Population Dynamics Ch. 8

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Population DynamicsCh. 8
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Big Picture

• Populations can be described by
• Distribution
• Numbers
• Age structure
• Density
• Populations cannot grow indefinitely (r) because
  there are limited resources and habitats (k)

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Distribution

• Clumping
• Uniform dispersion
• Random dispersion

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CLUMPINGSometimes 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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Clumping cont.

• 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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Uniform 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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Random dispersion

• 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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Size/Numbers

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

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Age Structure

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

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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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Population Size Change

• Birth (Natality)
• Death (Mortality)
• Immigration
• Emigration
• Population change (BI) (DE)

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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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J-Curve

• Density independent
• Results from sudden crash in population size
• Outstrip resource limit
• catastrophe

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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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Population Dynamics

• Populations tend to grow to the maximum extent
  possible given environmental conditions
• Biotic potential
• Dependent on innate biological principles
• Intrinsic rate of increase (r) rate of
  population growth if unlimited resources available

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Not Sustained

• Biotic potential cannot be sustained
• Environmental resistance
• Negative feedback
• Snowshoe hare and lynx

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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 or Sigmoid Curve

• Density dependent
• Recycling and renewal of resources
• Establishes equilibrium around carrying capacity

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Sigmoid Curve

• A lag phase
• B acceleration phase
• C exponential growth
• D deceleration phase
• E equilibrium
• G dynamic fluctuations

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Changing the carrying capacity

• Adaptive traits
• Technological advances

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Dynamic Fluctuation Phase

• Density- dependent factors
• Intraspecific competition
• Predation
• Disease
• Density-independent factors
• Natural disasters
• Pollution
• Habitat destruction (deforestation)

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Factors cont.

• 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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Patterns of population size

• Stable
• Rainforest species
• Irruptive
• Insects
• Cyclic
• Lemmings
• http//www.youtube.com/watch?vpDqlZjpSJCc
• Wolf-moose interactions
• Irregular
• Due to catastropies

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Predator- Prey Effect

• Delayed density dependence

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Reproduction

• Sexual vs. Asexual

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Reproductive Patternsr- selected vs K-selected

• r-selected
• Reproduce early and put most of their energy into
  reproduction
• Many small offspring
• High growth rate Population size fluctuates
  wildly around carrying capacity
• Low ability to compete
• Most offspring die before reaching reproductive
  age

• K-selected
• Fewer, larger offspring
• High parental care
• Most offspring survive to reach reproductive age
• Lower growth rate
• High ability to compete
• Population size fairly stable around carrying
  capacity

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Factors that Influence Population Dynamics

• Positive enhance population growth
• Negative reduce population growth

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Symbiotic Relationships

• Can be positive or negative
• Lichens
• Coral and zooxanthelae algae
• Wood termites and protozoa
• Humans and gut flora

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Behavior Patterns

• Can be positive or negative
• Migration
• Territory behavior
• Societies and hierarchies
• Mating and courtship
• Colors, patterns, physical characteristics

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Biological Controls

• Predators
• Diseases
• Pheromones

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Survivorship Curve

• Type I large animals immune to predation, live
  to old age
• Type II mostly prey, predation is constant
  throughout lifespan
• Type III large numbers of young because most
  will be eaten, only few adults survive