Population Ecology

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Population Ecology

• What is a population?
• Life Histories
• Population Growth
• Population-Limiting Factors

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A population is a group of individuals of a
single species that simultaneously occupy the
same general area.What do these individuals
share in common as a result?Part of a
population of African buffalo (Syncerus caffer)
in the Serengeti of East Africa
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Figure 52.3 Individuals within populations can
be distributed in a random, clumped, or uniform
pattern.
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Population Ecology

• What is a population?
• Life Histories
• Population Growth
• Population-Limiting Factors

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Variation in Life Histories

• An organisms traits that affect its schedule of
  reproduction and survival make up its Life
  History
• Such traits include frequency of reproduction,
  number of offspring (i.e., seeds for plants),
  size of offspring, lifespan

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• Life histories vary a great deal among species,
  but there are patterns
• One pattern semelparity vs. iteroparity
• Semelparity (or big-bang reproduction) one
  large single reproductive effort before death
• Iteroparity repeated reproduction during a
  lifetime

Figure 52.6 An example of semelparity Agave
(century plant)
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Another pattern of life history variation annual
vs. biennial vs. perennial

• Annual plants complete their life cycle in 1 year
  or less
• Biennial plants have 2 year cycle Usually 1 year
  with vegetative growth and one year with
  flowering, with a winter in between
• Perennial plants live many years

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The importance of trade-offs

• Semelparity vs. iteroparity illustrates an
  important theme in the ecology evolution of
  life histories they are driven by trade-offs
• Because resources are limited, organisms cannot
  maximize everything (lifespan, frequency of
  reproduction, number of offspring, size of
  offspring)
• Thus, the question is What situations favor
  strategies that maximize one or more of these
  variables at the expense of the others?

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One of the most important tradeoffs for mature
organisms reproduction vs. survival. Producing
offspring now may reduce a females chance of
surviving into the future.Cost of reproduction
in female red deer on the island of Rhum, in
Scotland
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Figure 52.67 Experimental demonstration of the
survival/reproduction tradeoff. Probability of
survival over the following year for European
kestrels after raising a modified brood (number
of chicks).
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Figure 52.5 Different species display
characteristic patterns of mortality vs. age
(survivorship curves). Which type would most
favor early reproduction? Which could favor
delayed reproduction?
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Figure 52.8 Another important tradeoff Number
vs. size of offspring. Is it best to produce
many small offspring or few large offspring?
Pictured below Variation in seed crop () and
seed size in plants Dandelion vs. coconut
palm.
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Summary How tradeoffs shape life histories

• Three basic, trade-off driven, life history
  choices facing organisms
• At what age to begin reproduction?
• How often to reproduce?
• How many offspring and what size?
• Answers to these questions can be behavioral or
  plastic choices, but will also be molded by
  natural selection over longer timescales

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Population Ecology

• What is a population?
• Life Histories
• Population Growth
• Population-Limiting Factors

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Population Growth Changes in N over time

• The size of a population (N) will increase with
  every birth or immigration event, and will
  decrease with every death or emigration event
• For now, well ignore immigration and emigration
• Thus, over a given time interval

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-Next, we express births and deaths on a per
capita basis, i.e. average number of births (b)
and deaths (d) per individual during a time
interval, then
B bN and D dN -For example, if a population
of size 1000 experiences 36 deaths per year, then
the annual per capita death rate (d) is 36/1000,
or 0.036 -If d is 0.036, how many deaths (D)
would you expect per year in a population of size
500?
Answer dN D or 0.036 x 500 18
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Thus

• If what were interested in is NET change in
  population size, we can lump b and d into one
  term r b d
• If r is greater than zero, then there are more
  births per unit time than deaths, and the
  population will increase
• r is the per capita growth rate of a population

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If resources are unlimited, we expect r to be
large, and the population will grow
exponentially, according to the exponential
model
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Example of exponential population growth in
nature the whooping crane.

• Would you expect real populations in nature to
  continue to grow exponentially forever?
• No. We would be buried in a worldwide heap of
  bacteria and other organisms.
• Rather, each habitat has a maximum number of
  individuals of a species it can support the
  carrying capacity (K)

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Figure 52.11 Therefore, we need a more realistic
model that incorporates this concept Reduction
of population growth rate with increasing
population size (N)
(K N) K
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Figure 52.12 An example of population growth
predicted by the logistic model is shown in red.
Note the assumed mechanism is a decrease in b
and/or increase in d with population size.
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Figure 52.13 How well do these populations fit
the logistic population growth model? What
incorrect assumtions might be contained in the
logistic model?
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Relationship between the logistic model and life
history traits

• Natural selection may favor different life
  history traits when populations are at low
  density compared to when they are at high density
• Traits favored by natural selection in crowded
  conditions (i.e. near K), are called K-selected
  traits, and tend to favor stable populations
  near the carrying capacity (Example large trees)
• Traits favored by natural selection at low
  densities are called r-selected, and favor
  rapid increases in population size. These traits
  are more common in species living in variable
  environments or colonizing open spaces after
  disturbances (e.g., weedy plants)

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Population Ecology

• What is a population?
• Life Histories
• Population Growth
• Population-Limiting Factors

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What limits the size of populations?

• Answer Negative feedback on population growth,
  i.e. density-dependent death and/or birth rates.
• Figure 52.14 is a graphic model showing how
  equilibrium may be determined for population
  density.
• Equilibrium is attained when the birth rate
  equals the death rate. The population then stops
  growing or shrinking in numbers (i.e., dN/dt 0).

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Figure 52.15 One kind of negative feedback on
population growth Decreased fecundity (i.e. per
capita birth rate, or b) at high population
densities.
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Figure 52.15 Another kind of negative feedback
on population growth Decreased survivorship
(i.e. increased per capita death rate, or d) at
high population densities.
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Figure 52.18 Fluctuations in population size are
often due to a combination of biotic and abiotic
factors. Shown below Long-term study of the
moose (Alces alces) population of Isle Royale,
Michigan
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Figure 52.21. Some populations, such as the lynx
and the hare, fluctuate in strikingly regular
cycles.Why might predator and prey numbers
cycle?
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A purely mathematical explanation of
predator-prey cycles.

• Simple math can explain or even predict complex
  ecological patterns.
• You dont need to completely solve a set of
  equations to gain insight from them.
• Graphs are a good way to get information out of
  unsolved equations.

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Changes in number of hares through time
dNhare
rhareNhare - pNlynxNhare

dt

• Ignore hare carrying capacity (assume lynx keep
  hare numbers too low to really deplete their
  resources).
• (Same explanation works if you dont ignore hare
  carrying capacity, math is just harder.)

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Changes in number of lynx through time
dNlynx
rlynxNhareNlynx - sNlynx

dt

• Can ignore lynx carrying capacity, since this
  model makes lynx starve when they run out of
  hares.

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System so far
dNhare
rhareNhare - pNlynxNhare

dt
dNlynx
rlynxNhareNlynx - sNlynx

dt
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When is hare population constant?
dNhare
0

dt
dNhare
rhareNhare - pNlynxNhare

dt
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When is hare population constant?
rhareNhare - pNlynxNhare 0
rhareNhare pNlynxNhare
rhare pNlynx
Nlynx p/rhare
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Nlynx
Nhare shrinks if Nlynx gt p/rhare
p/rhare
Nhare grows if Nlynx lt p/rhare
Nhare
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Nlynx
p/rhare
Nhare
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When is lynx population constant?
dNlynx
0

dt
dNlynx
rlynxNhareNlynx - sNlynx

dt
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When is lynx population constant?
rlynxNhareNlynx - sNlynx 0
rlynxNhareNlynx sNlynx
rlynxNhare s
Nhare s/rlynx
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Nlynx
p/rhare
Nhare
s/rlynx
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Nlynx
p/rhare
Nhare
s/rlynx
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Nhare
time
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Figure 52.22 Human population growth has been
nearly exponential for 300 years, and now causes
most of our major environmental problems. Until
the current U.S. administrations policies
started kicking in, it was showing signs of
slowing down due to increased empowerment of
women around the world