Ecology

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Ecology

• Chapters 9-11-Population Ecology

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What is ecology

• Study of interactions of organisms with one
  another and their physical environment
• How these interactions determine the distribution
  and abundance of organisms

3
Levels of ecology

• Population (Population ecology)
• Interbreeding group of individuals
• Community (Community ecology)
• Interacting group of species inhabiting same area
• Ecosystem (Ecosystem ecology)
• Community plus nonliving parts of environment

4
Populations

• A population is a group of interbreeding
  individuals of the same species
• Populations may stay the same, increase, or
  decrease
• Population ecology is the science that describes
  how and why populations change

5
Density Distribution

• Number of individuals in some specified area of
  habitat
• Crude density information is more useful if
  combined with distribution data

clumped
nearly uniform
Figure 45.2 Page 808  
random
6
Determining Population Size

• Direct counts are most accurate but seldom
  feasible
• Can sample an area, then extrapolate
• Capture-recapture method is used for mobile
  species

7
Population growth

• Populations may grow or decline
• Unrestrained growth or decline is exponential
• Results in a J-shaped curve plotting numbers of
  individuals versus time
• Exponential growth is proportional to number of
  individuals present in population

8
Exponential Growth Equation

• G rN
• G is population growth per unit time
• r is net reproduction per individual per unit
  time
• N is population size

9
Population Growth
net population growth rate per individual per
unit time
population growth per unit time
number of individuals

X
10
Growth rate

• Growth is the number of births - number of deaths
  in a population
• Birth rate is number of births/1000 individuals
• Death rate is number of deaths/1000 individuals

11
Population Growth

• Populations grow when birth rategtdeath rate, stay
  the same when equal, decline when birth
  rateltdeath rate
• r is the maximum growth rate

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Limits to exponential growth

• Populations cannot grow indefinitely
• Resources (space, food, light, etc) limit growth
• Logistic growth model (S-shaped curve) describes
  limited growth

14
Logistic Growth Equation

• G rmax N (K-N/K)
• G population growth per unit time
• rmax maximum population growth rate per unit
  time
• N number of individuals
• K carrying capacity

15
Exponential Growth vs. Logistic Growth
16
Role of movement of individuals

• In addition to births and deaths
• Emigration removes individuals and immigration
  adds individuals to a population
• Change in population is births plus immigrants
  minus deaths plus emigrants

17
Density-dependent and independent factors

• Some factors that affect birth and death rates
  are dependent on the size of the population
  (density-dependent factors)
• Larger populations may mean less food/individual,
  fewer resources for survival or reproduction
• Other factors that may affect these rates may be
  independent of population size (fire or hurricane)

18
Natural populations

• Studies of natural populations show that
  exponential growth can occur when populations are
  small and resources abundant
• But all populations stop growing when resources
  run out
• Sometimes populations overshoot carrying
  capacity, begin fluctuating around the carrying
  capacity

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Life-history traits of species

• Each species has its own unique capacity to grow
  rapidly
• Some species can experience rapid exponential
  growth
• R-selected species, weeds, cockroaches, many
  insects
• Other species grow very slowly
• K-selected species, elephants, large trees
• Most species show aspects of both types

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Life-history traits

• Mortality curves show how quickly individuals die
• Type I (low juvenile mortality, high adult
  mortality)-humans
• Type II (constant mortality)-birds
• Type III (high juvenile mortality, low adult
  mortality)-insects

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Population Age Structure

• Divide population into age categories
• Populations reproductive base includes members
  of the reproductive and pre-reproductive age
  categories

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Age structure of a population

• Fraction of individuals of different ages
• Each group born at the same time forms a cohort
• Age structure related to growth rate
• Pyramid shape is rapid growth, many young (India)
• Diamond shape is slower growth (USA)
• Rectangular shape is zero growth (Sweden)

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Demography (Human populations)

• Annual rate of increase percentage change in a
  population
• Completed family size average number of
  children born to each family that reach
  reproductive age
• Replacement reproduction same number of
  children as their parents (2.1)
• Population momentum-due to many young not yet
  reproducing

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Demographic Transition Model

• Based on historical data from western Europe
• Postulates that as countries become
  industrialized, first death rates drop, then
  birth rates drop

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Demographic Transition Model
Stage 1 Preindustrial
Stage 2 Transitional
Stage 3 Industrial
Stage 4 Postindustrial
relative population size
births
deaths
low
increasing
very high
decreasing
low
zero
negative
Figure 45.16 Page 822
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Earths carrying capacity

• Have humans exceeded the carrying capacity of the
  earth?
• 6 billion and rising
• Use 40 of all the energy of the earth for
  ourselves
• Inability to feed everyone
• Reliance on unsustainable energy reserves (oil,
  coal, and gas)
• Extinction of wild species of plants and animals