Population Ecology

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Chapter 52

• Population Ecology

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Population Ecology

• Population ecology is the study of the
  populations and their interactions with the
  environment.
• It also explores how the environment influences
  these populations in terms of size, age
  structure, and distribution.

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Population Ecology

• Ecologists usually begin an investigation of a
  population by defining appropriate parameters
  such as density and dispersion.

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Density

• How many individuals live within a given area.
• To determine the density of individuals, it is
  possible to count all of the organisms within a
  given area, but it is not likely.

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Dispersion

• Dispersion is the spacing patterns among
  individuals within the boundaries of a population.

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Sampling Techniques

• Usually a wide variety of them are used.
• Scientists can count all individuals in a given
  area.
• They can do this in a number of different spots.
• Then, average all of the numbers together to make
  educated estimates about the population density.

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Sampling Techniques

• Scientists also employ the mark and recapture
  method.
• Animals are captured, marked, and released.
• The animals can then be tracked or captured at a
  later date.
• Density and distribution can be studied.

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Sampling Techniques

• These methods are okay, but sometimes the data
  becomes unreliable because the organisms you are
  studying behave differently during study.

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Population Density

• The density is always changing.
• Birth, death, immigration, and emigration are
  ways a population changes.

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Dispersal Patterns

• There are varying dispersal patterns of organisms
  within a populations geographic range.
• These variations in local populations are
  extremely important to ecologists.

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Three Common Patterns of Dispersal

• 1. Clumped
• 2. Uniform
• 3. Random

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1. Clumpled

• 1. Organisms are in uniform patches

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2. Uniform

• Organisms are evenly spaced.

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3. Random

• Organisms exhibit unpredictable spacing patterns.
  They could be grouped together, or there could
  be an uneven distribution pattern.

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Demography

• Demography is the study of the vital
  characteristics of a population.
• For example
• Ecological needs
• Spacing of individuals
• Interactions of individuals within a population.

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Demographers

• These are people who study populations.
• They develop life tables to determine the
  survival pattern of a population.
• The use a cohort--a group of individuals of the
  same age that are followed from birth to death.

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Life Tables

• These are difficult to build and maintain.
• It is easier to graphically depict a life
  table--a survivorship curve.

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Survivorship Curves

• These typically involve 1000 individuals from a
  population.
• The numbers are obtained by multiplying the
  surviving population by 1000 each year.
• Plotting these numbers vs. age indicates the
  death rate (or life expectancy).

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Survivorship Curves

• There are three types of survivorship curves
• 1. Type I
• 2. Type II
• 3. Type III

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1. Type I

• Type I curves start flat indicating a low death
  rate for early and middle life.
• They decline sharply as individuals get older
  indicating a high death rate.

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2. Type II

• Type II curves exhibit relatively constant death
  rates from birth to death.

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3. Type III

• Type III curves see death rates very high in the
  beginning but as the animals grow and mature the
  death rates level off.
• Example animals that produce many young and
  provide little or no care for them.

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Life Histories

• Life histories are products of natural selection.
  
• The traits that affect an organisms schedule of
  reproduction and survival comprise its life
  history.

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Life Histories

• There are three basic variables that life
  histories entail
• 1. When reproduction begins.
• 2. How often the organism reproduces.
• 3. How many offspring are produced during a
  reproductive cycle.
• For the most part, life histories are the product
  of evolutionary outcomes because most animals
  dont choose when to reproduce.

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Reproductive Modes

• In general, there are 2 reproductive modes that
  are followed
• 1. Big bang reproduction--semelparity.
• 2. Repeated reproduction--iteroparity.

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Reproductive Modes

• The evolutionary events that favor these are
  determined by the environment.

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Semelparity

• This is a one and done scheme for reproduction.
• The organism takes a big chance.
• Favored when the survival rate of the offspring
  is low.
• Occurs when an organism lives in a highly
  variable or highly unpredictable environment.
• Examples
• Salmon and agave plants.

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Iteroparity

• This is repeated reproduction. Organisms
  continually give rise to offspring throughout
  their lives.
• Iteroparity is favored when environments are more
  stable.

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Energy Constraints

• Time, energy, and nutrients cannot be used for
  one thing as well as something else.
• This is the tradeoff that prevents producing a
  large number of offspring very frequently.

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Population Growth

• Unchecked population growth is considered
  exponential.
• There are mechanisms that prevent exponential
  population growth.
• This can be estimated using mathematical
  equations to describe the per capita growth rate.

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Population Growth

• It essentially boils down to the rate being equal
  to the number of births minus the number of
  deaths.
• r b-m
• rgt0 the population is increasing
• rlt0 the population is decreasing
• r0 no change

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Population Growth

• Because resources are limited populations cannot
  grow exponentially forever.
• Ecologists try to identify the carrying capacity
  of an environment, K.
• K is the maximum population size an environment
  can support.

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Population Growth

• To account for changes in the environment,
  scientists have created a logistic growth model
  to explain how populations vary in size.

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Population Growth

• The exponential growth model is used as a
  starting point.
• We add information about the environment that
  acts to reduce the per capita rate of increase.
• If K is the maximum, K-N is the number of
  individuals the environment can accommodate.
• N is the population size.

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Population Growth

• (K-N)/K is the fraction of the carrying capacity
  available for population growth.
• Multiplying by the maximum rate of increase of
  the population, rmax, allows us to modify the
  growth rate of the population as its size
  increases.
• rmax N (K-N)/K

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Population Growth

• When N K, the population stops growing.
• The logistic model will produce an S-shaped
  (sigmoid) growth curve when population size is
  plotted over time.

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Population Growth

• New individuals are added at the highest rate at
  intermediate population sizes.
• This is when the breeding population is of
  substantial size and space and resources are
  abundant.
• As N approaches K, the population size slows.

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Population Growth

• The logistic model.
• This incorporates the idea that every individual
  added to the population has the same negative
  effect on population growth.
• This is not true.

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Population Growth

• Certain populations exhibit the Allee effect.
• This describes a situation where individuals may
  have a difficult time surviving and reproducing
  when the population gets too small.
• The logistic model fits few, if any, real
  populations.
• It serves as a good starting point.

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Population Growth

• There are two general questions that are asked
  when studying population growth
• 1. What environmental factors stop a population
  from growing?
• 2. Why do some populations show radical size
  fluctuations while others stay stable?

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Population Growth

• To understand the answers to these questions, we
  have to
• Examine the birth and death rates
• Immigration and emigration
• How these factors affect population density.

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Population Growth

• If immigration and emigration are equal, then
  birth and death rates will affect population size.

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Population Growth

• A birth rate or death rate that does not change
  with population density is said to be density
  independent.
• A death rate that rises when population density
  rises is said to be density dependent--an example
  of negative feedback--it halts population growth.

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Population Growth

• 1. What environmental factors stop a population
  from growing?
• 1. Competition for resources
• 2. Territoriality
• 3. Health
• 4. Predation
• 5. Toxic waste
• 6. Intrinsic Factors

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1. Competition for Resources

• This occurs when population density increases and
  organisms compete for resources.
• This results in a reduction in the number of
  offspring an individual produces.

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2. Territoriality

• Territory is a resource and when available space
  is limited, population density decreases because
  reproduction is limited.

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3. Health

• When population density is high, transmission of
  disease is easier and more likely.
• If more of the population contracts the disease
  and dies, the population density will decrease.

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4. Predation

• In terms of predation, as the amount of prey
  increases, the predator eats more and the prey
  population declines.

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5. Toxic Waste

• As levels of toxic wastes increases (waste that
  is toxic to the organism), the size of the
  population decreases.

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6. Intrinsic Factors

• There are intrinsic factors tied to the behavior
  of organisms that result in a change in behavior
  that alters reproduction rates.
• Examples include aggressive behavior and hormonal
  changes.

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Population Growth

• The second major question
• 2. Why do some populations show radical size
  fluctuations while others stay stable?
• To understand population stability, researchers
  often look at population dynamics and how numbers
  vary from year to year.

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Population Growth

• Population dynamics focuses on the interactions
  between biotic and abiotic features that cause
  population sizes to vary.

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Population Growth

• For instance, stability and fluctuation are
  governed by complex interactions with the
  environment.
• For example the moose population fluctuates due
  to harsh winters, lots of snow, predation by
  wolves, disease, and parasites.

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Population Growth

• All populations go through cyclic fluctuations in
  size.
• Some cycles are very short, others are longer.
• A famous example is that of the 10 year cycle of
  the snowshoe hare and the lynx.

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Population Cycles

• Lynx are specialist predators, they feed on hares
  so their populations rise and fall with the hare.

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Population Cycles

• There are three hypotheses have been proposed to
  describe the 10-year cycle of these populations
• 1. The cycles may be caused by a shortage of
  food in the winter.
• 2. The cycles may be due to predator-prey
  interactions.
• 3. The cycles may be a combination of the two.

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Hypothesis 1

• This hypothesis has been discarded because for
  over 20 years researchers have studied the
  population dynamics.
• Adding food to the environment increased the
  numbers of hares, and the population still
  followed the natural fluctuations.

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Hypothesis 2 3

• These two hypotheses are supported because
  experiments have revealed that nearly all hares
  were killed by predators and none died of
  starvation.
• Also, when predators were eliminated, food was an
  added factor, especially in the winter.

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Another Factor

• Another factor contributing to the crash of the
  predators is that when food becomes scarce, lynx
  often turn on themselves.
• This shows that this cycle is not just a
  hare-lynx cycle.