Chapter 8 Understanding Population Change

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Chapter 8 Understanding Population Change

• Population ecology refers to the study of a
  defined population of organisms, in a given area,
  and how and why population changes occur over a
  period of time. Among issues of interest are how
  individuals compete for food and other resources,
  and how environ-mental pressures, e.g.,
  predation, disease, may affect the population.
  Also of interest is the reproductive health of
  the population and Does this population seem to
  be inter-acting with its ecosystem in a normal
  manner?. We could look at age distribution w/in
  the population, too.

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• In order to define characteristics of a
  population, physical boundaries of the population
  need to be defined.
• Population density number of individuals per
  unit area (land) or unit volume (water).
  Variable between different ecosystems
  seasonally depending on availability of food
  other needs.
• On a large scale, population changes occur by
  births (birth rate/1000/year) deaths (death
  rate/1000/year). The growth rate is the birth
  rate minus death rate.

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• In local ecosystems, in addition to births
  deaths, dispersal also affects population
  changes. Types of dispersal - immigration
  (influx of individuals) and emigration (departure
  of individuals). Births immigration growth.
  Deaths emigra-tion decrease of population.
• When presented with ideal conditions a population
  could undergo a maximum growth rate, known as its
  Biotic potential (Intrinsic rate of increase)
  dependent on age of reproductive maturity, length
  of reproductive life, how many individuals per
  litter, how many litters per year.

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• When a population undergoes rapid, seemingly
  unrestricted growth (biotic potential), it
  produces an Exponential growth curve (J curve)
  (see p. 164).

The human population curve (19th 20th
centuries) is an example of a semi-exponential
growth curve, due to better medical care food
production/distribution. Smaller organisms
generally produce sharper curves (Figure 8.3)
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• Organisms that fit the template of Intrinsic
  Rate of Increase are called r-Adapted Species
  they depend on high reproductive rate to survive.
  Other characteristics short life, rapid
  growth, many small offspring, generalists, low
  trophic levels. Example dwarf Siberian
  hamsters.
• When a population initially grows exponentially,
  then meets Environmental Resistance (Carrying
  Capacity), population stabilizes as S curve.

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Environmental Resistance includes Availability
of resources (food, water, shelter) Disease
Predation. Environmental Resistance smaller
birth rate is an example of a Negative Feedback
Loop see Chapter 6.
Carrying capacity how many of a particular
species can an eco-system support? When a
population overshoots the carrying capacity, a
population crash occurs.
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• Larger organisms are usually K-Adapted species
  long life, slow growth, few larger offspring,
  specialists, high trophic level.
• Survivorship probability that an individual in
  a given population will survive to a particular
  age.

Type I Late Loss K strategists Type II
Constant Loss Type III Early Loss r
strategists
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• Structure spatial distribution of individuals
  and populations the communitys relation to its
  surroundings. Some plants, e.g., creosote
  bushes, privet hedge, may release toxins that
  discourage other plants from adjacent growth.
  Others cluster for protection, reproduction,
  etc..
• Some animals are solitary, e.g., Syrian hamsters
  or colonial, e.g., dwarf Siberian hamsters.
• Population size influences 1) Density-Dependent
  Density Independent.

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Predation, disease, competition are
Density-Dependent Factors. With greater
population density, encounters with predators are
more common, individuals greater proximity more
disease greater competition for food, shelter,
water,
See Case-In-Point (p. 169) for explana-tion of
Fig. 8.9 and reasons for population curve changes.
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• Boom or Bust Population curves probably
  more easily seen in rapidly - reproducing
  r-strategists (lemmings, p.169). Density
  Dependent Population Controls - Causes may be
  over-consumption of food by increasing lemming
  population and/or parasite exchange during times
  of greater density.

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• Density-Independent Population Controls
  include climatic weather events that affect
  food supplies, Ranges of Tolerance, etc..
• Sometimes higher densities of animals may help
  population survive (or at least part), in
  cold-related weather events. Example sheep
  huddling during a blizzard.
• Conversely, higher densities of animals may
  hinder population survival during drought events.
  Example animals clustering around a single
  water hole (over-using it during a drought).

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• Human population issues (Slide 4) show-ing
  human population growth since beginning of
  Industrial Revolution.
• Thomas Malthus suggested that human population
  growth would overshoot food resources, resulting
  in famine, disease, war. His predictions did
  not account for improvements in farming practices
  food distribution methods.
• Paul Ehrlich, modern-day Malthusian - "The battle
  to feed humanity is over. In the 1970s the world
  will undergo famines . . . AND hundreds of
  millions of people including Americans are
  going to starve to death." (1968)

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• During the past 200 years, the worlds birth
  rate has declined. Population growth is because
  of the decline of the death rate, due to better
  medical care, cleaner water, better food
  production distribution.
• Some estimates suggest that the J-curve of
  human population growth will level-off and
  become an s-curve in the late 21st century,
  with a world population of about 8 to 10 billion.
• Earths carrying capacity is estimated at 4
  billion to 16 billion, depending on quality of
  life estimates. Debates vary over what to do
  about it.

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• Terms related to population changes
• Zero population growth birth rate death rate,
  immigration not withstanding.
• Negative population growth birth rate is below
  death rate. Desired by some deep ecologists
  (discussed more in Chapter 9).
• Replacement-level fertility number of children
  couples must have to replace themselves.
• Total fertility rate average number of children
  born to each woman during her lifetime.

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• National Demographics branch of Sociol-ogy
  dealing with population statistics.
• Classifications
• Developed countries (U.S., Canada, Japan, Western
  Europe, Sweden, Australia) low population growth,
  low infant mortality rates, high per capita
  GNP/GDP, longer life expectancy. Each country
  varies, but reasons for wealth include relative
  political stability, relative freedom, low taxes,
  rights of land ownership, favorable climate for
  agriculture, cultural characteristics that favor
  entrepreneurship and productivity that help
  develop sources of commerce.

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• Developing countries fall into 2
  subcate-gories Moderately developed countries,
  e.g., Mexico, Turkey, most of South America,
  India Less developed coun-tries, e.g.,
  Bangladesh, Niger, Ethiopia.
• Progress towards development are related to
  political stability (more democracy generally
  more stability) and degree of conditions cited on
  previous slide, e.g., India is undergoing growth
  because of efforts to lessen bureaucracies that
  stifle creativity entrepreneurship. Continued
  political strife major reason for poverty.

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• As nations proceed through demographic
  transition, four demographic stages occur as they
  become industrialized and urban-ized (pp.
  174-175, Figure 8.15, next slide).
• Finland as an example Preindustrial stage
  first settlements to late 1700s with high birth
  and death rates, intermittent famines, other
  problems exist slow population growth.
  Transitional stage lower death rate, initial
  industrialization, better conditions growing
  population mid-1800s. Industrial stage
  decline in birth rate, population growth slows
  early 1900s.

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• Postindustrial stage population growth
  stabilizes or declines. Higher standard of
  living generally smaller families (for reasons
  discussed on p. 175).
• At this stage, in some nations, immigrants may be
  needed to offset low birthrate.

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• Because of attitudes towards immigrants
  other cultures, some nations, e.g., Japan, face
  future difficulties because of their aging
  populations.
• Other future issues facing individual nations
  include cultural differences between natives
  and immigrants. Are the immigrants willing to
  assimilate? What traditions will be preserved?