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Population and Community
Ecology
Chapter 6
Organization of Nature
1. Individual
Organization of Nature
1. Individual
2. Population: composed of
all individuals that belong
to the same species and
live in a given area
• Evolution occurs at
this level
Organization of Nature
1. Individual
2. Population: composed of
all individuals that belong
to the same species and
live in a given area
• Evolution occurs at
this level
3. Community: Incorporates
all of the populations of
organisms within a given
area
• Study species
interactions
Organization of Nature
1. Individual
2. Population: composed of
all individuals that belong
to the same species and
live in a given area
• Evolution occurs at
this level
3. Community: Incorporates
all of the populations of
organisms within a given
area
• Study species
interactions
4. Ecosystem: consists of all
the biotic and abiotic
components in a particular
location
Organization of Nature
1. Individual
2. Population: composed of
all individuals that belong
to the same species and
live in a given area
• Evolution occurs at
this level
3. Community: Incorporates
all of the populations of
organisms within a given
area
• Study species
interactions
4. Ecosystem: consists of all
the biotic and abiotic
components in a particular
location
5. Biosphere
Population Characteristics
Population Size
• Given the variable “N”
• Number of individuals within a defined area
• Important to study
– Can help prevent extinction in species
Population Characteristics
Population Density
• The number of individuals per unit area (or
volume) at a given time
• Can help scientists estimate the abundance of
a species
• Useful for setting hunting/fishing regulations
Population Characteristics
Population Distribution
• How individuals are distributed with respect
to one another
– Highlights how the species interact with one
another
– Three types of distribution
Population Characteristics
Population Distribution
1. Random Distribution
– No pattern to locations where individuals grow
– Minimal interaction between the species
Population Characteristics
Population Distribution
1. Uniform Distribution
– Individuals are evenly spaced
– Common among territorial andimasl
Uniform Distribution
Population Characteristics
Population Distribution
1. Clumped Distribution
– Common among herding animals, flocking birds,
schooling fish,
– Common among territorial andimasl
Clumped Distribution
Population Characteristics
Population Age Structure
• A description of how many individuals fit into
particular age categories.
• Helps ecologists predict whether a population
is growing, staying the same, or declining
Population Characteristics
Population Age Structure
• Large number of individuals past reproductive
age
– Population will decline
• Large number of individuals that are young
– Population will increase
• Even numbers of old and young
– Population will remain the same size
Population Characteristics
Population Age Structure
• Ecologists use age-structure diagrams and age
pyramids to study age structure
Population Characteristics
Population Age Structure
Population Characteristics
Population Age Structure
Which population is will experience
rapid growth?
Factors that Influence Population Size
Why don’t populations grow indefinitely?
Two different types of population control factors
1. Density dependent
2. Density independent
Factors that Influence Population Size
Density Dependent
• Factors that influence an individual’s probability of survival
and reproduction in a manner that depends on the size of the
population
– Ex: available food
– Called Limiting Resources:
• A resource that a population cannot live without
• occurs in quantities lower than the population would
require to increase in size
Factors that Influence Population Size
Density Dependent
• Limiting Resources:
– Plants:
• Water
• Nutrients (Phosphorus & Nitrogen)
– Animals:
• Food
• Water
• Nest sites
Factors that Influence Population Size
Density Dependent
• Given a small population with an abundance
of Limiting Resources
– How would the population respond?
Factors that Influence Population Size
Density Dependent
• Given a small population with an abundance
of Limiting Resources
– How would the population respond?
• The population grows rapidly
• Given a dense population, what will happen to
the growth?
Factors that Influence Population Size
Density Dependent
• Given a small population with an abundance
of Limiting Resources
– How would the population respond?
• The population grows rapidly
• Given a dense population, what will happen to
the growth?
– Increased competition for resources
• Slow population growth
Factors that Influence Population Size
Density Dependent
• The growth slowed due to the lack of
resources
– Carrying Capacity: the number of individual an
ecosystem can sustain
• Given variable K
Growth Models
• Growth Rate: the number of offspring an individual can
produce in a given time period, minus the deaths of the individual
or its offspring during the same period
– Intrinsic Growth Rate (r):
A populations maximum potential for
growth
• Deer: 2 fauns
• Hogs: 10 piglets
• Bullfrogs: 20,000 eggs
• If we know the:
– intrinsic growth rate and
– the number of reproducing individuals
• we can estimate the future population size
Growth Models
Exponential Growth Equation
Where:
N = future size of population
N0 = current size of population
r = Intrinsic growth rate
t = amount of time the population grows
Growth Models
Exponential Growth
J shaped curve forms when
the population is not
limited by resources
Growth Models
Calculating Exponential Growth:
• A population of rabbits has an initial population size of 10 individuals
• The intrinsic rate of growth for a rabbit is 0.5
– Which means each rabbit produces a net increase of 0.5 rabbits each year
• Predict the size of the rabbit population in one year’s time
•
•
•
•
N = N0e^(rt)
N = 10*e^(0.5*1)
N = 10 * 1.6
N = 16 rabbits
• How large will the rabbit population be in 5 years?
• N = 122
• How large will the rabbit population be in 10 years?
• N=1,484
Growth Models
Assume that the intrinsic rate of growth is 1.0 for
rabbits.
Calculate the predicted size of the rabbit population
with an initial population of 10 after 1, 5, and 10
years
1 year: 27.2 rabbits
5 years: 1,484.1 rabbits
10 years: 220,264.7 rabbits
Growth Models
Logistic Growth Model
• Populations cannot grow exponentially
indefinitely
– Must take limiting resources into account
• Logistic Growth Model:
– Describes a population whose growth is initially
exponential, but slows as the population
approaches the carrying capacity (K) of the
environment
Growth Models
Logistic Growth Model
• As population reaches approx ½ the carrying
capacity
– Resources become limited
– Population stops growing
– Model produces an S-shaped curve
– Population reaches a stable state
Growth Models
Logistic Growth Model
Growth Models
Variations on Logistic Growth Models
• A population may Overshoot the carrying
capacity
– There will not be enough food
– Population will experience a Die-off or population
crash
Growth Models
Logistic Growth Model
Population may not recover OR will oscillate
Growth Models
Predator & Prey Relations
• Aside from limiting resources, Predator/prey
relations affect population sizes
1. Hares increase in
number, food abundant
2. Increase number of
hares reduces
population
(competition)
3. With less predation an
more food, hares
increase
Reproductive Strategies and
Survivorship Curves
• Two different reproductive strategies
– K-selected Species
• Have low intrinsic growth rate
– Population increases slowly
• Small population fluctuations
• The Typical K-selected species
–
–
–
–
Large
Mature relatively late
Produce a few, large offspring
Substantial parental care
• K-selected species that are endangered cannot respond
quickly to restoration efforts
Reproductive Strategies and
Survivorship Curves
• R-Selected Species
– Rapid population growth followed by over-shoots
and die-offs
– Typical R-Selected Species
•
•
•
•
Small
Reproduce frequently
Produce many offspring
Little to no parental care
Reproductive Strategies and
Survivorship Curves
• In our simulation, which animal was the Kselected species?
• The R-selected species?
– What evidence do you have to support this