Population Ecology

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Population Ecology

• Reading Freeman, Chapter 52

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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 spectrum.

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This is the geographic range of a species of
damselfly.
campus.greenmtn.edu/dept/NS/Dragonfly/Vermont
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Factors Determining the Geographic Range of a
Species

• History
• Biological Tolerances
• Other Species
• A combination of the above

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Historical Factors Determining Range Example

• 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 continent.
• 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.

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The geographic ranges of various ratite birds
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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.

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Populations are groups of individuals of the same
species living in the same place

• 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.

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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).

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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

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Size

• Simply the number of individuals in the
  population at any given time. Sometimes called
  abundance.

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Density

• 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.

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Example Problem

• There are 10,400 mice living in a 1000m x 1000m
  field. What is the density of this population?

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Answer

• The area of the field is 1,000,000 square meters
  (m2).
• The density of mice is therefore 10,400
  mice/1,000,000m2.0104/m2.

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Patterns of Dispersion

• Populations follow into three different patterns
  of dispersion, generally.
• Clumped
• Regular
• Random

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Clumping is probably the most common
situation.Sometimes clumping occurs because some
areas of habitat are more suitable than others

• 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

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Sometimes species clump for other reasons.

• 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

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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.

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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.

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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.

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Age structure

• This is the relative number of individuals at
  different ages.

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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
  populations
• 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.

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Variability is differences among individuals in
the population.

• Most populations show differences among
  individuals.
• 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.

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It is a fundamental characteristic of living
things that all organisms reproduce.

• Every species is capable of population growth
  under some set of possible conditions

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Demographic Processes

• Birth (Natality)
• Death (Mortality)
• Immigration
• Emigration

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Exponential Growth

• This is probably the best, simple, model of
  population growthit predicts the rate of growth,
  or decay, of any population where the per capita
  rates of growth and death are constant over time.
• 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 plentiful

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DN/DTbN-dN

• Where
• b is the per capita birth rate
• d is the per capita death rate
• ignoring immigration and emmigration.
• dN/dTrN (define r as the instantaneous
  population growth rate rb-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.

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Exponential growth formula

• N(t)N0ert
• where r is the exponential growth parameter
• N0 is the starting population
• t is the time elapsed
• r0 if the population is constant, rgt0 if
  population is increasing, rlt0 if the population
  is decreasing.

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Example problem

• The human population of the earth is growing at
  approximately 1.8per year.
• 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?

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Answer

• N(t)N0ert
• r .018
• t100 years
• N06 billion
• N(100)N0ert
• N(100)6x109ert 6x109e1.8
• N(100) 6x1096.04 36.3 billion

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Limits to Growth

• No population can continue growing forever.
• Even organism that reproduce very slowly, such as
  elephants, rhinos, whales, and humans, would
  outstrip their resources if they reproduced
  indefinitely.

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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.

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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.
• NThe Number of individuals in the population at
  a given time
• rmaxis the maximum population growth rate
• dN/dTrmaxN(K-N)/K
• Question What is dN/dT when NK?

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Answer

• When NK, dN/dT0
• Likewise, when N is small,
• dN/dT approximately rmax
• When NgtK the population declines.

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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.

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Demography

• 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.

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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 density
• mark and recapture
• index of relative abundance (e.g., pheremone
  baited insect traps)

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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?

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Answer

• First Sample Marked Recaptures
• TOTAL POP Total Recaptures
• 50/T13/100 (where T is the population of bats
  under the bridge)
• multiply both sides by 100T
• 500013T
• T385

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Life Tables

• A persons 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 persons 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.

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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.

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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

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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

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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 individuals
  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.

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http//www.io.uwinnipeg.ca/simmons/1116/
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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 individuals chance of living a given
  amount of time.
• Rapidly growing populations will have a
  disproportionate number of young individuals.

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Age Structure and Human Population

• -Europeans many old people, few young people
  populations of some countries will decrease in
  the future.
• -Africa many young people, few old people
  populations will increase in the future.
• m(x), age specific fecundity, is highest in 20
  year olds.
• 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..

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Life History Strategies

• Life history is the timing of an organisms
  reproduction and death.
• An organisms 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 reproduction.
• This is because reproduction can be dangerous,
  and involves the expenditure of resources the
  organism could use for growth or maintenance.

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For most organisms, life histories are timed to
maximize an organisms expected fitness.
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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.

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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.

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An Organisms 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
  reproduce.
• 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.

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Reproductive Episodes Per Lifetime

• Semelparity is one large reproductive effort
  (most insects, annual plants). Examples,
  grasshoppers, mayflies, octupi and squids Agave.
• Iteroparity is fewer offspring and many
  reproductive episodes. Example, perennial
  plants, most large mammals, sharks, most birds
  such as gulls and terns.

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Clutch Size

• Clutch size (or seed set in plants) is the number
  of offspring produced per reproductive episode.
• Clutch size varies depending on resource
  availability.
• Semelparous organisms expend all their resources
  on a single clutch.
• Iteroparous organisms must save some for growth
  and survival.

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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 individuals fitness more than
  many offspring later. This is because of the
  compound interest effect.
• Dying before reproduction also reduces an
  organisms 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
  offspring.

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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.

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Parental Care

• Parental care, in its many manifestations, is an
  adaptation by which an organism may potentially
  increase the survivorship of its offspring.
• 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.

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Parental Care and Development in Birds

• Birds (vertebrate class Aves) provide parental
  care, but different birds provide different
  amounts of parental care.
• 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
  incubation.

http//people.eku.edu/ritchisong/parentalcare.html