Transcript Pop Ecology - Mr. Wells` wikispace

Population Ecology
Population – n. a group of individuals of a single
species that occupies the same general area.
How are Populations
Measured and Distributed?
• Is it possible to count
EVERY individual in a
population?
• Scientists perform
“mark and recapture”
experiments to
estimate population
sizes
• Ex. “Something’s
Fishy”
• Randomly
– No pattern at all
• Clumped
– Groups of pop.
concentrations
• Uniform
– Evenly spread out or
spaced in the
environment
Population Dispersion
Why Different Types?
How Do Populations Grow?
There are several mathematical models…
• Exponential growth model – the rate of
expansion of a population under “ideal”
conditions
• Population-limiting factors – hunting,
amount of space suitable for breeding,
restricted population growth, food
availability, etc.
• Logistic growth model – idealized
population; growth slowed by limiting factors
as the population size increases
• Carrying capacity – the maximum
population size that an environment can
support at a particular time, without
degradation of the habitat
Exponential growth of
bacteria
Growth Without Limits
r = 1.0
r = 0.5
r = population growth rate
What if it stops growing?
Logistic growth, compared to
exponential growth
K = carrying capacity
Maximizing Yield
dN/dt is maximized when N*r is maximized
Impact of Limits
Imposition of limits
dN/dt = r  N  (K-N)/K
New or Changing
Environment (no
competition / limits)
Growth of a population of fur
seals (logistic growth model)
What does the logistic growth model
suggest about real populations in
nature?
• A population’s growth rate will be small
when the population size is either small or
large and highest when the population is at
an intermediate level relative to the carrying
capacity.
• Limiting factors make the birth rate
decrease, the death rate increase, or both
• Eventually the population will stabilize at the
carrying capacity, when the birth rate equals
the death rate
• (These are mathematical models and no
wild population fits either model perfectly!)
Some factors that limit population
growth
• As density of song
sparrows increase, the
number of eggs laid
decreases because of
food shortages
• Plants grown under
crowded conditions tend
to be smaller and less
likely to survive
• Disease transmission or
accumulation of toxic
waste products can
increase mortality
Continued……
• A predator may capture
more of a particular kind
of prey as the prey
becomes abundant
• White-footed mice stop
reproducing at a colony
size of 30-40 even when
food and shelter are
provided. Stress?
• The graph shows aphids
which feed on the phloem
sap of plants; increase in
population in the summer
and then die-off in the fall
and winter
Continued….
• Some populations remain
fairly stable in size close to
carrying capacity
• Most populations fluctuate
as seen at the left
• This graph shows song
sparrow populations, with
periodic catastrophic
reductions due to severe
winter weather
Boom and bust cycles
• Hare cycles may be
caused by increasing
food shortages during
winter caused by
overgrazing
• They may be due to
predator-prey interactions
• Cycles could be affected
by a combination of food
resource limitation and
excessive predation
• Predators reproduce
more slowly than their
prey so they always lag
behind prey in population
growth.
Why Does Population Size
Change?
• Density Independent Forces
– Forces that are at work irrespective of the
population density
– Doesn’t matter how many individuals there are in
an area: ALL are affected the same way
• Density Dependent Forces
– Forces that vacillate depending on the population
density
– May be caused by either low or high densities
Density Independent
Forces
• Examples
–
–
–
–
–
–
Climate
Topography
Latitude
Altitude
Rainfall
Sunlight
• In Sum: Abiotic factors
– Exceptions do exist!
Density-Independent Factors (e.g., weather)
Good
Times!
(in Australia)
Density Dependent Forces
• Examples
– Within species
•
•
•
•
Breeding spaces
Food
Mates
Foraging spots
– Between species
•
•
•
•
Predation
Parasitism
Pollinators
Competition
• In Sum: Biotic factors
– Exceptions do exist!
Density-Dependent Limits (to max = K)
Competition increases
Indeterminate Factors
• Most influences are pretty constant
and Deterministic
• Opposite of deterministic factors is
Stochastic forces
• Examples
– Environmental: Droughts, floods,
asteroids, volcanoes, fires, etc.
– Demographic: Crash in effective
population size (ex. passenger Pigeon),
series of single sex born (ex. Alligators).
Growth Matters!
• r-selected species
– Why most weeds are
“weedy”
– Edge species are typically
r-selected
– Invasive species are often
r-selected
Growth Matters!
• K-selected
species
– Why don’t we get
many species of oaks
in most young
forests?
– Climax communities
– Susceptible to habitat
fragmentation
Boom and then Bust
r-like
Water flee (Daphnia magna) is adapted to exploit new
environment: high growth rate, resistant eggs produced
before crash.
Boom and then really Bust
r-like
Reindeer
introduced to
Pribilov island.
Initial
exponential
growth, crash,
then complete
extinction.
Boom and sort of Bust
K-like?
r-like?
Predators were
removed from
Kaibab plateau.
Mule deer
population size
increased from
4,000 to
hundred
thousand, then
dropped and
stabilzed at
10,000.
Boom but not much Bust
r & K-like
Sheep
introduced to
Tasmania: rapid
initial growth,
overshoot, drop,
fluctuation
around carrying
capacity.
Boom & Bust
& Boom & Bust
& Boom & Bust
Hare r tendencies
kept under control
by predation or by
their food supply?
The familiar 10-11 year hare-lynx cycle might not be
true. Biased data. (http://www.behav.org/ecol/wildlife/w_06_populations.htm)
Exponential Population
Growth Equation
Derivation
• Which measured
population growth
components can
change?
• They are:
–
–
–
–
Birth
Death
Immigration
Emigration
• Relationship between
these?
• No + B + I - D – E
Exponential Population
Growth Equation
Derivation
• The equation for population change over a unit t
(time)
• N / t = No + B + I - D – E
• Simplify the equation
– Assume a closed population
– Eliminate migration (I, E)
• N / t = No + B - D
– Create a growth rate (r) = (B-D)/t
• N / t = (r)(No)
– This is the basic exponential growth equation
Exponential Population Growth
Equation - Implications
• N / t = (r)(No)
• What can be experimentally
changed here and how does our
close-to-home example apply?
• Only r can change
– r in humans has been continually
increasing with technology
• When r = 0, the population
growth has stopped
– What is this time-point called?
Logistic Population
Growth Equation
Derivation
• Add the Carrying Capacity (K)
– how?
• N / t = (r)(No)
– Base Expon. Equation
• N / t = (r)(No)(1-(N/K))
– Base Logistic equation
– (1-(N/K)) is the unoccupied
portion of the carrying capacity
Logistic Population
Growth Equation Implications
• N / t = (r)(No)(1-(N/K))
– Base Logistic equation
• Implications:
– As N becomes approx. equal
to K, population stops
increasing
– Logistic is a special case of
Exponential, when K = infinity
Large Variation in Pop. Size
Life tables – compiled by life
insurance agents
Survivorship curves
• Type I curve –
parents produce few
offspring and give
them good care
• Type III curve – high
death rates for infants
then a period when
death rates are lower
for those who survive
to a certain age
Exponential growth of the
human population
• Throughout human
history parents had many
children but only two on
average survived to
adulthood
• Estimates that by 2025
the world will have to
double food production,
2/3 of the available fresh
water on earth will be in
use, 60,000 plant species
will be lost to support the
population
• Issues: overgrazing,
rivers running dry,
decrease in groundwater,
energy?
Human Population Growth
Population Density
Human carrying capacity
estimates
• Ecological footprint (food,
fuel & water consumption,
housing size, and waste
production)
• Calculates current
demand on world
resources by each
country, in hectares of
land per person
• World ecological capacity
is approx. 1.7 ha per
person alive in 1997
How to achieve population
stability?
• Zero population growth –
when birth rates equal
death rates
• Two ways to reach ZPG.
High birth and death rates
or low birth and death
rates.
• Demographic transition is
moving from the first to
the second. Most
developed countries have
made the transition
• See the demographic
transition in Mexico at the
left.
Question:
Why do locusts destroy crops?
nymph
aggregating nymphs
swarming
adults feeding
destructamundo
Limits: Locust Freedom Without Responsibility
I’ve got my rights!
It’s a
free country!
Who’s going to stop me?
= Destroyed Crops
(destruction of environment)
©Phage et al.
Question:
Why do some microbes make us sick?
Limits: Freedom Without Responsibility
It’s a free country
Who’s going
to stop me?
I’ve got my rights!
= Disease!
Bacterial
pathogens
(destruction of the body environment)
Human Freedom Without Responsibility
lack of cooperation
destruction
TEOTWAWKI = “the end of the world as we know it”
air polution
Who’s going to stop me?
the bottom line
greed
It’s a free country
NIMBY
pesticides
= “not in my
backyard”
loss of
habitat
toxic algal
blooms
radical anti-environmentalism
overfishing
I’ve got my rights!
loss of farmland
short-term thinking
bigger is better
might makes right
global warming
deforestation
overconsumption
fish kills
loss of wetlands
out-of-control
materialism
loss of topsoil
urban sprawl
overpopulation
= Destructamundo!
(destruction of environment)
conspicuous consumption
special interests
greenhouse effect
monoculture
desertification
erosion
ozone hole
water polution
mass extinction
lack of cooperation