Population Ecology

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Population Ecology
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Populations

• Population ecology is the study of populations in
  relation to the environment
• A population is a group of individuals of the
  same species that live in the same area

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Population Ecology Vocabulary

• To describe populations, we have to talk about
  how they are situated
• Density
• The number of individuals per unit area/volume
• Example 47 elephants/km2
• Dispersion
• The pattern of spacing among individuals in a
  population
• Clumped
• Uniform
• Random

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Uniform

• Environmental conditions are uniform
• Causes COMPETITION or antagonism between
  organisms (territoriality)

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Clumping

• Most common
• Reproductive patterns favor clumping
• Social behaviors lead to clumping
• Optimal density is usually intermediate (medium)

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Random

• No competition
• No tendency to group/clump
• Conditions are uniform
• Rarely happens!

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Estimating Population Size

• Rarely, it is possible to count the number of
  species in a population
• Usually, a statistical method is needed to
  determine population size
• The mark-recapture method can be used to estimate
  the size of a population
• Capture, mark, release
• Recapture and count
• Equation
• N Number marked x Total catch 2nd time
• Number of marked recaptures

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Factors That Influence Population Size

• There are 3 major factors that influence
  population size
• 1. the number of births
• 2. the number of deaths
• 3. the number of individuals that enter or
  
• leave a population
• - immigration
• individuals entering an existing population
• - emigration
• individuals leaving an existing population

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DEMOGRAPHICS

• In looking at populations, biologists must also
  look at demographics (the vital statistics of a
  population and how they change over time)
• Birth rates
• Death rates
• Life tables
• Survivorship curves
• Reproduction rates
• Growth rates
• Age structures

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DEMOGRAPHICS

• One tool in demographics is to estimate the life
  span of organisms
• To do this they often create a life table, age
  summary of a population
• A cohort (group of individuals of the same age)
  are studied to see what percentage of the
  population dies
• This can determine life expectancy, survivorship
  at each age category, or male vs. female
  survivorship

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Survivorship Curves

• A way to represent a life table is a survivorship
  curve, a plot of the proportion of individuals
  that survive at each age group
• When discussing survivorship curves, there are 3
  general categories

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Survivorship Curves

• Type I- live to old age die (most large
  mammals)
• Type II- constant mortality rate (rodents,
  lizards, hydra)
• Type III- high mortality at young age, but if
  they survive they live a long life (sea turtles).

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DEMOGRAPHICS

• Another important aspect of demographics is
  reproductive rates
• For obvious reasons, biologists who study
  reproductive rates only focus on females
• Age of fertility
• Number of offspring for each age group
• Time of year
• Spawning cycles
• Reproductive tables summarize this data

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Life Histories

• Life history are traits that affect an organisms
  schedule of reproduction and survival
• Clutch size
• Number of offspring produced at each reproductive
  episode
• Semelparity
• A life history in which an organism spends most
  of its energy in growth and development, expend
  their energy in one large reproductive effort,
  and then die
• Many insects, annual plants, salmon, etc.

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Life Histories

• Iteroparity
• A life history pattern in which organisms produce
  fewer offspring at a time over a span of many
  seasons
• Example humans, panda bears, etc.

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Life Histories (Semelparity vs. Iteroparity)

• Many factors contribute to the life history of an
  organism
• Finite resources
• Reproduction vs. survival
• Number and size of offspring
• Paternal investment in offspring

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POPULATION GROWTH MODELS

• Different models of how populations grow

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Formulas off your Cheat Sheet

• Rate dY/dt
• Generic expression for change of some variable
  (Y) over time (t)
• Population Growth dN/dt (B D)
• The change in population size (N) over time (t)
  is the same as the birth rate (B) minus the death
  rate (D)
• Exponential Growth dN/dt rmax N
• The change in population size over time is equal
  to the growth rate (rmax) multiplied with the
  current population size

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Formulas off your Cheat Sheet

• Logistic Growth
• dN/dt rmax N (K-N)/K
• The growth rate over time is the same as
  exponential growth adjusted for carrying capacity
  (K)

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Types of Population Growth (start here)

• One of the biological imperatives is to reproduce
  and pass on genetic material to succeeding
  generations.
• Yet population growth is controlled by the
  environment and limited resources
• This causes different patterns of population
  growth

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Patterns of Population Growth

• Exponential Growth
• Occurs in ideal conditions with unlimited
  resources
• J shaped curve
• Example
• 1 bacterium (reproducing every 20 minutes) could
  produce enough bacteria to form a 1-foot layer
  over the entire surface of the Earth in 36 hours

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Exponential Growth

• Exponential growth is a useful model when
  studying populations that are introduced into a
  new, unfilled, environment
• Recovery after a catastrophe
• Exponential Growth
• dN/dt rmax N
• rmax is the maximum rate of population growth for
  the species

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Patterns of Population Growth

• Exponential growth cannot continue indefinitely
• It is characteristic of populations who are
  entering a new environment OR those whose numbers
  are rebounding from a catastrophic events

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Patterns of Population Growth

• Logistic Growth
• Pattern of population growth which takes into
  account the effect of population density on
  population growth
• Occurs when resources become more scarce
• Characterized by an S-shaped curve

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Patterns of Population Growth

• dN/dt rmax N (K-N)/K
• Carrying capacity (K)
• The maximum number of individuals that a
  particular environment can support over a long
  period of time
• Determined by such limiting factors as crowding
  and food resources
• Graph levels off at carrying capacity
• K-selected populations (equilibrial populations)
  live near or at the carrying capacity

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Carrying Capacity
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K-strategists (Life history)

• Density stays near carrying capacity.
• Large, slow growing organisms
• Small population sizes
• Long life span slow maturation
• Few young/small clutch size
• Reproduce late in life
• Parental care
• Most large mammals endangered species

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r- strategists (Life history)

• Grow exponentially when environmental conditions
  allow when conditions worsen, population size
  plummets.
• Short life span
• Reproduce early in life
• Many offspring/large clutch size
• Usually small in size
• Little or no parental care
• Bacteria, some plants, insects

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Environmental Factors

• Abiotic and biotic influences on population size

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Limiting Factors

• There are a number of factors that limit the size
  of populations
• Density-dependent limiting factors (depends on
  the size of the population)
• Density-independent limiting factors (does not
  matter the size of the population)

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Density-Dependent Limiting Factors

• The effect of density-dependent limiting factors
  intensifies as the population increases
• Intraspecific competition
• Food, space, etc.
• Territoriality
• Predation
• Waste build up
• Disease (if caused by pathogen/contagious)

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Density-Independent Limiting Factors

• The occurrence and severity of density-independent
  limiting factors are unrelated to population
  size
• Climate
• Disease (if not caused by pathogen/not
  contagious)
• Pollution

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The Interaction of Limiting Factors

• Density-dependent and density-independent
  limiting factors often work together to regulate
  the size of a population
• Deer in snowy winter
• Starve from lack of food (density-dependent)
• Severity of winter/depth of snow determines
  access to food (density-independent)

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Population Dynamics

• Population dynamics is the study of the
  environmental factors that cause variations in
  the population size
• Looks at fluctuations in population over time to
  examine stability
• Immigration and emigration also affect population
• Metapopulations are when you have several
  interconnected populations

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Boom-and-bust cycle

• Another phenomena that affects population are
  predator-prey relationships.
• Each population is interdependent and causes a
  boom-and-bust cycle
• The prey population increases which causes the
  predator population to increase
• The prey are over hunted and their population
  crashes
• This causes the predator population to crash
• Now, with fewer predators, the prey population
  can again increase (recovery gives a geometric
  increase)

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Boom-and-Bust Cycles
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Human Population Growth
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Increase in Human Population

• Agricultural Revolution - Major period of
  population growth began when humans started to
  cultivate crops and domesticate animals
• Industrial Revolution Improved food production
  and distribution
• Health Care germ theory lead to improved
  hygiene, better waste removal and water treatment
  

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Decrease in Human Population

• Plague disease that greatly reduces the size of
  population (Black Plague in 1300s reduced the
  population in England by 50)
• Famine a severe food shortage causing starvation
  and death (Potato Famine of 1840s/China
  1870-1890)
• War death by combat, disease, cut off from food
  supply (Germany 1618-1648/WWI/WWII)

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Human Population Growth

• The human population is unlike any other organism
• Since about 1650, we have remained in an
  exponential population increase
• Population increases by about 201,000 people/day
  worldwide

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Human Population Growth
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Human Population Growth

• Even though there is a tremendous increase in
  human population, it is not evenly distributed
  around the globe
• Regional areas have different population trends
• Some regions have stable regional human
  populations (birth rate is the same as death
  rate)
• Other regions show incredible growth rates

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Industrialized Nations

• An emerging nation usually has a very high birth
  rate, but also a high death rate (disease, lack
  of modern medical treatment, famine)
• An industrialized nation usually has a low death
  rate, but also a low death rate
• Moving from an emerging nation to an
  industrialized nation is known as a demographic
  transition

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Human Population Growth

• In the 1950s, mortality rates began to rapidly
  drop (advances in medicine and sanitation)
• Yet, the birth rates have not always dropped
• Has caused a huge increase in population in some
  nations
• About 80 of the worlds population lives in
  emerging nations

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Age Structure

• One way to determine human population growth is
  to look at the nations age structure, relative
  number of individuals at each age
• By looking at the age structure of a population,
  you can determine the population growth

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Age-Structure Diagrams
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Human Population Growth

• Implications of exponential human population
  growth
• Lack of food supplies
• Lack of space
• Lack of natural resources (metals, fossil fuels,
  etc)
• Lack of sites for waste disposal
• Ecologists cannot agree on a carrying capacity
  for Earth (2 40 billion)
• Are we going to reach carrying capacity through
  individual choices and/or government programs?
• OR
• Is Earths population going to level off as a
  result of mass deaths?