Chapter 20: Populations

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Chapter 20 Populations
20-1 Understanding Populations
20-2 Measuring Populations
20-3 Human Population Growth
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20-1 Understanding Populations
I. Properties of Populations

• SIZE, DENSITY, DISPERSION, and GROWTH RATE are
  DRIVEN by abiotic and biotic forces of the
  environment.

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(A) Population Size (estimate a sampling size ?
distribution constant)

• Size is a REPRODUCTIVE function of SPACE and
  RESOURCES in an ecosystem.

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(B) Population Density

• of individuals PER unit of area of volume.
• (Ex In U.S., population density of humans is
  about 30/square kilometer In Japan, 330/square
  kilometer however, BOTH has similar pop size.)

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Critical Thinking
(1) Because we have more power to alter our
environment than other animals do, we can affect
the carrying capacity of our environment. How do
we increase or decrease the carrying capacity of
our local area?
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(C) Dispersion (influenced by species BEHAVIOR
and RESOURCES)

• SPATIAL distribution of individuals of a
  population WITHIN a geographical range.

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(1) Clumped Dispersion

• Occurs when resources are clumped OR may occur
  due to species social behavior, including herds
  and flocks.

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(2) Even Dispersion

• Usually result from TERRITORIALITY ? space is
  CLAIMED and DEFENDED against others who wish to
  use it.

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(3) Random Dispersion (no pattern visible)

• Results from seed dispersal (in wind-influenced
  plants) and is apparent in fields of wildflowers
  or forests.

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II. Population Dynamics

• Populations change in size and range over time.
  (i.e., this is the field of Population Ecology)

NOTE To understand HOW change occurs, we need
to look to forces BEYOND size, density, and
dispersion.
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(1) Birth Rate (In U.S. 2003, 4 million/year)

• Number of births occurring within a population
  over a period of time.

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(2) Mortality Rate (In U.S., 2.4 million/year)

• Number of deaths occurring within a population
  over a period of time.

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(3) Life Expectancy (In U.S., 72 years male 79
years female)

• Age an individual of a population is expected to
  live (i.e., longevity).

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(A) Age Structure

• Distribution of individuals among AGE RANGE in a
  population.

NOTE In many species, including humans, the
very old do NOT reproduce, thus populations with
a greater proportion of YOUNGER individuals may
have greater potential for a BURST of rapid
growth.
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(B) Patterns of Mortality

• Mortality rate of different species tend to
  CONFORM to one of THREE CURVES on a graph.
  (i.e., survivorship curves)

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(1) Survivorship Curves (based on reproductive
strategies, evolution)

• Displays the LIKELIHOOD of survival at DIFFERENT
  ages throughout the lifetime of the organism.

• Type I Ex Humans, the likelihood of dying is
  SMALL until late in life.

• Type II Ex Squirrels, the likelihood of dying
  is LINEAR to lifespan.

• Type III Ex Oysters or Insects, the likelihood
  of dying young is HIGH, however if the young
  survives, it will likely LIVE OUT its lifespan.

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20-2 Measuring Populations
I. Population Growth Rate

• Charles Darwin (1821) calculated that a SINGLE
  pair of elephants COULD increase to a population
  of 19 MILLION within 750 years.

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Critical Thinking
(2) Explain TWO difficulties an ecologist might
have in counting a population of migratory birds.
Develop and explain a method for estimating the
size of such a population.
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(1) Growth Rate (influenced by TWO processes, in
addition to BR/DR)

• The AMOUNT of change a population SIZE has over
  time.

NOTE By the end of this class PERIOD, the
GLOBAL growth rate of H. sapiens will have
increased by approximately 10,000 individuals.
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Critical Thinking
(3) Differentiate between growth rate and birth
rate, and evaluate the statement that a
DECREASING birth rate may lead to a DECREASING
growth rate.
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(2) Immigration (insignificant to global human
population growth?)

• Forces can drive members INTO a population ?
  INCREASING its size.

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(3) Emigration (also insignificant to global
human population growth)

• Forces can drive members OUT of a population ?
  DECREASING its size.

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Critical Thinking
(4) Support the idea that immigration and
emigration are insignificant factors when
studying GLOBAL human population growth.
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EX Population ecologists divide large
populations into groups of 1,000 and present
their data PER CAPITA.

• If, in ONE year, there are 52 BIRTHS and 14
  DEATHS per 1,000 individuals in a large
  population, the PER CAPITA birth rate would be
  52/1000 or 0.052 BIRTHS per individual per year.
  The PER CAPITA death rate would be 14/1000 or
  0.014 deaths per individual per year.

• THUS, The per capita GROWTH rate can be found
  by
• BIRTH RATE DEATH RATE GROWTH RATE

0.052 (births per capita) 0.014 (deaths per
capita) 0.038 ( growth)

• Therefore, if the sample population is 50,000
  individuals, then in ONE year, the population
  should INCREASE by(0.038 x 50,000) 1,900.

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II. The Exponential Model (ONLY occurs with NO
limiting factors present)

• The LARGER the population becomes, the MORE
  RAPIDLY it grows.

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(1) Exponential Growth (binary fission)

• The population increase of BACTERIA in the lab
  produces a J-SHAPED graph of UNLIMITED growth.

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(A) Predictions Based on the Exponential Model

• Assumes birth/death rates remain CONSTANT and NO
  limiting factors exist on the population.

Ex Bacteria colony estimated size at time (t) 0
? 1,000, at (t) 4, what SHOULD the colony
population size be approximating?
16,000 ? (1K-2K-4K-16K) An example of
predicting exponential growth
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(B) Limitations of the Exponential Model

• Do populations grow exponentially? Yes, but only
  under RARE conditions and ONLY for a SHORT period
  of time.

NOTE An abundance of space and nutrition must
be supplied, and waste must be adequately
neutralized.
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(1) Limiting Factor (abiotic and biotic limiting
factors exist in nature)

• A factor that RESTRAINS the growth of a
  population and PREVENTS prolonged exponential
  growth.

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III. The Logistic Model (BIRTH rate dec/DEATH
rate inc as pop. GROWS)

• LIMITING FACTORS establish a CARRYING CAPACTIY
  (K) for the population (i.e., Results in a
  stretched out, S-SHAPED graph).

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(1) Carrying Capacity (K ? influenced by
limiting factors)

• The SIZE of a POPULATION that an environment can
  SAFELY SUPPORT over a LONG period of time.

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(2) Logistic Growth

• Graph describes a population STABILIZING around
  K. (At K, birth and death rates are EQUAL and
  growth rate LEVELS OFF (plateau).

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IV. Population Regulation

• Size is KEPT IN CHECK by TWO classes of
  limiting factors, grouped by their relationship
  to population DENSITY.

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(1) Density-Independent Factors (hurricanes,
drought, floods, eruptions)

• Include climate and natural disasters ?factors
  REDUCE the population by the SAME proportion,
  REGARDLESS of the populations SIZE.

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(2) Density-Dependent Factors

• Include RESOURCE limitations, such as SHORTAGES
  of food or territory and are TRIGGERED by
  increasing population DENSITY.

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Critical Thinking
(5) Explain how disease could be a
density-dependent factor in a population.
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(A) Population Fluctuations

• CHANGES are LINKED to the biotic and abiotic
  forces of the environment. (e.g., food
  availability, disease, predation)

• Ex Charles Eltons research of the HARE and
  LYNX populations. (i.e., each species REGULATED
  the other species GROWTH cycle i.e., the
  cycles fluctuate TOGETHER)

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(B) Perils of Small Populations (safety WITHOUT
numbers)

• Threatened by (1) habitat destruction, (2)
  inbreeding (a decrease in genetic variability),
  and (3) genetic disease.

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(1) Inbreeding (blood lines of kin crossing)

• Reduction of offspring, increases genetic
  disease, and PREVENTS a population from being
  able to EVOLVE.

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20-3 Human Population Growth
I. History of Human Population Growth

• Until about 10,000 years ago, human populations
  grew SLOWLY, living and migrating in small,
  nomadic tribes (as other species lived).

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(1) Hunter-Gatherer Lifestyle

• The LOW rate of GROWTH results from SMALL
  populations and a HIGH MORTALITY rate among the
  YOUNG (surviving modern HG societies).

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(A) The Development of Agriculture

• The hunter-gatherer lifestyle began to CHANGE
  about 10,000 years ago when humans discovered
  DOMESTICATION and CULTIVATION.

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(1) Agricultural Revolution

• Period where a critical resource (food supply)
  became STABILIZED and the human population began
  to EXPLODE.

NOTE Estimates claim that about 10,000 years
ago, there were between 2-20 MILLION people on
Earth. In the following 10,000 years after
agriculture, the human population had increased
to about 170-330 million, or by approximately
1600.
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(B) The Population Explosion

• Improvements in SANITATION and hygiene, CONTROL
  of disease, increased food supplies, and
  increased mobility.

NOTE IT took MOST of HUMAN HISTORY for the
human population to reach 1 BILLION
BUTin JUST the 27 years (POST WW II) between
1960 and 1987, pop. grew by an ADDITIONAL 2
billion people. Advances raised BIRTH rates and
dropped MORTALITY rates (but we have SLOWED since
the 1960s).
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(C) Population Growth Today

• In 1970, 3.7 billion people, and GROWTH rate
  was 0.0196. Therefore, 73 million people
  were ADDED in 1971 to global population.

• In 12003, there were about 6.5 billion people,
  and the growth rate has dropped slightly to about
  0.014. HOWEVER, about 92 million people were
  STILL added this year to the global population,
  despite a drop.

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(1) Developed Countries (LEAST FIT in Darwinian
sense? ? slowed growth)

• Contain about 20 of the global population, and
  on average, hold HIGHER life expectancies (than
  developing countries).

• Include the U.S., Japan, Germany, France, the
  United Kingdom, Australia, Canada, and Russia.

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(2) Developing Countries (most fit in Darwinian
sense? ? rapid growth)

• Contains about 80 of the global population, and
  currently are experiencing a much FASTER growth
  rate, at more than 0.02 per capita.

• Include MOST countries in Asia, Central and
  South America, and Africa.

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Extra Slides AND Answers for Critical Thinking
Questions
(1) Humans have increased carrying capacity by
improving crop yields, health care, and
sanitation. Pollution and overexploitation of
resources have reduced carrying capacities in
some areas.
(2) Crowding promotes contagion. Accumulated
waste promotes lack of hygiene and sanitation.
(3) Some populations of migratory birds are
difficult to count because the flocks are too
numerous, too widespread, or too mobile.
Students may have some ideas for determining the
population size of migratory birds. Answers
should include a method of isolating and counting
a few and then using a proportion to obtain an
estimate of the full population.
(4) As far as anyone has been able to prove,
immigration and emigration to and from Earth has
not yet occurred. It has not yet been proven
that even microorganisms exist elsewhere in the
universe, although some scientists believe there
is evidence of ancient bacterial life in
meteorite fossils from Mars.
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(5) Growth rate is the change in size of a
population over a period of time. Fertility rate
is the average number of children the females in
a population have it does not consider deaths,
as growth rate does. A lower fertility rate
results in a lower birth rate. A lower birth
rate will lead to a lower growth rate if the
death rate remains constant.
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Revisiting Interdependence of Organisms

• In order for populations to grow and survive,
  they must compete with each other and with other
  populations for food and space.

• Carrying-capacity fluctuations may be due to
  species interactions, such as predation, or
  abiotic conditions like drought.

Assessing Prior Knowledge

• What is meant by a populations gene pool?

• What are some of the variables that may make a
  population more vulnerable than others?

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