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Demography
• birth, death, immigration, and emigration.
• Populations grow due to birth and immigration, which occurs
when individuals enter a population by moving from another
population.
• Populations decline due to deaths and emigration, which occurs
when individuals leave a population to join another population.
• Demography is the study of factors such as these that determine
the size and structure of populations through time.
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Demography
• If a population consists primarily of young individuals with a high
survival rate and reproductive rate, the population size should
increase over time.
• On the other hand, if a population comprises chiefly old individuals
with low reproductive rates and low survival rates, then it is almost
certain to decline over time.
• To understand a population’s dynamics, biologists turn to the data
contained in a life table.
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Life Tables
• A life table summarizes the probability that an individual will
survive and reproduce in any given time interval over the course of
its lifetime.
• The lizard Lacerta vivipara is a common resident of open, grassy
habitats in western Europe. Most populations give birth to live
young.
• Biologists were able to construct a life table for this species
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© 2011 Pearson Education, Inc.
Survivorship
• Survivorship is the proportion of offspring produced that survive,
on average, to a particular age.
• To recognize general patterns in survivorship and make
comparisons among populations or species, biologists create a
graph called a survivorship curve.
• The survivorship curve is a plot of the logarithm of the number of
survivors versus age.
• There are three general types of survivorship curves.
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Survivorship
• In a type I curve, survivorship throughout life is high, and most individuals
approach the maximum life span of the species (e.g. humans )
• In a type II curve, most individuals experience relatively constant survivorship
over their lifetimes; songbirds have this curve.
• Type III curves result from high death rates early in life, with high survivorship
after maturity; many plants have type III curves.
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Fecundity
• Fecundity is the number of female offspring produced by each
female in the population.
• Age-specific fecundity is the average number of female offspring
produced by a female in a given age class—a group of individuals
of a specific age.
• Data on survivorship and fecundity allow researchers to calculate
the growth rate of a population.
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The Role of Life History
• In many species, key aspects of the life table vary dramatically
among populations.
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What Are Fitness Trade-Offs?
• Fitness trade-offs occur because every individual has a restricted amount of time
and energy at its disposal―its resources are limited.
• If a female devotes a great deal of energy to producing a large number of
offspring, it is not possible for her to devote that same energy to her immune
system, growth, nutrient stores, or other traits that increase survival.
• A female can maximize fecundity, maximize survival, or strike a balance
between the two.
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Life History Is Based on Resource Allocation
• An organism’s life history describes how an organism allocates its
resources to growth, reproduction, and activities or structures
related to survival.
• Traits such as survivorship, age-specific fecundity, age at first
reproduction, and growth rate are all aspects of an organism’s life
history.
• Understanding variation in life history is all about understanding
fitness trade-offs.
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Patterns across Species
• Life-history traits form a continuum.
• In general, organisms with high fecundity tend to grow quickly, reach sexual
maturity at a young age, and produce many small eggs or seeds.
• In contrast, organisms with high survivorship tend to grow slowly, and invest
their energy and time in traits that reduce damage from enemies and increase
their own ability to compete for resources.
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Predicting Population Growth
• The most fundamental questions that biologists ask about
populations involve growth or decline in numbers of individuals.
• For conservationists, analyzing and predicting changes in
population size is fundamental to managing threatened species.
• A population’s overall growth rate is a function of birthrates, death
rates, immigration rates, and emigration rates.
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Quantifying the Growth Rate
A population’s growth rate is the change in the number of
individuals in the population (N) per unit time (t).
• If no immigration or emigration is occurring:
growth rate = N  r
• The per-capita rate of increase (r) is the difference between the
birthrate and death rate per individual.
r=b−d
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Quantifying the Growth Rate
• If b>d, r>0
• If b<d, r<0.
• Within populations, r varies through time. Its value can be positive,
negative, or 0.
• When birthrates per individual are as high as possible and death
rates per individual are as low as possible, r reaches a maximum
value called the intrinsic rate of increase, rmax.
• When this occurs, the population's growth rate is expressed as:
N/t = rmaxN
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Quantifying the Growth Rate
• Each species has a characteristic rmax that does not change. But at
any specific time, the per-capita rate of increase of each population
of that species is likely to be much lower than rmax.
• A population’s r is also likely to be different from r values of other
populations of the same species and to change over time.
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Exponential Growth
• Exponential population growth occurs when r does not change
over time. It does not depend on the number of individuals in the
population (it is density independent.)
• In nature, exponential growth is observed in two circumstances:
1. A few individuals found a new population in a new habitat.
2. A population has been devastated by a storm or some other
type of catastrophe and then begins to recover, starting with a
few surviving individuals.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Exponential Growth
• Exponential growth cannot continue indefinitely. (Environments
have finite resources)
• When population density—the number of individuals per unit
area—gets very high, the population’s per-capita birthrate should
decrease and the per-capita death rate increase, causing r to decline.
– This type of growth is density dependent.
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Logistic Growth
• Carrying capacity, K, is the maximum number of individuals in a
population that can be supported in a particular habitat over a
sustained period of time. K can change depending on conditions.
• The carrying capacity of a habitat depends on a large number of
factors: food, space, water, soil quality, and resting or nesting sites.
Carrying capacity can change from year to year, depending on
conditions.
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Predicting pop size: A Logistic Growth Equation
• If a population of size N is below the carrying capacity K, the
population should continue to grow. Specifically, a population’s
growth rate should be proportional to (K – N)/K:
N/t = rmaxN((K – N)/K)
• This expression is called the logistic growth equation (density
dependent).
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Graphing Logistic Growth
• In a hypothetical population, density-dependent growth has three distinct stages:
1.
Initially, growth is exponential (r is constant).
2.
Growth rate begins to decrease (N increases) when competition for
density-dependent factors begins to occur.
3.
Growth rate reaches 0 at the carrying capacity (N vs. t is flat).
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What Limits Growth Rates and Population Sizes?
• Population sizes change as a result of density-independent and
density-dependent factors.
• Density-independent factors are usually abiotic; they change
birthrates and death rates irrespective of population size.
• Density-dependent factors are usually biotic; they change in
intensity as a function of population size.
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A Closer Look at Density Dependence
• An experimental study of a coral-reef fish called the bridled goby showed a
strong density-dependent relationship in survivorship.
• Likewise, a long-term study of song sparrows on Mandarte Island, British
Columbia, showed a strong density-dependent relationship in fecundity.
• Density-dependent changes in survivorship and fecundity cause logistic
population growth.
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Carrying Capacity Is Not Fixed
• K varies among species and populations.
• K varies because for any particular species, some habitats are better
than other habitats due to differences in food availability, space,
and other density-dependent factors. Stated another way, K varies
in space.
• It also varies with time, as conditions in some years are better than
in others.
• In addition, the same habitat may have a very different carrying
capacity for different species.
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Population Dynamics: changes in pops thru time
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How Do Metapopulations Change through Time?
• If individuals from a species occupy many small patches of habitat so that they
form many independent populations, they represent metapopulations―a
population of populations.
• Because humans are reducing large, contiguous areas of forest and grasslands to
isolated patches or reserves, more and more species are being forced into a
metapopulation structure.
• Glanville fritillaries—an endangered species of butterfly native to the Åland
islands off the coast of Finland—exist naturally as metapopulations.
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Metapopulations Should Be Dynamic
• Ilkka Hanski and colleagues determined
the number of Glanville fritillary
breeding pairs in each patch within a
metapopulation.
• Over time, each population within the
larger metapopulation is expected to go
extinct due to any number of potential
causes.
– migration (rescue effect)
• There is thus a balance between
extinction and recolonization within a
metapopulation. Subpopulations may
blink on and off over time, but the overall
population is maintained at a stable
number of individuals.
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Metapopulations Should Be Dynamic
• Experimental studies supported the dynamic nature of
metapopulations in Glanville fritillaries; the overall population size
was relatively stable even though constituent populations came and
went.
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Why Do Some Populations Cycle?
• Some populations exhibit population cycles—regular fluctuations in size.
• Most hypotheses put forward to explain population cycles depend on some
density-dependent factor. Predation, disease, or food shortages intensify
dramatically at high population density and cause population numbers to crash.
• A long-term study of the hare and lynx illustrates population cycles:
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Is It Food or Predation?
• To explain the hare-lynx cycle, biologists proposed two hypotheses
based on density dependent factors:
1. Hares use up all their food when their populations reach high
density and starve; in response, lynx also starve.
2. Lynx populations reach high density in response to increases
in hare density. At high density, lynx eat so many hares that
the prey population crashes.
• So the question was, do hares control lynx population size, or do
lynx control hare population size?
– bottom-up control or top-down control?
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A Field Experiment
• To test the hypotheses that predation, food availability, or a
combination of those two factors controls the hare-lynx cycle,
researchers set up a series of study plots in similar boreal forests.
• Three plots were used as controls, one plot excluded lynx but not
hares, two plots provided additional food for hares, and one plot
both excluded predators and provided extra food for hares.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
How Does Age Structure Affect Population Growth?
• A population’s age structure—the proportion of individuals that are
at each possible age—has a dramatic influence on the population’s
growth over time.
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Age Structure in a Woodland Herb
• The common primrose lives in
sunny gaps in shady woodlands.
• Its age structure varies over time.
– Rapidly growing juveniles
dominate populations in sunny
new gaps.
– As trees grow to shade these
gaps, the growth of the
population declines, leaving
mostly adults in the
population.
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Age Structure in a Woodland Herb
• The dynamics of primrose populations are expected to follow three
trends:
1. Populations that are dominated by juveniles should
experience rapid growth followed by a decline due to
shading by trees.
2. The long-term trajectory of the overall primrose population
in an area may depend primarily on the frequency and
severity of windstorms that knock down trees and create
sunlit gaps.
3. In a large tract of forest, the primrose population will have
metapopulation structure.
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Age Structure in Human Populations
• The age structure of human populations in different countries,
which varies dramatically, can be represented by age pyramids—
graphs with horizontal bars representing the numbers of males and
females of each age group.
• The age structure of a population tends to be uniform in developed
countries and bottom-heavy in developing countries.
• Analyzing an age pyramid can give biologists important
information about a population’s history, and also help them predict
a population’s future.
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© 2011 Pearson Education, Inc.
Analyzing Change in Human Population Growth Rate
• The rate of human
population growth has
increased over the past 250
years, leading to a very
steeply rising curve over
the past few decades.
• It is almost impossible to
overemphasize just how
dramatically the human
population has grown
recently.
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© 2011 Pearson Education, Inc.
How Large Is the Current Human Population?
• In 2009, the world population is estimated at over 6.8 billion, and
about 77 million additional people are being added each year.
• It is not possible to overstate the consequences to us and to the
planet of these recent and current increases in human population.
• In addition to being the primary cause of habitat loss and species
extinction, overpopulation is linked to declines in living standards,
political instability, and acute shortages of basic resources.
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How Large Is the Current Human Population?
• The one encouraging trend in the data is that the growth rate of
the human population has begun to decline.
• Since 1970, the growth rate of human populations has been
dropping. Between 1990 and 1995, the worldwide growth rate
averaged 1.46 percent per year; currently the annual rate is 1.2
percent.
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Will Human Population Size Peak in Your Lifetime?
• The UN has projected human population growth to the year 2050
based on current fertility rates, and 2.5 (high), 2.1 (medium), or 1.7
(low) children per woman.
• When fertility at the replacement rate is sustained for a
generation—each woman producing exactly enough offspring to
replace herself and her offspring’s father—zero population
growth (ZPG) results.
• The future of the human population hinges on fertility rates—on
how many children each woman living today decides to have.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Can Population Ecology Help Endangered Species?
When designing programs to save species threatened with
extinction, conservationists draw heavily on concepts and
techniques from population ecology.
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Using Life-Table Data
• Collecting and analyzing demographic data such as age-specific
survivorship and fecundity are important for saving endangered
species and for other applied problems.
• Life-table data can be used to project the future of a population.
• Projecting a population based on life-table data allows biologists to
alter values for survivorship and fecundity at particular ages and
assess the consequences. (also: for humans, life insurance
premiums)
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Altering Values for Survivorship and Fecundity
• Analyses like these allow biologists to determine which aspects of
survivorship and fecundity are especially sensitive for particular
species, and plan accordingly.
• For example, many endangered species have high juvenile
mortality, low adult mortality, and low fecundity. In these species,
the fate of a population is sensitive to increases in adult mortality.
• Based on this, conservationists have begun an intensive campaign
to reduce the loss of adult female sea turtles in fishing nets.
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Altering Values for Survivorship and Fecundity
• In humans and other species with high survivorship, rates of
population growth are sensitive to changes in age-specific
fecundity.
– Programs to control human population growth focus on
lowering fertility rates and delaying the age of first
reproduction.
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Population Viability Analysis
• Although traditional population growth models such as the
expressions for exponential and logistic growth are simple and
elegant, the factors they ignore—immigration and emigration—are
crucial to understanding the dynamics of most populations.
• Because metapopulation structure is common, biologists must use
more sophisticated models to predict the fates of populations.
© 2011 Pearson Education, Inc.