Transcript Ecology Intro

Introduction to Ecology
CERC Certificate Program
Columbia 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
• Before all that however…
An Exercise…
• From a study of your own design:
– Work through the 11 steps of the scientific
method
– What did you do?
Scientific Method - Steps 1-5
1. Observe or suspect pattern
2. Posit cause or significance of observed
difference
3. Create answerable question to explain
pattern
4. Create testable hypotheses
– Null (Ho ) and alternate hypotheses (Ha)
5. Design experiment
Scientific Method - Steps 6-11
6. Collect data (descriptive stage)
7. Analyze data, primarily using statistics
8. Evaluate hypotheses, reject Ho?
9. Make conclusions based on data
10. Note problems in current work
11. Predict future directions for research
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
– Rosenbaum’s work with whales
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
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
• 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 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?
– Think close to home
• 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 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
Assignment for Next week:
• Population Growth Models
– Learn more about the consequences of these models
• Instructions are all online, and available here
• Turn in at beginning of class next week
– We will discuss it then
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