Chapter 52: Population Ecology

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Chapter 52PopulationEcology
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Population Characteristics

• Population ecology studies organisms from the
  point of view of the size and structure of their
  populations both are properties of populations,
  not individuals
• Population ecology also is the study of
  interactions within populations (i.e.,
  intraspecific interactions)
• Recall that populations are groups of interacting
  conspecifics (i.e., inter-mating individuals)
• We can characterize individual populations in
  terms of their
• Size (average vs. variation)
• Density ( impacts on size density dependence)
• Patterns of Dispersion
• Demographics (age structure, sex ratios)
• Rates of growth (or decline)
• Limits on population growth

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Population Dynamics
Addition of individuals to populations
Removal of individuals from populations
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Clumped Dispersion of Population
Clumped dispersion implies some sort of cohesive
force, e.g., either individuals seek other
individuals out, or individuals are limited in
where then can reside
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Uniform Dispersion of Population
Uniform dispersion implies some sort of
antagonistic interaction, e.g., either
individuals actively repel other individuals
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Random Dispersion of Population
Random dispersion implies a minimum of
interspecific interactions that impact where
individuals reside
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Why Different Types?
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Population Demographics
Evolution will tend to maximize the
representation in a population of those
individuals who display those combinations of
life history traits that maximize the number of
surviving progeny they produce
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Life History

• The traits that affect an organisms schedule of
  reproduction and survival (from birth through
  reproduction to death) make up its life history.
  (p. 1156, Campbell Reece, 2002)
• "In many cases there are trade-offs between
  survival and traits such as clutch size (number
  of offspring per reproductive episode), frequency
  of reproduction, and investment in parental care.
  The traits that affect an organism's schedule of
  reproduction and death make up its life history.
  
• In other words, the Darwinian goal is to maximize
  lifetime reproductive output, and this can be
  achieved by having babies more rapidly or living
  longer, or some combination of the two, as well
  as by varying many additional details having to
  do with survival and reproduction

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Age-Structure Pyramids
Note sex ratios are not always 11
Rectangle Cohort group of individuals of the
same age
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Survivorship
Later in life simply fewer individuals left for
selection to act upon
Selection here is stronger earlier in life
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Idealized Survivorship curves
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Idealized Survivorship curves
Exponential declines are due to accidents and
predation at rates that do not appreciably change
as a function of age
Type II curves are a simple exponential decline
with age
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Idealized Survivorship curves
Because individuals tend to die exponentially due
to accidents or predation, it often is a good
strategy to reproduce relatively early in a life
span rather than relatively late
Species that combine maximum fecundity with early
ages typically do so at the expense of their
ability to survive long periods (this is an
example of a genetic principle of allocation)
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Idealized Survivorship curves
A survivorship curve of such individuals may
display a relatively shallow slope while
individuals are younger (i.e., maximally robust
and maximally reproductive) but then show an
abrupt increase in death rate at ages that are
coincident to declines in fecundity
Humans, of course, have a type I survivorship
curve evolution makes us get married young and
have lots of babies before a saber toothed tiger
comes along and picks us off
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Idealized Survivorship curves
Organisms that produce large numbers of cheap
progeny and which display minimal declines in
fecundity with age, if they survive their youth,
can display type III survivorship curves
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Idealized Survivorship curves
Type III survivorship species have a very large
rate of mortality when young, but should they
survive their youth, they put significant energy
into continued survival since the longer they
survive, the more progeny they will produce
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More-Complicated Curve
II or III
High early mortality, accidents or predation in
the middle, and aging later in life
I or II
I (senescence)
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Reproduction

• Survivorship considers death
• Births, of course, also impact population
  densities
• A number of factors are relevant to birth rates
• Age of first reproduction (sexual maturity)
• Clutch size
• Investment in individual progeny
• Tradeoff between reproduction and survival
• Number of reproductive episodes per lifetime
• Semelparity means that an organism can experience
  only one reproductive episode per lifetime
• But as a consequence may be able to produce more
  progeny sooner
• Iteroparity means that an organism can experience
  more than one reproductive episode per lifetime
• But as a consequence may be able to produce more
  progeny over life span

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Reproductive Table
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Quantity rather than Quality
With iteroparity, offspring also will be made
during chance good years for offspring survival
Minimal investment per offspring but lots of
offspring
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Big-Bang Reproduction
Big-Bang reproduction is one reproductive episode
per life time, but very large numbers of
offspring per that episode
Also know as semelparity
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Semelparity
All remaining resources devoted reproduction
rather than continued survival
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Iteroparity
The advantage of iteroparity is that it allows
organisms to display more than one statistical
shot at producing a successful litter
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Emphasizing Quality
Significant investment per offspring, but fewer
offspring
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Resource Allocation

• The life history we observe in organisms
  represent a resolution of several conflicting
  demands. An important part of the study of life
  histories has been understanding the relationship
  between limited resources and competing
  functions Time, energy, and nutrients that are
  used for one thing cannot be used for something
  else.
• "These issues can be phrased in terms of three
  basic questions
• How often should an organism breed?
• When should it begin to reproduce?
• How many offspring should it produce during each
  reproductive episode?
• The way each population resolves these questions
  results in the integrated life history patterns
  we see in nature."

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Life History Tradeoffs
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Population Size

• Population ecology studies organisms from the
  point of view of the size and structure of their
  populations, which are properties of populations
• A population's size depends on how the population
  is defined
• If a population is defined in terms of some
  degree of reproductive isolation, then that
  population's size is the size of its gene pool
• If a population is defined in terms of some
  geographical range, then that population's size
  is the number of individuals living in the
  defined area
• Ecologists typically are more concerned with the
  latter means of defining a population since this
  is both easier to do and is a more practical
  measure if one is interested in determining the
  impact of a given population on a given
  ecosystem, or vice versa

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Exponential Pop. Growth
Population growth rates are a function of
fecundity (birth rate), survivorship, and
generation times
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Exponential Pop. Growth
Greater intrinsic growth rate (r)
r 1.0
r 0.5
r intrinsic population growth rate
Dont worry about actual numbers
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Population Density

• Given that a population is defined in terms of
  some natural or arbitrarily defined geographical
  range, then population density may be defined as
  simply the number of individual organisms per
  unit area
• Different species, of course, exist at different
  densities in their environments, and the same
  species may be able to achieve one density in one
  environment and another in a different
  environment
• Population densities may additionally be
  determined in terms of some measure other than
  population size per unit area such as population
  mass per unit area
• Although we can determine an average population
  size or density for many species, the average
  is often of less interest than the year-to-year
  or place-to-place trend in numbers. (p. 1166,
  Campbell Reece, 2002)

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Density-Dependent Limits on r
K Carrying capacity
No growth
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Density-Dependent Limits on r
N/K environmental resistance
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Logistic Growth
Environmental limits result in logistic growth
Carrying capacity
No limits
New or changed environment
Logistic growth curve
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Maximizing Yield
dN/dt is maximized when Nr is maximized
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Smooth Transition
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Overshooting K
Delayed environmental feedback
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r versus K Selection
Tendency to overshoot K or to poorly compete with
other organisms except in terms of population
growth rates
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r versus K Selection
K-selected organisms are good competitors, dont
overshoot carrying capacities, but tend to have
low population growth rates
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r versus K Selection
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Density Independence?
Logistic growth does not consider predators or
interspecific competition so fails to predict the
complexities of the density of many natural
populations as a function of time
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Impact of Environment/Predators
Logistic growth
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Varying Environment?
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Equilibrium Birth vs. Death Rates
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Declining Birth Rates with Density
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Enforced Density Limitations
By being territorial, individuals can maintain
population densities below carrying capacities
set by other resources (e.g., food supplies)
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Predator-Prey Cycling
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QuestionWhy are humansdestroying the earth?
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QuestionWhy do locusts destroy crops?
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Limits Locust Freedom Without Responsibility
Ive got my rights!
Its a free country!
Whos going to stop me?
Destroyed Crops (destruction of environment)
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QuestionWhy do some microbes make us sick?
(hint it often has much to do with selfish greed
causing environment destruction)
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Limits Pathogen Freedom Without Responsibility
Its a free country
Whos going to stop me?
Ive got my rights!
Disease! (destruction of the body environment)
Bacterial pathogens
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Ecological Footprints
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Paul Ehrlich and the Population Bomb

• Limits on human population growth will be set by
  the carrying capacity of our environment
• Human impact on Earths environment
• Impact Population Affluence Technology
• Impact Population Affluence Efficiency
• Consumption per Baby Resource Efficiency or
  Affluence Technology or Affluence Efficiency
• Consumption Damages Resources (a.k.a., the
  environments
• There are only so many resources to use up!
• And not all of them are renewable
• And many that are renewable are being used up
  faster than they can be renewed!
• So far we have averted these problems by having
  cheap energy, large buffers in the environment to
  human impact (e.g., reserve), and a relatively
  benign climate

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Human Population Growth
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Human Freedom Without Responsibility
Whos going to stop me?
Its a free country
Ive got my rights!
Destructamundo! (destruction of environment)
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