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Chapter 52 Notes
Earth’s Fluctuating Population
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No population can continue to grow indefinitely
Evolution remains our central theme as we now view populations in the context of ecology
Population ecology: the study of populations in relation to the environment, including environmental influences
on population density and distribution, age structure, and variations in population size
52.1
Dynamic biological processes influence population density, dispersion, and demography
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Population: a group of individuals of a single species living in the same general area
o Members rely on the same resources, are influenced by similar environmental factors, and have a high
likelihood of interacting with and breeding with one another
o Can evolve through natural selection acting on heritable variations among individuals
Density and Dispersion
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At any given moment, every population has specific boundaries and a specific size
Usually start by defining boundaries appropriate to the organisms under study and to the questions being asked
o Boundaries could be natural or arbitrary
Density: the number of individuals per unit area or volume
Dispersion: the pattern of spacing among individuals within the boundaries of the population
Density: A Dynamic Perspective
o Can count number of individuals to determine density, but usually impractical
 Instead, ecologists use a variety of sampling techniques to estimate densities and total
population sizes
 Ex: counting in a grid square and multiplying to get an estimate; or count nests, tracks,
burrows, etc.
 Another method: mark-recapture method
o Place traps and mark animals with tags, collars, band, or dye; then after a few
weeks recapture marked and unmarked animals
o Assumes that every marked individual has the same probability of being trapped
as each unmarked individual
o Density is not a static property of a population but is a result of interplay between processes that add
individuals to a population and those that remove them
o Immigration: influx of new individuals from other areas
o Emigration: the movement of individuals out of a population
 To increase population: immigration and birth
 To decrease population: emigration and death
Patterns of Dispersion
o Within a population’s geographic range, local densities may vary substantially
 Provide insight into the environmental associations and social interactions of individuals in the
population
o Most common pattern: clumped
 Individuals aggregate in patches
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 May be associated with mating behavior
 May increase the effectiveness of certain predators
Uniform pattern may result from direct interactions between individuals in the population
 Usually exhibit territoriality: the defense of a bounded physical space against encroachment by
other individuals
 Not as common as clumped
Random: occurs in the absence of strong attractions or repulsions among individuals of a population or
where key physical or chemical factors are relatively homogeneous across the study area
 Not as common in nature as you would expect  least common
Demography
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Demography: the study of the vital statistics of populations and how they change over time
o Particular interests: birth rate and how they vary among individuals; and death rates
Life Tables
o Life tables: age-specific summaries of the survival pattern in a population
o Cohort: a group of individuals of the same age, from birth until all are dead
o To build a life table, we need to determine the number of individuals that die in each age group and
calculate the proportion of the cohort surviving from one age to the next
 Difficult to construct for wild animals
 Life table shows the proportions of females and males in the cohort that are still alive at each
age
 Also compare death rates
Survivorship Curves
o Survivorship curve: a graphic way of representing the data in a life table
 A plot of the proportion or numbers in a cohort still alive at each age
o Multiply the proportion alive at the start of each year by the hypothetical beginning cohort
o Three general types:
 Type I: flat at the start, reflecting low death rates during early and middle life, then drops
steeply as death rates increase among older age groups
 Ex: humans and many other large mammals
 Type II: constant death rate over the organism’s life span
 Ex: squirrels, rodents, some invertebrates, some lizards, and some annual plants
 Type III: drops sharply at the start, reflecting very high death rates for the young, but then
flattens out as death rates decline for those few individuals that have survived to a certain
critical age
 Usually associated with organisms that produce very large numbers of offspring but
provide little or no care
 Ex: long-lived plants, fish, marine invertebrates
o Many species fall somewhere between these basic types of survivorship or show more complex patterns
o In populations without immigration or emigration, survivorship is one of the two key factors
determining changes in population size
Reproductive Rates
o Demographers usually ignore males and focus on females because females are the ones who reproduce
o Reproductive table: age-specific summary of the reproductive rates in a population
 Best way is to measure the reproductive output of a cohort from birth until death
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Reproductive output is the product of the proportion of females of a given age that are breeding
and the number of female offspring of those breeding females
Reproductive tables vary greatly, depending on the species
52.2
Life history traits are products of natural selection
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Natural selection favors traits that improve an organism’s chances of survival and reproductive success
Life history: the traits that affect an organism’s schedule of reproduction and survival
o Entail three basic variables:
 When reproduction begins
 How often the organism reproduces
 How many offspring are produced during each reproductive episode
Remember: organisms do not choose consciously when to reproduce or how many offspring to have (except
humans)
Life History Diversity
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Semelparity (big-bang reproduction: “one-shot” reproductive pattern; produce several thousand offspring and
then die
Iteroparity (repeated reproduction): produce only a few offspring a year, but reproduce for several years
What factors contribute to the evolution of semelparity versus iteroparity?
o Critical factor is the survival rate of the offspring
 When survival rate of offspring is low, semelparity is favored
 Production of large numbers of offspring in such environments increases the probability
that at least some will survive
 Iteroparity favored in more dependable environments where competition for resources may be
intense
 A few relatively large, well-provisioned offspring will have a better chance of surviving
to reproductive age
“Trade-Offs” and Life Histories
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Time, energy, and nutrients that are used for one thing cannot be used for something else
o As a consequence, natural selection cannot maximize all these reproductive variables simultaneously
Organisms have finite resources, and limited resources mean trade-offs
o There is a trade-off between reproduction and survival
Selective pressures also influence the trade-off between the number and size of the offspring
Extra investment on the part of the parent greatly increases the offspring’s chances of survival
52.3
The exponential model describes population growth in an idealized, unlimited environment
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The concepts of life history provide a biological foundation for a quantitative understanding of population
growth
Unlimited population increase does not occur indefinitely for any species, either in the lab or in nature
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A population that begins at a small size in a favorable environment may increase rapidly for a while, but
eventually, its numbers must stop growing
So, it is useful to study population growth in an idealized, unlimited environment because such studies reveal
the capacity of species for increase and the condition in which that capacity may be expressed
Per Capita Rate of Increase
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Define a change in population size during a fixed time interval with the following equation (ignoring immigration
and emigration):
o Change in population size during time interval = births during time interval –deaths during time interval
 Want to more concisely represent this
 N represents the population size
 T represents time
 ∆N = change in population size
 ∆T = time interval
o Equation is now:
 ∆N/∆t = B-D
 B is the number of births in the population during the tem interval
 D is the number of deaths
The per capita birth rate is the number of offspring produced per unit time by an average member of the
population
o Ex: if there are 34 births per year in a population of 1000 individuals, the annual per capita birth rate is
34/1000 = .034
 Can use the formula B = bN to calculate the expected number of births per year in a population
of any size
 Ex: If annual per capita birth rate is .034 and the population size is 500:
o B =bN
o B = .034 x 500
o B = 17 per year
The per capita death rate (m) allows us to calculate the expected number of deaths per unit time in a population
of any size
o Ex: if m = .016 per year, we would expect 16 deaths per year in a population of 1,000 individuals
Change the equation from:
o ∆N/∆t = B-D
To:
o ∆N/∆t = bN – mN
One more simplification
o Population ecologists are most interested in the difference between the per capita birth rate theat per
capita death rate:
 R = b—m
 Shows if a population is growing (r>0) or shrinking (r<0)
o Zero population growth: occurs when the per capita birth and death rates are
equal (r=0)
Final equation for change in population size:
o ∆N/∆t = rN
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Exponential Growth
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Exponential population growth: population whose members all have access to abundant food and are free to
reproduce at their physiological capacity
o Under these conditions, the per capita rate of increase may assume the maximum rate for the species,
called the intrinsic rate of increase (rmax)
Equation for exponential population growth:
o ∆N/∆t = rmaxN
 Results in J-shaped growth curve when population size is plotted over time
 Population growth depends on N as well as rmax, and larger populations experience more births
than small ones growing at the same per capita rate
 Also, a population with a higher intrinsic rate of increase will grow faster than one with
a lower rate of increase
J-shaped curve of exponential growth is characteristic of some populations that are introduced into a new or
unfilled environment or whose numbers have been drastically reduced by a catastrophic event and are
rebounding
52.4
The logistic growth model includes the concept of carrying capacity
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Carrying capacity: symbolized as K; the maximum population size that a particular environment can support
o Not fixed, but varies over space and time with the abundance of limiting resources
 Energy, shelters, refuges from predators, soil nutrients, water, an suitable nesting sites can all
be limiting factors
Crowding and resource limitation can have a profound effect on population growth rate
o If individuals cannot obtain sufficient resources to reproduce, the per capita birth rate (b) will decline
o If they cannot find and consume enough energy to maintain themselves, the per capita death rate (n)
may increase
 A decrease in b or an increase in m results in a lower per capita rate of increase, r
The Logistic Growth Model
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Logistic population growth: the per capita rate of increase declines as carrying capacity is reached
o To make a model, start with the exponential population growth model and add an expression that
reduces the per capita rate of increase as N increase
 If the maximum sustainable population size is K, then K – N is the number of additional
individuals the environment can accommodate, and (K – N)/K is the fraction of K that is still
available for population growth.
 By multiplying the exponential rate of increase rmaxN by (K—N)/K, then the new equation is:
 ∆N/∆t = rmaxN(K—N)/K
When N is small compared to K, the term (K—N)/K is large, and the per capita rate of increase is close to the
intrinsic rate of increase
When N is large and resources are limiting, then (K—N)/K is small, and so is the per capita rate of incrase.
When N=K, the population stops growing
The logistic model of population growth produces a sigmoid (S-shaped) growth curve when N is plotted over
time
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New individuals are added to the population most rapidly at intermediate population sizes, when there
is not only a breeding population of substantial size, but also lots of available space and other resources
in the environment
 The population growth rate slows dramatically as N approaches K
For a population’s growth rate to decrease, either the birth rate b must decrease, the death rate m must
increase, or both
The Logistic Model and Real Populations
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In most natural populations, there is a lag time before the negative effects of an increasing population are
realized
o Reproduction can be reduced, but the birth rate may not decline immediately
 This may cause the population to overshoot its carrying capacity before settling down to a
relatively stable density
The logistic model also incorporates the idea that regardless of population density, each individual added to a
population has the same negative effect on population growth rate
Although the logistic model fits few, if any, real populations closely, it is a useful starting point for thininga bout
how populations grow and for construction more complex models
o Also useful in conservation biology for estimating how rapidly a particular population might increase in
numbers after it has been reduced to a small size
The Logistic Model and Life Histories
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The logistic model predicts different per capita growth rates for populations of low or high density relative to
the carrying capacity of the environment
o At high densities, each individual has few resources available, and the population grows slowly, if at all
o At low densities, per capita resources are relatively abundant, and the population can grow rapidly
At high population density, selection favors adaptations that enable organisms to survive and reproduce with
few resources
At low population density, adaptations that promote rapid reproduction should be favored
K-selection: selection for life history traits that are sensitive to population density; AKA density-dependent
selection
o Tends to maximize population size and operates in populations living at a density near the carrying
capacity
R-selection: selection for life history traits that maximize reproductive success in an uncrowded environment
o Tends to maximize the rate of increase and occurs in environments in which population densities
fluctuate well below carrying capacity
52.5
Populations are regulated by a complex interaction of biotic and abiotic influences
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Two questions about regulation of population growth:
o 1: what environmental factors stop a population from growing?
o 2: why do some populations show radical fluctuations in size over time, while others remain more
stable?
 Questions have many practical applications
 In agriculture
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In protecting endangered species
Population Change and Population Density
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First step in understanding why a population stops growing to study how the rates of birth, death, immigration,
and emigration change as population density rises
o If immigration and emigration offset, then a population grows when the birth rate exceeds the death
rate and declines when the death rate exceeds the birth rate
Density independent: birth rate or death rate that does not change with population density
Density dependent: death rate that rises as a population density rises, as is a birth rate that falls with rising
density
Density-Dependent Population Regulation
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Example of negative feedback
o Negative feedback: accumulation of an end product of a process slows that process
Without some type of negative feedback between population density and the vital rates of birth and death, a
population would not stop growing
o However, at increased densities, birth rates decline and/or death rates increase, providing the negative
feedback that halts continued population growth
 Once we know how birth and death rates change with population density, we need to
determine the mechanisms causing these changes
Competition for Resources
o In crowded populations, increasing population density intensifies intraspecific competition for declining
nutrients and other resources, resulting in a lower birth rate
Territoriality
o In many vertebrates and some invertebrates, territoriality may limit density
 In this case, territory space becomes the resource for which individuals compete
Health
o Population density can also influence the health and thus the survival of organisms
 If the transmission rate of a disease depends on a certain level of crowding in a population, the
disease’s impact may be density dependent
Predation
o Predation may be an important cause of density-dependent mortality for some prey populations if a
predator encounters and captures more food as the population density of the prey increases
 As a prey population builds up, predators may feed preferentially on that species, consuming a
higher percentage of individuals
Toxic Wastes
o The accumulation of toxic wastes can contribute to density-dependent regulation of population size
Intrinsic factors
o For some animals, intrinsic (physiological) factors, rather than the extrinsic (environmental) factors
we’ve just discussed, appear to regulate population size
o Ex: mice
 Although the exact mechanisms by which aggressive behaviors affects reproductive rate are not
yet understood, researchers have learned that high population densities in mice induce a stress
syndrome in which hormonal changes can delay sexual maturation, cause reproductive organs
to shrink, and depress the immune system
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These various examples of population regulation by negative feedback show how increased densities cause
population growth rates to decline by affecting reproduction, growth, and survivorship
o Helps answer first question
Now look at second question
Population Dynamics
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While some populations appear to be more stable in size than others, over the long term, all populations for
which we have data show some fluctuation in numbers
Population dynamics: focuses on the complex interactions between biotic and abiotic factors that cause
variation in population size
Stability and Fluctuation
o Although large mammal populations are more dynamic than once thought, they show much more
stability than other populations
Metapopulations and Immigration
o Immigration and emigration can also influence populations
 Especially true for metapopulations
 Metapopulation: group of populations is linked
o How might immigration influence population dynamics
 The metapopulation concept underscores the significance of immigration and emigration in the
contrasting song sparrow populations, and is important for understanding populations in patchy
habitats
Population Cycles
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While many populations fluctuate at unpredictable intervals, others undergo regular boom-and-bust cycles,
fluctuating in density with a remarkable regularity
52.6
Human population growth has slowed after centuries of exponential increase
The Global Human Population
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Exponential growth for humans since 1650
o Singular case: unlikely that any other population of large animals has ever sustained so much growth for
so long
o Human population increases by about 73 million people a year (201,000 a day)
Even though the global population is still growing, the rate of growth began to slow during the 60s
o Shows that human population is not growing exponentially anymore
o Due to voluntary population growth and diseases
Regional Patterns of Population Change
o 0 population growth = high birth rate – high death rate
o 0 population growth = low birth rate – low death rate
 Movement from the first toward the second is called demographic transition
 Associated with an increase in the quality of health care and sanitation as well as
improved access to education, especially for women
o After 1950, mortality rates declined rapidly in most developing countries, but birth rates have declined
in a more variable manner
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How do such variable birth rates affect the growth of the world’s population?
 Some are below replacement
 Most of the population growth is found in developing countries
o A unique feature of human population growth is the ability to control it with family planning and
voluntary contraception
 Reduced family size is the key to the demographic transition
Age Structure
o Age structure: the relative number of individuals of each age
 Commonly represented in a pyramid
 Small base means that individuals younger than reproductive age are relatively
underrepresented in the population: population decrease
 Large base means there are young individuals who will grow up and may sustain the explosive
growth with their own reproduction: rapid growth
 Relatively even: population distributed pretty evenly all the way through the age ranges: slow
growth
o Age structure diagrams not only predict a population’s growth trends, but they can also illuminate social
conditions
 Schooling and education
 Social Security and Medicare availability
Infant Mortality and Life Expectancy
o Infant mortality: the number of infant deaths per 1,000 live births
o Life expectancy: the predicted average length of life at birth
 Reflect the quality of life faced by children at birth
o Infant mortality rate higher and life expectancy lower in developing countries
Global Carrying Capacity
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The projected worldwide population size depends on assumptions about future changes in birth and death rates
o For 2050, United Nations projects a global population of approximately 7.5 to 10.3 billion people
Estimates of Carrying Capacity
o What is the carrying capacity of Earth for humans?
 First estimate: 13.4 billion made in 1679
 Since then, anywhere from less than a billion to over a trillion people; average of 10-15 billion
o Difficult to estimate, and scientists who provide these estimates use different methods to get their
answers
 Some use logistic curves
 Some generalize from existing “maximum” population density and multiply this by the area of
habitable land
 Some are based on a single limiting factor, like food, and require many assumptions
Ecological footprint
o Humans have multiple restraints: we need food, water, fuel, building materials, clothing, transportation,
etc.
o Ecological footprint: summarizes the aggregate land and water area appropriated by each nation to
produce all the resources it consumes and to absorb all the waste it generates
o Six types of ecologically productive areas are distinguished in calculating the ecological footprint:
 Arable land
 Pasture
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Forest
Ocean
Built-up land
Fossil energy land
 All measure are converted to hectares per person
o Adding up all the ecologically productive land on the earth is about 2 ha per
person
 1.7 ha is the benchmark because some are reduced for parks and
conservation means
Two key conclusions:
 Countries vary greatly in their individual footprint size and in their available ecological capacity
 Ecological capacity: the actual resource base of each country
o US: ecological footprint of 8.4 ha per person but has only 6.2 ha per person of
available ecological capacity
 Above carrying capacity
o New Zealand: ecological footprint of 9.8 ha per person but an available capacity
of 14.3 ha per person
 Below carrying capacity
 In general, the world was already in ecological deficit when the study was conducted
 Suggests that the world is now at or slightly above its carrying capacity
Can only speculate about Earth’s ultimate carrying capacity for the human population or about what
factors will eventually limit our growth
 Food?
 Suitable space?
 Nonrenewable resources (metals and fossil fuels)?
 Renewable resources (water)?
NO POPULATION CAN CONTINUE TO GROW INDEFINITELY
 Exactly what the world’s human carrying capacity is and under what circumstances we will
approach it are topics of great concern and debate