Population Ecology

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Population Ecology
Caribou herd Alaska
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Life History Characteristics

• Growth for at least part of their life history,
  all organisms grow by assimilating energy and
  nutrients growth rate is critical
• Change of form - many organisms have dramatically
  different forms or stages in their life cycle
• Dispersal - at some time in their lives, most
  organisms go through dispersal
• Timing of reproduction has a strong influence on
  population growth - typically the earlier a
  species starts reproduction, the faster the
  population will grow
• Age distribution - Populations also have a
  characteristic age structure or distribution -
  there will be certain numbers of young
  individuals, mature individuals and old
  individuals
• Size at birth or germination
• Number, size and sex of offspring
• Age at Death

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Life History Trade-Offs

• As we saw in the discussion of timing of
  reproduction, there are frequently trade-offs in
  life history characteristics

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Black backed gull
Females that lay large clutches have less
survival than females with smaller clutches
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Semelparous - Species with a single reproductive
event
Pacific Salmon
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By Ray Troll
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Agave Century Plant Reproduces Once
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Samoan Palolo Worm
Palolo epitokes swimming
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Australian Red-Backed SpiderGenus Latrodectus
Female Male
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Iteroparous - Few offspring produced at one time,
but many reproductive events in a lifetime
African elephant
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White Oak
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General Relationship between Offspring Size And
Number of Offspring
Many
Number Of Offspring
Few
Small Large
Offspring Size
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Trade off between longevity and number of
offspring
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Phylogenetic constraints

• Evolution of species life history is limited by
  the species evolutionary history

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Phylogenetic constraints on offspring number in
Laysan Albatross
Brood patch accommodates one egg
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Phylogenetic constraints on offspring number in
primates
Gorilla nursing infant
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Population Dynamics
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Sampling to collect population data

• Most basic sampling a census - count and
  determine age of all individuals in population,
  count again later
• Almost always have to sample sub-populations and
  estimate population size - several different ways
• 1. determine total area in which population
  occurs, count all individuals in small plots,
  multiply average number in plots to get total,
  repeat at later dates - works best for sessile
  organisms
• 2. Mark-recapture methods - use on animals -
  catch a bunch and mark them and release, later
  recapture a bunch
• 3. Catch per unit effort - tells relative size
  but says nothing about absolute size
• 4. various sampling methods - traps for animals,
  count fecal pellets, count number of
  vocalizations (birds, frogs, crickets), feeding
  damage on plants (for herbivorous insects),
  roadside spottings at standard distance (bird
  counts), fur or pelt records

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Demography

• Demography is the study of processes that
  influence population size - it is the way we
  study changes brought about by births, deaths and
  dispersal

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Fundamental Equation of Ecology

• ?N B - D I E
• Change in number births deaths immigration
  emigration
• Or - N1 N0 B D I E
• Where N1 population in the future N0
  population now
• We can group immigration and emigration together
  as dispersal, often considered to be equal to
  zero as many come in as leave, e.g. I E
• N1 N0 B D

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Demographic study of Beldings Ground Squirrels
at Tioga Pass
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Survivorship curves for Beldings Ground Squirrels
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Generalized Survivorship Curves
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Types of General Survivorship Curves

• Type I - low early mortality, high late in life -
  humans in developed countries - heavy investment
  in parental care
• Type II - equal probability of mortality
  throughout life songbirds
• Type III - very high early mortality, then
  survivors live a long time - oysters, many fish
  with millions of eggs

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Population Growth
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Exponential Growth

• The equation for exponential growth is
• dN/dt (b - d)N
• we usually ignore dispersal with unlimited growth
• this amount of growth only occurs if there are no
  limits to growth
• b births, d deaths, N Population size

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Exponential Growth

• We can simplify the equation this way dN/dt rN
  
• Where r b d
• We call little r - the intrinsic rate of natural
  increase or the per capita rate of increase
• r allows us to calculate the amount of new
  individuals added to a population each generation
  
• if little r gt 0, the population is growing
• if little r lt 0, the population is declining
• if little r 0, the population is stable

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Exponential Growth Example

• If
• b 30/1000 .03
• d 10/1000 .01
• r b - d .03 - .01
• r .02
• N 10,000

• Then
• dN/dt rN .02(10,000) 200 new individuals
• N2 10,200
• if we do this again, N3 10,200 204 new, etc.

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Exponential Growth Curves
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Exponential Growth in Elephants? Kruger
National Park
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Exponential Growth in Elephants in Kruger
National Park
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Thomas Malthus

• most populations have capacity to grow faster
  than resources necessary to support their growth
  - 1798

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Pests which show exponential growth
Spruce Budworm Gypsy Moth
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Constraints to Population Growth

• 1) density independent effects - growth
  constraints that are not effected by population
  size - usually abiotic - weather, storms,
  volcanos
• 2) density dependent effects - growth constraints
  whose effects change as population size increases
  - usually biotic - competition, predation,
  parasitism, disease

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Effects of Growth Constraints on Population
Growth
Equilibrium density often symbolized By the
letter K and called carrying capacity
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Logistic Population Growth

• Lets go back to our exponential growth equation
  and add in carrying capacity K
• dN rN(K-N)
• dt K

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With Logistic Growth We Find

• dN rN(K-N)
• dt K
• if the population is small so that K gt N, then
  K-N/K approx. 1, the equation almost reduces to
  dN/dt rN, so growth is nearly exponential at
  small population sizes
• if population is large, so that K N, then
  K-N/K approx. 1/K or 0, so growth is much less
  than exponential or 0

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Logistic Growth in Yeast
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Logistic Growth in Yeast
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Logistic Growth in Several Species
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Still More Logistic Growth
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Effects of Growth Constraints on Population
Growth
Equilibrium density often symbolized By the
letter K and called carrying capacity