Download Population Dynamics

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Populations can be described by
 Distribution
 Numbers
 Age structure
 Density
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Populations cannot grow indefinitely (r)
because there are limited resources and
habitats (k)
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Clumping
Uniform dispersion
Random dispersion
 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
 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
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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This pattern occurs in the absence of strong
attraction or repulsion among individuals.
 It is uncommon.
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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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Simply the number of individuals in the
population at any given time. Sometimes
called abundance.
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This is the relative number of individuals at
different ages.
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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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There are 10,400 mice living in a 1000m x
1000m field. What is the density of this
population?
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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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Birth (Natality)
Death (Mortality)
Immigration
Emigration
Population change= (B+I) – (D+E)
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This is probably the best, simple, model of
population growth…it 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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Density
independent
Results from
sudden crash in
population size
Outstrip resource
limit
catastrophe
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N(t)=N0ert
where r is the exponential growth parameter
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.
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The human population of the earth is
growing at approximately 1.8%per 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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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
Populations tend to grow to the maximum
extent possible given environmental
conditions
 Biotic potential
 Dependent on innate biological principles
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 Intrinsic rate of increase (r)= rate of
population growth if unlimited
resources available
 Biotic potential cannot be
sustained
 Environmental resistance
 Negative feedback
▪ Snowshoe hare and lynx
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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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Density dependent
Recycling and
renewal of
resources
Establishes
equilibrium around
carrying capacity
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A: lag phase
B: acceleration
phase
C: exponential
growth
D: deceleration
phase
E: equilibrium
G: dynamic
fluctuations
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Adaptive traits
Technological advances
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Density- dependent factors
 Intraspecific competition
 Predation
 Disease
 Density-independent factors
 Natural disasters
 Pollution
 Habitat destruction (deforestation)
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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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Stable
 Rainforest species
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Irruptive
 Insects
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Cyclic
 Lemmings
▪ http://www.youtube.com/watch?v=pDqlZjpSJCc
 Wolf-moose interactions
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Irregular
 Due to catastropies
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Delayed density
dependence
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Sexual vs. Asexual
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
 Positive: enhance population
growth
 Negative: reduce population
growth
 Can be positive or negative
 Lichens
 Coral and zooxanthelae algae
 Wood termites and protozoa
 Humans and gut flora
 Can be positive or negative
 Migration
 Territory behavior
 Societies and hierarchies
 Mating and courtship
 Colors, patterns, physical
characteristics
 Predators
 Diseases
 Pheromones
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
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