Chapter 48: Population Growth and Regulation

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Chapter 48 Population Growth and Regulation
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Scope of Ecology

• Ecology is the study of the interactions of
  organisms with other organisms and with the
  physical environment.
• The study of ecological interactions can be
  undertaken at many levels the individual
  organism, populations, communities, ecosystems,
  and the biosphere.
• A population is all the members of the same
  species interacting with the environment at a
  particular locale.

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• A community consists of all the various
  populations in an area.
• An ecosystem is the community plus its nonliving
  habitat, including abiotic (nonliving) and biotic
  (living) components.
• The biosphere is the portion of the entire
  earths surface, including air, water, and land,
  where living things exist.
• Ecology describes the environment and tests
  models.

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Ecological levels in a coral reef
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Two terrestrial communities
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Community Stability and Diversity

• A change in community composition over time is
  called ecological succession.
• Primary succession starts on areas devoid of soil
  and secondary succession starts with pioneer
  species in areas where there is already soil such
  as an abandoned field.
• Succession also occurs in aquatic communities.

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Secondary succession in a forest
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Models of Succession

• The climax-pattern model predicts that a
  particular area will always lead to a climax
  community characteristic for that area (i.e.,
  tropical rain forest at the equator).
• The facilitation model says that each proceeding
  stage facilitates the development of the next
  stage.
• The inhibition model says that each preceding
  stage tries to prevent the arrival of the next
  stage.

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• The tolerance model says that plants from various
  stages try to colonize at the same time and
  chance arrival of seeds determines the outcome.
• The length of time it takes trees to develop
  gives the impression of a series of plant
  communities from the simple to the complex.
• These models are not mutually exclusive, and
  succession is probably a complex process.

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Population Characteristics and Growth

• Population density is simply the number of
  individuals per unit area or volume.
• Distribution of these individuals can be uniform,
  random, or clumped.
• Most members of a population are clumped, as are
  the members of a human population.

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Patterns of distribution within a population
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• Abiotic factors such as water, temperature, and
  availability of organic nutrients often determine
  a populations distribution.
• Biotic factors, such as the availability of food,
  or presence of disease, affect the distribution
  of populations.
• Limiting factors are those factors that determine
  whether an organisms lives in an area.

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Patterns of Population Growth

• Each population has a particular pattern of
  growth.
• The per capita rate of increase is calculated by
  subtracting the number of deaths from the number
  of births and dividing by the number of
  individuals in the population.
• Rate of Increase Births Deaths
• Total Population
• It is assumed that immigration and emigration are
  equal.

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• Every population has a biotic potential, the
  greatest possible per capita rate of increase
  under ideal circumstances.
• Two possible patterns of population growth are
  considered.
• Exponential growth results in a J-shaped curve
  because as the population increases in size so
  does the expected increase in new members.

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Biotic potential
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Exponential growth
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• Environmental resistance occurs when most
  environments restrict growth, and exponential
  growth cannot continue indefinitely.
• Under these circumstances logistic growth occurs
  and an S-shaped growth curve results with four
  phases lag, exponential growth, deceleration,
  and stable equilibrium.
• When the population reaches carrying capacity,
  the population stops growing because
  environmental resistance opposes biotic
  potential.

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Logistic growth
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Survivorship

• Populations are made up of individuals of
  different ages.
• Populations tend to have one of three types of
  survivorship curves, depending on whether most
  individuals live out the normal life span (type
  I), die at a constant rate regardless of age
  (type II), or die early (type III).
• Much can be learned about the life history of a
  species through its survivorship curve.

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Survivorship curves
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Human Population Growth

• The human population is expanding exponentially.
• The doubling time is the length of time it takes
  for a population to double, currently estimated
  at 53 years.
• Only when birthrate equals death rate will there
  be zero population growth.

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More-Developed Versus Less-Developed Countries

• Most of the expected increase in human population
  will occur in certain less-developed countries
  (LDCs) of Africa, Asia, and Latin America.
• Doubling time in more-developed countries (MDCs)
  is about 100 years because a decrease in death
  rate due to medical advances was followed by a
  decrease in birth rates.

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Standard of living
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World population growth
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• The relationship between decreased death rate
  followed by a slower birth rate is called
  demographic transition.
• Despite introduction of medical care, LDCs still
  have twice the MDC growth rate.
• Support for family planning, social progress, and
  delayed childbearing could help prevent an
  expected increase in population size.

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Regulation of Population Growth

• Two life history patterns exist in populations.
• Opportunistic populations have a short lifespan,
  small stature, and produce many offspring to take
  advantage of new resources.
• Equilibrium species live longer, are larger, and
  produce fewer young but have greater parental
  care they hold population size near carrying
  capacity.

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Life history patterns
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• Population growth is limited by both
  density-independent factors (e.g., weather) and
  density-dependent factors (predation,
  competition, and resource availability).
• Density-independent factors operate regardless of
  population density.
• Density-dependent factors increase in intensity
  as population size increases.

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Competition

• Competition occurs when two species try to use a
  resource that is in limited supply.
• According to the competitive exclusion principle,
  no two species can occupy the same ecological
  niche at the same time when resources are
  limiting.
• Resource partitioning occurs when resources are
  partitioned between two or more species.

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Competition between two populations of Paramecium
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Competition between two species of barnacles
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Predation

• Predation occurs when one living organism, the
  predator, feeds on another, the prey.
• Predators include lions, whales that filter feed,
  parasites that draw blood from hosts, and
  herbivores that eat grass, trees, and shrubs.

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Predator-Prey Population Dynamics

• Predator-prey interactions between two species
  are influenced by environmental factors.
• Cycling of population densities may occur, as in
  the case of the Canadian lynx and hare predators
  kill off prey and then the predator population
  declines when food is in short supply.
• Predator-prey systems are not usually simple
  two-species systems.

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Predator-prey interaction lynx and snowshoe hare
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Prey Defenses

• Coevolution occurs when two species adapt to
  selective pressures of each other.
• Prey defenses against predation take many forms
  camouflage, use of fright, and warning coloration
  are three prey defense mechanisms.

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Antipredator defenses
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Mimicry

• Mimicry occurs when one species resembles another
  that possesses an antipredator defense.
• Batesian mimicry occurs when one species has the
  warning coloration but lacks the antipredator
  defense of the species it mimics.
• Müllerian mimicry occurs when two species with
  the same warning coloration both have defenses.

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Mimicry
Has the appearance of a stinging wasp, but is a
harmless hover fly
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Symbiosis

• Symbiosis refers to close interactions between
  members of two populations.
• Three types of symbiosis occur parasitism,
  commensalism, and mutualism.
• Symbiotic associations do not necessarily fall
  neatly into these three categoties.

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Parasitism

• In the symbiotic relationship called parasitism,
  the parasite benefits and the host is harmed.
• Parasites derive nourishment from their host and
  the effect can be mild or fatal to the host.
• Many parasites use a secondary host to disperse
  or complete their stages of development, as is
  the case in the life cycle of a deer tick.

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Commensalism

• In commensalism, one species benefits and the
  other is neither benefited nor harmed.
• Often a host provides a home or transportation
  for another species.
• For example, barnacles attach to backs of whales,
  remoras attach to sharks, clown fishes live
  within the tentacles of sea anemones, and cattle
  egrets eat insects off large grazing mammals.

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Egret symbiosis
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Mutualism

• In mutualism, both members benefit.
• Lichens have traditionally been regarded as
  mutualistic but experiments suggest that the
  fungus may be parasitic on the algae.
• The bullhorn acacia tree provides a home for the
  ant Pseudomyrmex ferruginea, in swollen acacia
  thorns.
• Ants feed from nectaries at base of leaves and
  also eat Beltian bodies at leaf tips.

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• In return ants protect this tree from herbivores.
  
• Cleaning symbiosis involves crustaceans, fish,
  and birds that act as cleaners of a variety of
  vertebrate clients.
• In some cases, the cleaners may exploit the
  situation and feed on host tissues, but cleaning
  appears to improve the fitness of the client.

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Cleaning symbiosis