Transcript What is a population? - Midlands State University

Introduction to Ecology
Midlands State University
Session 2 – Population Ecology
Living organisms play an important role in the
recycling of many elements within an ecosystem.
Discuss how various types of organisms and their
biochemical reactions contribute to the recycling
of either carbon or nitrogen in an ecosystem.
Include in your answer one way in which human
activity has an impact on the nutrient cycle you
have chosen.
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
• 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
• Example:
Only minimal genetic
flow, at most
What is a Metapopulation?
• Components?
• Definition :
– One species
– Multiple areas
– Isolated from other
areas, further away
• 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
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
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
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
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
Exponential Population
Growth Equation Derivation
• Which measured population
growth components can
change?
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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
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
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
r versus K Selection
r-selected organisms usually
• mature rapidly and have an early age of
first reproduction
• have a relatively short lifespan
• have a large number of offspring at a time,
and few reproductive events, or are
semelparous
• have a high mortality rate and a low
offspring survival rate
• have minimal parental care/investment
K-selected organisms usually
• mature more slowly and have a later age of
first reproduction
• have a longer lifespan
• have few offspring at a time and more
reproductive events spread out over a
longer span of time
• have a low mortality rate and a high
offspring survival rate
• have high parental investment
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