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Population Ecology

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Title: Population Ecology


1
Population Ecology
2
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

3
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

4
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5
Uniform
  • Environmental conditions are uniform
  • Causes COMPETITION or antagonism between
    organisms (territoriality)

6
Clumping
  • Most common
  • Reproductive patterns favor clumping
  • Social behaviors lead to clumping
  • Optimal density is usually intermediate (medium)

7
Random
  • No competition
  • No tendency to group/clump
  • Conditions are uniform
  • Rarely happens!

8
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

9
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

10
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

11
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

12
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

13
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).

14
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

15
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.

16
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.

17
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

18
POPULATION GROWTH MODELS
  • Different models of how populations grow

19
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

20
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)

21
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

22
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

23
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

24
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

25
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

26
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

27
Carrying Capacity
28
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

29
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

30
Environmental Factors
  • Abiotic and biotic influences on population size

31
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)

32
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)

33
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

34
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)

35
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

36
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)

37
Boom-and-Bust Cycles
38
Human Population Growth
39
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

40
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)

41
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

42
Human Population Growth
43
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

44
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

45
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

46
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

47
Age-Structure Diagrams
48
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?
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