Chapter 4, Population Biology

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Chapter 4, Population Biology

• Try taking the online practice quiz at
  http//www.glencoe.com/qe/science.php?qi2495
• for practice.

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4.1 Population Dynamics

• Population growth an increase in size of
  population over time.
• Can be studied in bacterial increase on nutrient
  agar, or growth of organisms in a new
  environment.
• Most growth is linear, but populations increase
  exponentially (at first slowly, and then an
  increasingly rapid pace.

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Linear growth vs. Exponential growth
Linear growth a steady Increase over time
(no acceleration) like earning 6 each hour.
Exponential growth an increase that is
initially slow and then proceeds with increasing
rate (an accelerating growth). Two flies becomes
4, Then 8, then 16, 32, 64, 128, 256
See fig.4.2 and 4.3, p.96,97.
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• It has been estimated that a pair of reproducing
  fruit flies would bury us all (on earth) to a
  depth of around 991 million miles, if allowed to
  reproduce without limitations for a year. See
  math below (from L.Higley, UNL)
• Since we dont see this, what, then, prevents it
  from occurring?
• Limiting factors available food, space to live,
  competition, predation
• This growth vs. limits creates a carry
  capacitynumber of organisms of one species an
  environment can support.

e.g., fruit flies - one pair yields 30
generations/year with 40 eggs/ pair with a 11
sex ratio unlimited reproduction for a single
year would produce a layer of fruit flies over
the earth ca. 991 million miles deep 4029 2
2.882 x 1046 flies, assume each fly is 1 x 1 x 1
mm earth is 4/3 x Pi x 4000 miles x 1.852 x 106
mm/mile1.836 x 1015 cubic mm in volume solve
for radius in miles cube root of (2.882 x
1046)/(1.836 x 1016) x 3/4Pi) 1.836 x 1015 mm
(1.836 x 1015/1.852 x 106) - 4000 9.914 x 108
miles
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• When population is below carrying capacity,
  births exceed deaths until carrying capacity is
  reached.
• When population exceeds C.C., deaths tend to
  exceed births until C.C. is reached.
• Thus population tend to the C.C. as a stable
  equilibrium.
• Carrying capacity is often symbolize as K.
• There are two strategies or styles of living.

• (A) Organisms can have few young, care for each
  one extensively (invest a lot of energy in it)
  and thus give them a better opportunity to
  survive. (Elephants do this). Called a
  K-strategy (counting on the stable carrying
  capacity).
• (B) Organisms can have many young, use little
  energy/care on them, and plan for few to survive,
  counting on the numbers in reproduction to carry
  their survival. Called an r-strategy because
  they are counting on/emphasizing reproduction
  (ticks do this).

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• Organisms that live in a very stable environment,
  where competition advantages and parental
  investment are important do better with a
  K-strategy, especially if they are long-lived.
• Organisms that are invaders, live under
  unpredictable conditions or are short-lived do
  better with an r-strategy
• Different organisms have different degrees of
  these to suit their lives

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Population growthlimiting factors
Crowdingstress in some creatures Factors arent
well understood, but effects include agression,
lessened parental care, more disease and
decreased fecundity.

• Abiotic and biotic factors had to do with living
  or not.
• Another way to look at this is to consider
  factors to be Density dependent, or Density
  independent.
• Density dependent factors depend on how many of
  the organisms are around (ie., if the population
  is more crowded, more of them will die of
  disease). Other examples include competition,
  food supplies, predation, parasites.
• Predator/prey can follow a close pattern of
  cyclical relationships showing mutual effects,
  see p.102

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• Density independent factors act on the population
  regardless of numbers the same way (ie., a cold
  frost will kill the same proportion of chiggers,
  regardless of total numbers).
• Examples usually abiotic factors such as
  temperatures, storms, floods, drought, habitat
  disruption.

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4.2 Human population growth

• There were about 10 million people around 10,000
  years ago. Today there are over 6 billion.
• What the carrying capacity of earth is depends on
  many unknowns (increased food supply, crowding,
  etc. At some point, food is likely to be
  outstripped by growth if the population doesnt
  level off for other reasons.
• Demography is the study of human population
  growth characteristics.

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• Factors in demography growth rate, age
  structure, geographic distribution.
• Human population has increased exponentially over
  time thus far (this isnt sustainablewhat will
  limit it is the question in time. Our ability to
  kill competition, control disease and increase
  food supply have sustained the exponential
  increase thus far.
• Growth rate is difference between birth rate and
  death rate.
• Sometimes expressed as doubling rate (how long to
  double the population).

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• As the 21st Century dawns, the world faces the
  prospect of a new and complex food crisis that
  will require better ways of ensuring that the
  hungry and the malnourished will be able to meet
  their food needs (James D. Wolfensohn, President
  of the World Bank). To tackle this enormous
  challenge, the international community must
  launch a new "Green Revolution", more powerful
  and encompassing than the one that thirty years
  ago that doubled production of key crops such as
  rice and wheat. The power of science and
  information technology must be harnessed for the
  benefit of the worlds poor. "New scientific
  developments have the potential to radically
  reshape the worlds agriculture and food systems
  according to Mahendra Shah the co-author of the
  book, Food in the 21st Century From Science to
  Sustainable Agriculture. "We need to recommit to
  science and research to ensure that the poor are
  not excluded, and that biodiversity and the
  environment are not undermined."
• Today, there are more than 840 million people a
  number exceeding the combined population of
  Europe, US, Canada and Japan who do not have
  enough to eat. Every minute, some 30 people die
  of hunger in the developing world and half of
  these are infants and children. A medium
  projection is for world population to reach about
  8.3 billion by 2025, before hopefully stabilizing
  at about 10-11 billion toward the end of the 21st
  century. At least in the foreseeable future,
  plants and especially cereals will continue
  to supply much of our increased food demand, both
  for direct human consumption and as livestock
  feed to satisfy the rapidly growing demand for
  meat in the newly industrializing countries. It
  is likely that an additional 1 billion tonnes of
  grain will be needed annually by 2025. Most of
  this increase must be supplied from lands already
  in production, through yield improvements.
• Food production will have to increase by more
  than 50 percent to feed an additional two billion
  people by 2025. But the challenge is far more
  intricate than simply producing more food.
  Conditions are very different than they were on
  the eve of the Green Revolution. To prevent a
  crisis, the world community must simultaneously
  confront the issues of poverty, food insecurity,
  environmental degradation, and erosion of genetic
  resources.
• ..C.Reimers-Hild, UNL Module 1.5, 2003.
• http//www.worldbank.org/poverty/quiz/
• http//web.worldbank.org/WBSITE/EXTERNAL/EXTABOUTU
  S/0,,contentMDK20040558menuPK34559pagePK34542
  piPK36600,00.html

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• Declining death rate in industrialized countries
  has greater effect than increased birth rate.
• You are more likely to live a bit longer than I
  am (each generation living a little longer.
• Fertility (number of offspring produced by a
  female in reproductive years) is decreasing
  (mostly folks waiting longer to have childrenis
  this a K-strategy? fewer offspring, more time
  invested, more energy).
• Compare birth/death/fertility rates to overall
  population change in Table 4.2, p.105.

Mr. Gthe early years.
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• Age structure is important factor. How many of
  these people are teenagers or 65?
• Makes difference in likely population increase
  (see Fig.4.11, p.107 and compare Mexico to
  Germany to see what is meant by this).
• A wide base population means large percentage are
  young, and likely to reproduce and add more to
  population size.

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Mobility

• Humans move in and out of populations.
• Emigration moving out of a given population
  (think Exit).
• Immigration moving into a population (think In).
  The U.S. scutinizes immigrants b/c they are
  coming into the country.
• Mobility doesnt change total population, but
  alters local/national levels and makes prediction
  one step more complicated.
• Wars, economic opportunities or lack of them.
• E/Immigration on local scale cause changes in
  urban/rural populations, growth in some
  neighborhoods over othersdriven by
  necessity/opportunity or plain ol desire to live
  with others of like mind.

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The End

• But only the middle, on the human population
  growth curvewhat will the future bring?