Population Biology Demography_

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 Population Biology Demography_ Powered By Docstoc
					Population Ecology

Reading: Freeman, Chapter 52
 Every species has a geographic range
• A geographic range describes where individuals
  of a species might potentially be located.
• In the United states, most species have a range of
  4-24 states.
• Cosmopolitan species are an extreme, they are
  worldwide in distribution.
• Endemic species are found in only a small,
  restricted area, they represent the other side of the
    This is the geographic range of a species of damselfly.

Factors Determining the
Geographic Range of a Species
•   History
•   Biological Tolerances
•   Other Species
•   A combination of the above
    Historical Factors Determining Range:
• Many species have what is called a “Gondwanan” distribution.
  They occur in the Southern continents of Australia, South
  Africa, South America, and sometimes India.
   – These places are far away from each other now, but 150
     million years ago, they were all linked together in a massive
• Examples number in the thousands, across many different
  types of species, including the rattite birds, the bee family
  colettidae, and the southern beech tree, Nothofagus sp.
The geographic ranges of various ratite birds
  No species are truly ubiquitous, in the sense that
 all species are restricted to a particular habitat.

• Suitable habitats tend to be clustered within the
  geographic range of a population, therefore,
  most species are composed of discontinuous
  groups called populations.
• Clearly, the boundaries between populations can
  be somewhat subjective.
  – What constitutes a population depends upon the species
    in question, but in general, members of a population
    interact, mate, and compete with each other much more
    frequently than members of different populations.
Populations are groups of individuals
of the same species living in the same
• Individuals within a population occupy the
  same general area, rely on the same resources,
  and are influenced by the same general
  environmental conditions.
• Most of the interaction, including sexual
  reproduction, between individuals of a species
  is among members of the same population.
• A grassland in North Dakota may support a single
  population of Buffalo, many separate
  populations of prairie dogs, and hundreds of
  populations of frogs. Each population of frogs
  would be localized in a patch of wetland called a
  prairie sinkhole. Likewise, each prairie dog town
  would support its own population of fleas. There
  might be several streams bisecting the grassland,
  each would support an independent population
  of fish, such as the bullhead catfish, and several
  islolated populations of water loving (hydric)
           Some populations move
• A single population of salmon may spawn
  upstream in a river of the Pacific Northwest, and
  return to the ocean to feed and grow.
• Most of the interaction between individuals and
  exchange of alleles, occurs among individuals
  nesting in the same stream.
• Likewise, a population of monarch butterflies may
  migrate, en masse, to over-wintering grounds in
  Mexico for the winter, returning to the Midwest to
  reproduce during the summer (although no single
  individual makes the whole trip).
          Populations have certain
          emergent properties
• These properties are consequences of the way an organism
  interacts with the environment, and with other organisms,
  and influence its evolution.
   –   Size
   –   Density
   –   Patterns of Dispersion
   –   Age Structure
   –   Spatial Structure
   –   Sex Ratio
   –   Variability
• Simply the number of individuals in
  the population at any given time.
  Sometimes called abundance.
• The number of individuals in the population
  per unit area or unit volume.
  – For many organisms, it is the density of a
    population rather than its actual numbers, that
    exerts a real effect on the organism.
Example Problem
• There are 10,400 mice living in a 1000m x
  1000m field. What is the density of this
• The area of the field is 1,000,000 square
  meters (m2).
• The density of mice is therefore 10,400
     Patterns of Dispersion

• Populations follow into three different patterns of
  dispersion, generally.
   – Clumped
   – Regular
   – Random
• This is the most common pattern of
  distribution, it occurs when individuals
  aggregate into patches.
Sometimes clumping occurs because some
areas of habitat are more suitable than
– i.e., Plethodon sp. salamanders are found
  clumped under fallen logs in the forest
– the night lizard Xantusia sp. is found
  clumped within fallen Joshua trees in the
  Mojave desert
 Sometimes species clump for other
– Plants often clump because their seeds fall
  close to the parent plant or because their seeds
  only germinate in certain environments.
  Impatiens capensis seeds are heavy and usually
  fall close to the parent plant-this species grows
  in dense stands.
– Species may clump for safety, or social reasons.
  Ground nesting bees Halictus sp. prefer to nest
  in the presence of other bees, forming
  aggregations of solitary nests
         Random distribution
• This pattern occurs in the absence of strong
  attraction or repulsion among individuals.
  – It is uncommon.
• The trees of some forest species are
  randomly distributed within areas of
  suitable habitat.
  – For example, fig trees in the amazon rain forest.
    This random distribution might be due to seed
    dispersal by bats.
         Regular Distribution
• This generally happens because of interactions
  between individuals in the population.
• Competition: Creosote bushes in the Mojave
  desert are uniformly distributed because
  competition for water among the root systems of
  different plants prohibits the establishment of
  individuals that are too close to others.
• Territoriality: The desert lizard Uta sp. maintains
  somewhat regular distribution via fighting and
  territorial behavior
• Human Intervention: I.e., the spacing of crops.
                 Spatial Structure
• The scale matters a great deal in describing the
  spatial distribution of a species.
• A species may be clumped on the large scale,
  but evenly distributed on a finer scale.
  – Example: Ground nesting wasps, Sphex sp. are
    clustered in areas of suitable nesting substrate
    (packed sand). Within these areas, their nests are
    evenly distributed because of aggressive interactions.
              Age structure
• This is the relative number of individuals at
  different ages.
                      Sex ratio
• Sex ratio is the proportion of individuals of
  each sex. The number of females is more
  important in the overall growth rate of
  – Examples: elk; fewer males of reproductive age than
    females; males breed with more than one female.
• Wasps: Melittobia sp. may have as many as a
  hundred females per male. These males never
  leave the nest and mate with their sisters.
  Population growth is essentially independent of
  the number of males.
 Scale is important in determining how
organisms respond to their environment.
– A habitat is called course grained if the scale
  of variation in the environment is large
  compared to the size of individuals.
– A habitat is called fine grained if the scale of
  environmental variation is small compared to
  the movements of individuals.
– I.e., to a foraging butterfly, a field of
  wildflowers would be course grained        .. to a
  zebra, the same field would be fine grained.
 Variability is differences among individuals
               in the population.
• Most populations show differences among
  – Some variation has a genetic basis.
  – Other variation is largely environmental.
  – In many cases, variability is caused by both
    genes and the environment.
• Sexual Dimorphism is when the two sexes
  differ greatly in appearance.
• Metamorphosis is when individuals differ
  in appearance because of a dramatic
  transformation as they age.
It is a fundamental characteristic
of living things that all organisms
• Every species is capable of population
  growth under some set of possible
 Demographic Processes
• Birth (Natality)
• Death (Mortality)
• Immigration
• Emmigration
          Arithmetic Growth
• Imagine a species where all the births occur
  at once (natality).
• All the deaths occur in the interval before
  the births (mortality).
• In the same interval, individuals can leave
  the population by emigration, and enter by
• This is called arithmetic growth.
• Some species exhibit this kind of growth,
  i.e., annual grasses and grasshoppers.
        N(t+1)=N(t) + B - D + I -E
•   Where:
•   N(t+1)=populalation at time (t+1)
•   N(t)=population at time t
•   Where B=Number of Births (natality)
•          D=Number of Deaths (mortality)
•          E=Number of Emmigrants
•          I=Number of Immigrants
             Example Problem:
• A population of field mice, Peromyscus sp.
  Consists of 371 indiviuduals at the start of
• That year, 115 individuals die, 201 are born, 37
  immigrate and 75 emigrate.
• What is the population at the start of 2000?
• N(t)=371
• N(t+1)=N(t) + B - D + I -E
• N(t+1)= N(t)+ 201 (natality) -
  115 (mortality) - 75
  (emigration) + 37
  (immigration) =371+48=419
    l is an arithmetic growth
  parameter, it describes the
 amount of population growth
        per generation
• N(t+1)/N(t)=l
¨ if l is=1 the population is
  constant, if l is<1 if the
  population is decreasing,if l >1 if
  the population is increasing

• t is the elapsed time in
• proof:
• N(t)=lN(t-1)
• N(t)=l * l N(t-2)
• N(t)=l * l * l N(t-3) etc.
     Example Problem
• A population of grasshoppers
  grows at 23% per year.
• At the start of 1991, the population
  is 144 individuals.
• What is the population at the start
  of 2001?
• t=10 (10 years is 10 generations
  for an annual species).
• N(t+1)/N(t)=l
l =1.23 (1.23 is 1.00 plus 23%)
• N(t)=N(0)lt;
• N(t)=144*(1.23)10=144*7.93
• N(t)=1141
Exponential Growth
• In exponential growth models, births
  deaths, emigration and immigration
  take place continuously
  – This is a good approximation for the
    growth of most biological populations
• I.e., human populations grow
  exponentially when resources are
• Where:
• b is the per capita birth rate
• d is the per capita death rate
• ignoring immigration and emmigration.
• dN/dT=rN (define r as the instantaneous
  population growth rate; r=b-d)
• you can integrate this to get the exponential
  growth formula that follows…
• Note that we are assuming that there is no
  emigration or immigration.
 Exponential growth formula
• N(t)=N0      e rt

• where r is the exponential growth
• N0 is the starting population
• t is the time elapsed
• r=0 if the population is constant, r>0 if
  population is increasing, r<0 if the
  population is decreasing.
        Example problem
• The human population of the earth is
  growing at approximately 1.8%per
• The population at the start of 2001 was
  approximately 6 billion.
• If nothing were to slow the rate of
  population growth, what would the
  population be in the year 2101?
•   N(t)=N0ert
•   r= .018
•   t=100 years
•   N0=6 billion
•   N(100)=N0ert
•   N(100)=6x109ert =6x109*e1.8
•    N(100)= 6x109*6.04 = 36.3 billion
          Limits to Growth
• No population can continue growing
• Even organism that reproduce very slowly,
  such as elephants, rhinos, whales, and
  humans, would outstrip their resources if
  they reproduced indefinitely.
           Carrying Capacity
• Populations grow until one or several
  limiting resources become rare enough to
  inhibit reproduction so that the population
  no longer grows.
• The limiting resource can be light, water,
  nesting sites, prey, nutrients or other factors.
• Eventually, every population reaches its
  carrying capacity, this is the maximum
  number of individuals a given environment
  can sustain.
      Logistic Growth Model
• The logistic growth model accounts for
  carrying capacity.
• K= Carrying Capacity, this is the maximum
  number of individuals that the population
  can sustain.
• N=The Number of individuals in the
  population at a given time
• rmaxis the maximum population growth rate
• dN/dT=rmaxN(K-N)/K
• Question: What is dN/dT when
•   When N=K, dN/dT=0
•   Likewise, when N is small,
•   dN/dT =approximately rmax
•   When N>K the population declines.
  How Well Does the Logistic
  Model Fit Actual Populations?
• For laboratory populations of Paramecia,
  crustaceans, etc.., the logistic provides a
  pretty good fit.
• For actual populations, the logistic does not
  provide such a good fit.
• There are usually other factors involved.
• One factor: lag time is the time it takes
  between reaching carrying capacity and the
  slowdown in reproduction.
• Demography is the study of the age structure
  and growth of populations, especially as it
  relates to the births and deaths of individuals.
• The study of human mortality goes back a
  long way, to the Middle Ages and the early
  Renaissance. Thomas Malthus was a rather
  famous early demographer/economist. In his
  Essay on the Principal of population, 1798,
  he was the first to reach the conclusion that
  human populations tended to grow until they
  outstripped their available food supply.
       Counting Individuals
• Complete enumeration: count every
  individual in the population
• Sample the Population:
  – count individuals in many small portions
    of the area (e.g., quadrats) then calculate
  – mark and recapture
  – index of relative abundance (e.g.,
    pheremone baited insect traps)
     Mark-Recapture Example
• A population of bats lives under a bridge in
  Texas. On Monday, 50 bats are netted and
  marked with ear tags. On Tuesday, the
  same researchers return and net 100 bats.
  Of these, 13 are marked.
• How many bats live under the bridge?
• First Sample       =     Marked Recaptures
• TOTAL POP                  Total Recaptures

• 50/T=13/100 (where T is the population of bats under
  the bridge)
• multiply both sides by 100T
• 5000=13T
• T=385
• They discover that the population is growing at a rate
  of 3.2% per day.
• If it is June 1, if the population continued to grow
  exponentially, what would the density of aphids be at
  the end of the growing season, 100 days later?
• A. 7139
  B. 3313 aphids
• C. 7.138966 x103 aphids per m2
• D. About 7000 aphids per m2
• E. 93120 aphids per m2
• Clicker Question: A population of cabbage aphids is
  infesting the field of a farm in Nebraska. The farm is 500
  hectares (a hectare is 100m x 100m).
• The owners of the farm select 10, 1m x 1m quadrats,
  within the farm, as randomly as possible, and count the
• Their data are as follows. 214, 515, 604, 11, 946, 12, 0,
  31, 514, and 66 aphids per plot, respectively.
• Estimate the abundance of aphids on this farm
• A) 2339
• B) 291.3 aphids per m2
• C) 2.91 aphids per hectare
• D) 1.42 x 109 aphids per farm
• E) 1.5 x 109
                Life Tables
• A person’s chance of death is of tremendous
  interest to a person selling life insurance.
  Life tables are based on the actuarial tables
  insurance companies keep.
• By tabulating deaths, causes of death, and
  ages of death, it is obvious that a person’s
  chance of dying is not constant over time.
  For humans, it increases as we get past a
  certain age.
• Biologists have adopted Life Tables and
  applied them to other species.
      Constructing a Life Table
• A Cohort is a group of organisms born at the
  same time.
• S(0) is the number of individuals born into the
  cohort. S(x) is the number of individuals alive at
  the start of interval x.
• D(x) is the number of individuals dying during
  interval x.
• l(x) is the proportion of individuals alive at the
  start of interval x. l(x)=S(x)/S(0)
• m(x) is the expected number of female offspring
  born to a female during interval x.
        Life Table for a Field Grasshopper
•             X     s(x)  d(x) l(x)  m(x)      l(x)*m(x)
•   eggs      0    44000 40487 1.000   0            0
•   Instar I 1     3513 984 .080       0            0
•   Instar II 2    2519 597 .057       0             0
•   Instar III 3   1922   461 .044      0            0
•   Instar IV 4    1461    161 .033     0            0
•   Imago 5        1300 1300 .030       17          .51
•                                               R0=.51
•                     adapted from Richards and Waloff, 1954
       Survivorship Curves

• A survivorship curve traces the
  decline of a group of newborns
  over time.
• Survivorship curves plot the
  probability of surviving to a certain
  age for a representative member of
  the population
   Types of Survivorship Curves
• Type I: A convex curve. Most individuals
  live to adulthood with most mortality
  occurring during old age. I.e., humans, red
  deer, elephants.
• Type II: A straight line. An individual’s
  chance of dying is independent of its age.
  I.e., small birds and mammals.
• Type III: A concave curve, few individuals
  live to adulthood, with the chance of dying
  decreasing with age. I.e., oysters, redwood
  trees, snapping turtles.
            Age Distributions
• The age distribution of a population is the
  proportion of individuals at different ages.
• It has a significant impact on future
  population growth.
• Populations that have remained constant for
  a long time have stable age distributions,
  which reflect an individual’s chance of
  living a given amount of time.
• Rapidly growing populations will have a
  disproportionate number of young
         Age Structure and Human Population
• -Europeans – many old people, few young people –
  populations of some countries will decrease in the
• -Africa – many young people, few old people
  populations will increase in the future.
• m(x), age specific fecundity, is highest in 20 year
• for humans, death rates are highest in the first year
  (babies) and in old age.
• Per capita death rates are actually higher in the U.S.
  than Guatemala, Mexico, etc..
            Life History Strategies
Life history is the timing of an organisms
 reproduction and death.
An organism’s fitness is how many offspring it
 produces that are ultimately able to produce their
 own offspring.
There is often a trade-off between survivorship and
This is because reproduction can be dangerous, and
 involves the expenditure of resources the organism
 could use for growth or maintenance.
    For most organisms, life
histories are timed to maximize
an organism’s expected fitness.
    Factors Limiting Populations
• Density-Dependent factors intensify as the
  size of a population increases.
• Examples: Suitable nesting sites for cliff
  nesting birds such as gannets, competition
  for light and water among prairie grasses.
• Density-Independent factors are
  independent of population size.
• Examples: Winter temperatures greatly
  affect the populations of yellowjackets,
  sawflies and Melanopus grasshoppers.
       Both Density-Dependent and Density-
            Independent Factors Affect
           Populations of Most Species
• Examples: For Neodiprion sawflies, winter
  surviorship is greatly affected by the weather, which
  is density-independent.
• During the summer, however, parasitic wasps
  impose very high density-dependent mortality.
• Pacific mussels, Mytillus sp., are largely limited by
  density-dependent competition for space on rocky
  outcrops. Occasionally, density -independent
  disturbance by floating logs decimates populations.
An Organism’s Life History is Adapted to
           its Environment
• Example: Pacific salmon hatch in the headwaters
  of streams and migrate to the open ocean. They
  eventually return to freshwater streams to
• Steelhead Trout are genetic variants of the same
  species that stay in estuaries rather than migrating
  to the open ocean.
• Life history is polymorphic, in these species. Each
  morph exploits a life history where it avoids
  competition with the other morph.
    Reproductive Episodes Per
• Semelparity is one large reproductive effort
  (most insects, annual plants). Examples,
  grasshoppers, mayflies, octupi and squids
• Iteroparity is fewer offspring and many
  reproductive episodes. Example, perennial
  plants, most large mammals, sharks, most
  birds such as gulls and terns.
               Clutch Size
• Clutch size (or seed set in plants) is the
  number of offspring produced per
  reproductive episode.
• Clutch size varies depending on resource
• Semelparous organisms expend all their
  resources on a single clutch.
• Iteroparous organisms must save some for
  growth and survival.
       Age at First Reproduction
• For growing populations, the timing of first
  reproduction greatly affects fitness.
• In a growing population, a few offspring early can
  increase an individual’s fitness more than many
  offspring later. This is because of the “compound
  interest effect”.
• Dying before reproduction also reduces an
  organism’s fitness to zero.
• But organisms often have more resources
  available to them as they age.
• There is therefore a tradeoff: reproduce early with
  a few offspring or reproduce later with more
A Trade-Off: Generation Time is
     Related to Body Size
Larger organisms are slower to reach
 reproductive age.
E. coli, about 20 minutes.
Paramecium, about 24 hours.
Elephant or human, 10-14 years.
                 Parental Care
• Parental care, in its many manifestations, is an
  adaptation by which an organism may
  potentially increase the survivorship of its
• It comes at a cost to the parents, however.
  – Time and resources spent investing in parental care
    mean that the resources that are available must be
    spread over fewer offspring.
Parental Care and Development in Birds
• Birds (vertebrate class Aves) provide parental care, but
  different birds provide different amounts of parental
   – Superprecocial birds, such as the megapodes, hatch and are
     able to fend for themselves immediately. Eggs are often laid
     in warm environments, and parents do not incubate them. In
     theory, this means more time to forage .
   – On the other end of the spectrum are the superaltrical birds,
     such as passerines and parrots. Young are born blind and
     require constant parental care, after a period of egg