Download Population Growth

Ecology of Populations
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Population ecology is the study of populations
in relation to environment
–
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Dynamic biological processes influence
population density, dispersion, and
demography
• A population is a group of individuals of a
single species living in the same general area
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Four main factors affecting population size
• Natality = number of births
•
• Immigration = number of individuals arriving
from other places
• Emigration = number of individuals leaving the
population
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• The exponential model describes population
growth in an idealized, unlimited environment
• It is useful to study population growth in an
idealized situation
–
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Exponential Growth
• Exponential population growth
–
• Under these conditions
– The rate of reproduction is at its maximum,
called the intrinsic rate of increase (rmax)
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• Exponential population growth
– Results in a J-shaped curve
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• The J-shaped curve of exponential growth
– Is characteristic of some populations that are
rebounding
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• Environmental factors that restrict population
growth are called population-limiting factors
• Environmental factors limit the growth of
populations, preventing exponential growth
•
• Produces an S-shaped or logistic growth curve
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•
• Exponential growth
– Cannot be sustained for long in any population
• A more realistic population model
– Limits growth by incorporating carrying
capacity
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• Carrying capacity (K)
– Is the maximum population size the
environment can support with no net increase
or decrease
– The value of K varies, depending on species
and habitat
–
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• Carrying capacity is defined by limiting factors
such as:
– Availability of resources
– Build up of wastes, CO2
– Predation
– Disease
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• The logistic model of population growth
– Produces a sigmoid (S-shaped) curve
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Sigmoid or Logistic Growth Curve
1. Exponential phase
2. Transitional phase
3. Plateau phase
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Causes of each phase
• Exponential phase - under ideal conditions,
population doubles on a regular basis, due to:
– Plentiful resources
–
– Favorable abiotic factors
– Little or no predation or disease
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Causes of each phase
• Transitional phase - population growth rate
slows because of :
– Increased competition for resources
– More predators
–
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Causes of each phase
• Plateau phase - population size has stabilized,
no more growth, this is due to:
–
– Predators and disease increase mortality and
growth curve levels off
– Births + immigration = deaths + emigrations
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The Logistic Model and Real Populations
• The growth of laboratory populations of
paramecia
– Fits an S-shaped curve
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• Some populations overshoot K
– Before settling down to a relatively stable
density
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• Some populations
– Fluctuate greatly around K
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• The logistic model fits few real populations
–
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The Logistic Model and Life Histories
• Life history traits favored by natural selection
– May vary with population density and
environmental conditions
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• K-selection, or density-dependent selection
–
• r-selection, or density-independent selection
– Selects for life history traits that maximize
reproduction
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Survivorship Curves
• A survivorship curve is a graphic way of
representing the data in a life table that shows
the differential mortality rates in relation to age
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• The survivorship curve for Belding’s ground squirrels
– Shows that the death rate is relatively constant
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• Survivorship curves can be classified into three
general types
– Type I, Type II, and Type III
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• Type I curve shows low infant mortality
– These species produce few offspring but
provide them with a high degree of parental
care. This increases the likelihood that they will
survive to maturity
–
– Show a low rmax
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• Type II curve mortality is more constant over
the life span
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• Type III curve have high death rates for very
young. Death rates drop as individuals survive
to increased ages
• Species produce very large numbers of
offspring but provide little or no care for them
•
• Have a high rmax
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• An organisms life history influences the growth
rate of a population
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Opportunistic life history or r-strategists
• Small bodied species, reproduce when young,
produce many offspring, populations tend to grow
exponentially when conditions are favorable
• Such populations typically live in unstable or disrupted
environments and are controlled by densityindependent factors
•
• Exhibit type III survivorship curves
• Use a tactic called r-selection
• Insects and other invertebrates
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Equilibrial life history or K-strategists
• Larger bodied species, produce few slowly
maturing offspring but provide care for their
young (nurturing)
•
• Occurs in stable, balanced environments
• Exhibit type I survivorship curve
• Use tactic called K-selection
• Large animals like mammals
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• Some plants produce a large number of small
seeds
– Ensuring that at least some of them will grow
and eventually reproduce
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• Other types of plants produce a moderate
number of large seeds
– That provide a large store of energy that will
help seedlings become established
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Population Cycles
• Many populations
– Undergo regular boom-and-bust cycles
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• Boom-and-bust cycles are influenced by
complex interactions between biotic and abiotic
factors
•
• Prey populations may be influenced by
predation, and by fluctuations in the availability
of the plants they feed on
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Population ecology is the study of populations
in relation to environment
– Including environmental influences on
population density and distribution, age
structure, and variations in population size
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Dynamic biological processes influence
population density, dispersion, and
demography
•
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Demographics of Populations
• Demography is the statistical study of a
population, which includes its density,
distribution, rate of growth
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Density and Distribution of Populations
• Population Density - Number of individuals per
unit area or volume
• Population Distribution - Pattern of dispersal of
individuals within a space of interest
– Ecologists analyze what causes the spatial
and temporal “patchiness” of organisms
– Affected by the availability of resources
•
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Density: A Dynamic Perspective
• Determining the density of natural populations
–
• In most cases
– It is impractical or impossible to count all
individuals in a population
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• Density is the result of a dynamic interplay
– Between processes that add individuals to a
population and those that remove individuals from it
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Patterns of Dispersion
• Environmental and social factors
– Influence the spacing of individuals in a
population
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• A clumped dispersion
– Is one in which individuals aggregate in
patches
– May be influenced by resource availability and
behavior
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• A uniform dispersion
– Is one in which individuals are evenly
distributed
–
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• A random dispersion
– Is one in which the position of each individual
is independent of other individuals
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Population Growth
• Exponential Growth
–
• Biotic Potential
– Maximum population growth that can possibly occur
under ideal circumstances
• Environmental Resistance
– All environmental conditions that prevent populations
from achieving biotic potential
• Biotic potential is having it’s full effect and birthrate is
at it’s maximum during exponential growth
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• Zero population growth
– Occurs when the birth rate equals the death
rate
• The population growth equation can be
expressed as
dN 
rN
dt
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G = rN
dN 
dt rN
G = growth rate of population
N = population size
r = intrinsic rate of increase
If r is constant, then the rate at which population
grows depends on the number of individuals
already in the population (N), value of r
depends on the kind of organism
G = dN/dt (∆N/∆t) = change in # of individuals over
a given time
r = (b-d) = birth rate - death rate
G = rN
or
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∆N/∆t = (b-d)N
• The equation of exponential population
growth is
dN 
dt rmaxN
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The logistic growth equation
– Includes K, the carrying capacity
(K  N)
dN
 rmax N
dt
K
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Environmental factors that restrict population
growth are called population-limiting factors
•
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Population Dynamics
• The study of population dynamics
– Focuses on the complex interactions between
biotic and abiotic factors that cause variation in
population size
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Population Change and Population Density
• In density-independent populations
–
• In density-dependent populations
– Birth rates fall and death rates rise with
population density
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Density-dependent factors
• Population limiting factors whose effects
depend on population density
• As the number of individuals increases, so
does the percentage of individuals affected
•
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Competition for Resources
• In crowded populations, increasing population
density
– Intensifies intraspecific competition for resources
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Competition
• Intraspecific competition is competition
between members of the same species for
resources that are limited supply (food, shelter,
mates)
•
• Interspecific competition is competition
between members of different species
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Food supply
• Individuals in a large population have a smaller
share of the limited food supply
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Territoriality
• In many vertebrates and some invertebrates
– Territoriality may limit density
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• Cheetahs are highly territorial
– Using chemical communication to warn other
cheetahs of their boundaries
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• Oceanic birds
– Exhibit territoriality in nesting behavior
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Health
• Population density can influence the health and
survival of organisms
•
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Predation
• As a prey population builds up predators may
feed preferentially on that species
• The major limiting factor for predator
populations is the availability of its prey
•
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Toxic Wastes
• The accumulation of toxic wastes
– Can contribute to density-dependent regulation
of population size
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Intrinsic Factors
• For some populations
– Intrinsic (physiological, anatomical, behavioral)
factors appear to regulate population size
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• Emigration –
• Mutualism - two species acting in a mutually
beneficial fashion may increase survival of both
species involved
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Density-independent factors
• Population limiting factors whose occurrence is
not affected by population density
•
• Limit population size well before resources or
other density-dependent factors become
important
• Can cause rapid crashes in populations
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• Most populations are probably regulated by a
mixture of density-independent and density
dependent factors
• Many populations are fairly stable and near
carrying capacity that is regulated by densitydependent factors
•
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Logistic Model and Life Histories
• Life history traits favored by natural selection
• Populations vary on factors such as number of
births per reproduction, age of reproduction, life
span of individuals, probability of living entire
life spans (survivorship curves)
• These factors are part of a species life history
• Natural selection shapes the life histories of
species
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Human Population
• Human population growth has slowed after
centuries of exponential increase
•
– And humans are no exception
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The Global Human Population
• The human population
– Increased relatively slowly until about 1650
and then began to grow exponentially
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• Though the global population is still growing
– The rate of growth began to slow approximately
40 years ago
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Human Population Growth
• Human population has an exponential growth
pattern
• Doubling time currently estimated at 53 years
• Population Size
– 1800
1 Billion
– 1930
2 Billion
– 1960
3 Billion
– 2000
6 Billion
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Country Development
• More-Developed Countries (MDCs)
– Slow population growth
– High standard of living
– Completed Demographic Transition (decreased
deathrate followed by decreased birthrate)
–
• Less-Developed Countries (LDCs)
– Rapid population growth
– Low standard of living
• Latin America
•
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Infant Mortality and Life Expectancy
• Infant mortality and life expectancy at birth
– Vary widely among developed and developing
countries but do not capture the wide range of the
human condition
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Age Distributions
•
• At least three structures possible
– Increasing (pyramid-shaped) - prereproductive
group is largest of three groups
– Stable (bell-shaped) - reproductive group
equals size of prereproductive group
– Decreasing (urn-shaped) - prereproductive
group becomes smaller than reproductive
group, postreproductive group is the largest
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Age Structure
• One important demographic factor in present
and future growth trends
–
– Divide populations into three age groups
• Pre-Reproductive
• Reproductive
• Post-Reproductive
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• Age structure
– Is commonly represented in pyramids
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• Age structure diagrams
–
– Can illuminate social conditions and help us
plan for the future
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Environmental Impact
• Environmental impact of a population is
measured in terms of:
– Population size
–
– Resultant pollution caused by population size
and increased resource consumption
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• Population growth is putting extreme pressure
on each country’s social organization, the
Earth’s resources, and the Biosphere
•
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Global Carrying Capacity
• Just how many humans can the biosphere
support?
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Estimates of Carrying Capacity
• The carrying capacity of Earth for humans is
uncertain
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U.S.Population
• 281.4 M (4/00)
– Most people are Caucasian (69.1%)
– Hispanic (12.5%) - 60% from Mexico
– Asian (3.7%) - ~36% live in California
– Black (12.1%)
• 40% of population under 18 belong to minority
group - minorities increasing
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• U.S. – 73 people/mi2
• Diamond Bar – 14.9 mi2 / 38.8 km2
• Population – 58, 763
• Density – 1,515 people/km2 or
3,944 people/mi2
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Mumbai, India - World’s Most Densely Populated City
• 14.35 million people/484 km2
• Density = 29,650 people/km2
–
(DB-1,515/km2)
75,925 people/mi2 (DB-3,944/km2)
• 29,650 people/km2 (density-Mumbai) X 38.8 km2
(D.B.) = 1,150,420 people (DB-58,763)
• Others:
– Shenzhen, China (5th) - 466 km2 / 8 million people
(17,150 people/km2)
– Seoul, South Korea - 16,700 people/km2
– Taipei, Taiwan - 15,200 people/km2
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