limiting factor
• Many factors can limit population growth – such
as food, mineral ions, refuge, safe nesting sites
• The first essential resource to be in short supply
is the limiting factor
Carrying Capacity
 The sustainable supply of resources in an
environment limits population size
 Carrying capacity
• Maximum number of individuals of a population
that an environment can sustain indefinitely
Logistic Growth
 Logistic growth
• A small population starts growing slowly in size,
then it grows rapidly, then its size levels off as
carrying capacity is reached
• Graph of logistic growth yields an S-shaped curve
 When a change in environment lowers carrying
capacity, population size declines
Logistic Growth Equation
Population growth per unit time =
Maximum per capita growth rate x
Number of individuals x Proportion of
resources not yet used
Logistic Growth
initial carrying capacity
new carrying capacity
Fig. 45-8, p. 802
Unregulated Population Growth
 A population of reindeer introduced to St.
Matthew island peaked, then collapsed
Fig. 45-9a, p. 803
carrying
capacity
Fig. 45-9a, p. 803
Two Categories of Limiting Factors
 Density-dependent factors such as disease
are related to crowding and competition
• Control population size through negative
feedback, resulting in logistic growth patterns
 Density-independent factors such as fire,
storms, and other natural disasters are unrelated
to crowding – no logistic growth pattern
45.4 Key Concepts
Limits on Increases in Number
 Over time, an exponentially growing population
typically overshoots the carrying capacity—the
maximum number of individuals of a species that
environmental resources can sustain
 Some populations stabilize after a big decline;
others never recover
45.5 Life History Patterns
 Each species has a life history pattern
• Life span, age at maturity, and number of
offspring produced vary widely among organisms
• Individuals at different stages of life require
different resources
• Natural selection influences life history traits
Cohorts and Life Tables
 Cohort
• Group of individuals born in the same time interval
• Tracking a cohort from birth until the last one dies
reveals patterns of reproduction, death, migration
 Data about cohorts are summarized in life tables
• Help determine how harvesting or environmental
changes may affect population numbers
• Insurance companies use human life tables
Life Table for an Annual Plant Cohort
Life Table for Humans in the US
Survivorship Curves
 Survivorship curve
• A graph line that emerges when a cohort’s agespecific survival in its habitat is plotted
• Reveals differences in age-specific survival
among species or populations of the same
species
• Three types are common in nature
Type I Survivorship Curve
 Type I: Survivorship is high until late in life
Type II Survivorship Curve
 Type II: Death rate varies little with age
Type III Survivorship Curve
 Type III: Death rate peaks early in life
Reproductive Strategies
 Different environments and population densities
can favor different reproductive strategies
 r-selection favors traits that maximize number
of offspring
 K-selection favors traits that improve offspring
quality
45.6 Natural Selection and Life Histories
 Predation can serve as a selection pressure that
shapes life history patterns
Predation on Guppies in Trinidad
 Predators are agents of natural selection
• Killifish eat only small guppies
• Cichlids eat only large guppies
 Guppies in streams with cichlids grow faster,
reproduce earlier, are smaller at maturity, have
more offspring at a time, breed more frequently
• Traits can change rapidly if predators change
Two Guppies and Guppy Eaters
Experiment:
Predation Pressures on Guppies
Overfishing and Atlantic Cod
 Overfishing of mature
Atlantic cod led to
population collapse
 Early warning sign:
rapid decline in age at
first reproduction
45.5-45.6 Key Concepts
Patterns of Survival and Reproduction
 Resource availability, disease, and predation are
major factors that can restrict population growth
 These limiting factors differ among species and
shape their life history patterns
45.7 Human Population Growth
 The size of the human population is at its
highest level ever and is expected to increase
 Humans have temporarily evaded environmental
resistance to growth through expansion into new
habitats, cultural interventions, and technology
 Eventually, density-dependent controls will slow
human population growth
Human Population Growth
Extraordinary Foundations for Growth
 Geographic expansion
• Technology allows humans to occupy habitats
that would not otherwise support them
 Increased carrying capacity
• Agricultural productivity increased carrying
capacity for human population
 Sidestepped limiting factors
• Sanitation and medicine lowered death rates
Unequal Distribution of Resources
 More than 1 billion people lack clean drinking
water, more than 2 billion face fuel shortages,
and more than 800 million are malnourished
45.8 Fertility Rates and Age Structure
 Worldwide population is expected to peak at 8.9
billion by 2050 – increasing problems with
resources, food production, and pollution
 Risks posed by rising populations have led to
increased family planning in almost every region
Demographic Indicators for Countries
with Different Levels of Development
298 million
Population
in 2006
188 million
132 million
Population
in 2025
(projected)
Population
under
age 15
Population
above
age 65
349 million
211 million
206 million
20%
26%
42%
13%
6%
3%
2.1
1.9
Total fertility
rate (TFR)
5.5
Infant
mortality
rate
6 per 1,000 live births
29 per 1,000 live births
97 per 1,000 live births
Life
expectancy
78 years
72 years
47 years
Per capita
income
$43,740
$3,460
$560
Fig. 45-15, p. 810
Shifting Fertility Rates
 Total fertility rate (TFR)
• The average number of children born to women
of a population during their reproductive years
 Worldwide total fertility rate has been declining,
but is still above replacement level
Comparing Age Structure Diagrams
 More than one-third of the world population is in
a broad pre-reproductive base
 Even if TFR declines to replacement level
worldwide, population will continue to increase
Age Structure Diagrams
Fig. 45-16c, p. 811
Animation: Age structure diagrams
Animation: U.S. age structure
45.9 Population Growth
and Economic Effects
 Demographic transition model
• Describes population growth rate changes over
time as a country becomes economically
developed
• Four stages based on industrialization:
Preindustrial, transitional, industrial, postindustrial
The Demographic Transition Model
Animation: Demographic transition
model
Growth and Resource Consumption
 Population growth and resource consumption are
correlated with levels of economic development
• The most developed countries have the slowest
growth rates and use the most resources
 Growth rates are typically greatest during the
transition to industrialization
• As more countries become industrialized, pressure
on Earth’s resources will increase
Projected Effects of Population Growth
45.10 Rise of the Seniors
 In some developed countries, decreasing total
fertility rate and increasing life expectancy have
resulted in a high proportion of older adults
 In the US, the proportion of people over age 65
is projected to reach 20 percent by 2030
Social Implications
 In 2050, there could be 31 million Americans
over age 85
45.7-45.10 Key Concepts
The Human Population
 Human populations sidestepped limits to growth
by way of global expansion into new habitats,
cultural interventions, and innovative technology
 Even so, no population can continue to expand
indefinitely
Animation: Current and projected
population sizes by region
Animation: Distribution patterns
Animation: Effect of death on growth
Animation: Guppy charateristics
Animation: Life history patterns
Animation: Logistic growth
Animation: Mark-recapture method
ABC video: People's Explosion
ABC video: Easter Island
Video: The human touch
Video: Pelican nesting colony