Transcript What is a population? - Midlands State University

Introduction to Ecology
Midlands State University
Session 2 – Population Ecology
Population Ecology
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Goals for the day
Why is this field important?
What is a population?
Why does a population change in size?
Unlimited, exponential population growth
Logistic population growth
Exponential vs. Logistic growth
Population interactions
Population Ecology
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Goals for the day
Why is this field important?
What is a population?
Why does a population change in size?
Unlimited, exponential population growth
Logistic population growth
Exponential vs. Logistic growth
Important Applications of
Population Ecology - Local
• Growth rates of introduced
species
• Population Viability Analyses
of endangered species
– What is the minimum number of
individuals needed to ensure a
90% chance of survival for 100
years
• Population genetics of
endangered species
Important Applications of
Population Ecology - Regional
• Metapopulation
analyses
– Tracing the survival of
all component
populations
– More in a bit
• Captive Breeding
projects at zoos
– Applied metapopulation
analyses
Population Ecology
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Goals for the day
Why is this field important?
What is a population?
Why does a population change in size?
Unlimited, exponential population growth
Logistic population growth
Exponential vs. Logistic growth
What is a Population?
• Components?
• Definition :
– One species
– One area
– Isolated from other
areas
– Able to interbreed
• Example:
Only minimal genetic
flow, at most
What is a Metapopulation?
• Components?
• Definition :
– One species
– Multiple areas
– Isolated from other
areas, further away
– Able to interbreed
• Example:
Only minimal genetic
flow, at most
Population Characteristics
Population ecology is the study of interactions
within populations (i.e., intraspecific interactions)
Recall that populations are groups of interacting
conspecifics (e.g., inter-mating)
We can characterize individual populations in terms
of there…
 Size (average vs. variation)
 Density (& impacts on size; density dependence)
 Patterns of Dispersion
 Demographics (age structure, sex ratios)
 Rates of growth (or decline)
 Limits on population growth
Characteristics of a Population
• What features can we measure of a population?
• Features:
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Size
Age structure
Sex ratios
Effective population size
Birth rate
Death rate
Immigration
Emigration
Population Ecology
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Goals for the day
Why is this field important?
What is a population?
Why does a population change in size?
Unlimited, exponential population growth
Logistic population growth
Exponential vs. Logistic growth
Why Does Population Size
Change?
• Density Independent Forces
– Forces that are at work irrespective of the
population density
• Density Dependent Forces
– Forces that vacillate depending on the population
density
Density Independent Forces
• Types?
• Examples
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Climate
Topography
Latitude
Altitude
Rainfall
Sunlight
• In Sum: Abiotic factors
– Exceptions do exist!
Density Dependent Forces
• Types?
• Examples
– Within species
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Breeding spaces
Food
Mates
Foraging spots
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Predation
Parasitism
Pollinators
Competition
– Between species
• In Sum: Biotic factors
– Exceptions do exist!
Indeterminate Factors
• Most influences are pretty constant and
Deterministic
• Opposite of deterministic factors is
Stochastic forces e.g droughts, floods
volcanoes, asteroids, fires etc
• Examples
– Environmental: Droughts, floods, asteroids,
volcanoes, fires, etc.
– Demographic: Crash in effective population
size, series of single sex born, etc.
Small Populations
• Usually at great risk
• Why?
-Small population size
-Small genetic diversity
-Highly susceptible to
stochastic forces
-Poor competitors with resident
biota
•Severely limited adaptability
Population Dispersion
Why Different Types?
Age-Structure Pyramids
cohort
(Cohort) Survivorship Curves
Reproduction
 Age of first reproduction (~sexual maturity)
 Clutch size
 Investment in individual progeny
 Tradeoff between reproduction and survival
 Number of reproductive episodes per
lifetime
 Upside of Semelparity
 Upside of Iteroparity
Semelparity
Iteroparity
Population Ecology
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Goals for the day
Why is this field important?
What is a population?
Why does a population change in size?
Unlimited, exponential population growth
Logistic population growth
Exponential vs. Logistic growth
Types of Population Growth
• Exponential
– Unlimited, rapid growth
– Often called Malthusian
– Growth without bounds
• Logistic
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Growth within natural limits
What sets that limit?
What is the limit?
More in a moment…
Exponential Population
Growth
• Examples of this?
– Invasive alien species water
hycinth
• Often an unnatural
occurrence
• Conditions under which this
occurs naturally
– Introduced species
– Nutritionally enriched
environments
– Cultural innovations?
Exponential Population
Growth Equation Derivation
• Which measured population
growth components can
change?
• They are:
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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 timepoint called?
Population Ecology
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Goals for the day
Why is this field important?
What is a population?
Why does a population change in size?
Unlimited, exponential population growth
Logistic population growth
Exponential vs. Logistic growth
Carrying Capacity – Unique
to Logistic Growth
• Definition?
• A summary of all factors
regulating population sizes
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Density dependent
Density independent
Determinate
Stochastic
• Site and species specific
value
Logistic Population Growth
• What is added in this form of
population growth?
• The Carrying Capacity is
added
– What is it and what determines it?
– Typically summarized as K
• How would we modify the
exponential population growth
equation to reflect this?
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 ~ K, population increase
stops
– Logistic is a special case of
Exponential, when K = infinity
Growth Without Limits
r = 1.0
r = 0.5
r = population growth rate
Impact of Limits
Imposition of limits
dN/dt = r  N  (K-N)/K
New or Changing
Environment (no
competition /
limits)
Environmental Resistance
K
N
N/K = “Environmental Resistance”
Maximizing Yield
dN/dt is maximized when N*r is maximized
Fitting Curves to Real Pops.
 Logistic growth model does not consider the effects of
predators or interspecific competition, so fails to predict the
complexities of the density of many natural populations as a
function of time
 Nevertheless, it serves as good null hypothesis
Oscillations about K
overshoot
K
N
oscillations
Time
r versus K Selection
Growth Matters!
• How many humans can we expect?
– May be unlimited?
– What about implications of Ecological
Footprint exercise?
– Currently 6 billion people
– Hotly contested
Growth Matters!
• r-selected species
– Why most weeds are weedy
– Edge species are typically
r-selected
– Invasive species are often rselected
Growth Matters!
• K-selected species
– Why we don’t 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,
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 1predation 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)
Density-Dependent Limits (to max = K)
Competition increases
Density-Independent Factors (e.g., weather)
Good
Times!
(in Australia)
Density-Dependent vs. Independent Limits
Question:
Why are humans
destroying the earth?
destructamundo
destructamundo
destructamundo
destructamundo
Limits: Paul Ehrlich and the Population Bomb
Impact = Population * Affluence * Technology
Impact = Population * Affluence * Efficiency
Consumption per Baby = Resource * Efficiency
(or afluence * technology)
Consumption Damages Resources (a.k.a., the environment)
There are only so many resources to use up!
Human Population Growth
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!
out-of-control
materialism
loss of farmland
short-term thinking
bigger is better
might makes right
global warming
deforestation
loss of topsoil
overconsumption
overpopulation
= Destructamundo!
(destruction of environment)
conspicuous consumption
special interests
urban sprawl
fish kills
loss of wetlands
greenhouse effect
monoculture
desertification
erosion
ozone hole
water polution
mass extinction
lack of cooperation
Proximate Ecological Fields
- Revisited
• Trends down pyramid:
– Increase in geographic scale
Population
– From single species to multiple
species
Community
– Increasing number of ecological
factors that may be influential
Ecosystem
– Decreasing certainty in results
Next Week: The Tour of
Ecology Continues
• Population ecology
• Community ecology
– Next week’s emphasis
• Ecosystem ecology
• Conservation Issues