Chapter #11 – Population Regulation
(pg. 223 – 237)
Chapter #11 – Population Regulation
(pg. 223 – 237)
11.1 –
The Environment Functions to Limit Population Growth.
The Logistic Model of Population Growth
dN = r N ( k – N ) or
dt
k
(
dN = r N 1 – N
k
dt
)
Population Dynamics
Logistic Growth
•
•
Environmental Resistance can reduce
the reproductive rate and average life
span and increase the death rate of
young.
As Environmental Resistance increases,
population growth slows and eventually
stops, likely near (k).
Carrying Capacity
Finite Resources?
Carrying Capacity (k): The abundance (size)
of a species’ population
that a habitat can
support for a specified
period of time (an upper
limit).
Source: Bolen and Robinson (1999) Wildlife Ecology and Management
Environmental Resistance
? Finite Resources ?
Environmental Resistance: The limiting
influences of
environmental factors
on the numbers of
individuals in a
community.
Source: Bolen and Robinson (1999) Wildlife Ecology and Management
Chapter #11 – Population Regulation
(pg. 223 – 237)
11.2 – Population Regulation Involves Density-Dependence.

As population density increases, individuals compete for
space, energy, and nutrients.

At carrying capacity, each individual's share of resources is
just enough to allow it to replace itself in the next generation.

At carrying capacity the birth rate (b) = death rate (d).

Carrying capacity (k) is determined by the continuous
availability of resources.
Environmental
Resistance (ER)
• Environmental Resistance
(ER) can be classified into
two broad categories:
– Density-independent factors
– Density-dependent factors
Environmental
Resistance (ER)
A factor that causes
higher mortality or
reduced birth rates as a
population becomes more
dense is referred to as a
density-dependent factor.
Density-Dependence
• Density-dependent
factors become more
effective as population
density increases.
• Exert negative
feedback effect on
population size.
Density-Dependence
Density-Dependence (DD)
High
Per Capita
Birth Rate
Low
Low
“A factor that causes
higher mortality or
reduced birth rates as a
population becomes
more dense”
(Bolen & Robinson)
-disease, food supply,
-predation, and
-territorial behavior
Population Density
High
Population Dynamics
Population Cycles
• Predation
- involves a predator killing a prey organism to consume.
• Predators exert density-dependent controls on a
population.
– Increased prey availability can
increase birth rates and/or
decrease death rates of predators.
• Prey population losses will increase.
Population Regulation
Population Cycles
Prey Switching and
Availability - Interactions
Chapter #11 – Population Regulation
(pg. 223 – 237)
11.3 – Competition Results when Resources Are Limited.

Describes the interaction among individuals who attempt to
utilize a resource that is limited relative to the demand for it.

Competition intensifies as populations grow and near carrying
capacity.

For two organisms to compete, they must share the same
resource(s).
Competition for Resources
11.3 – Competition Results when Resources Are Limited.
 Competition may be divided into two groups
based on the species identity of the competitors:
– Interspecific competition is between individuals
of different species.
– Intraspecific competition is between individuals
of the same species.
Competition for Resources
 Competition may also be divided into two types
based on the nature of the interaction:
– Scramble (exploitative) competition is a free-forall scramble as individuals try to beat others to a
limited pool of resources.
– Contest (interference) competition involves
social or chemical interactions that limit a
competitor’s access to resources.
Competition for Resources
 Competition may also be divided into two types
based on the nature of the interaction:
– Scramble (exploitative) competition is a free-forall scramble as individuals try to beat others to a
limited pool of resources (direct or indirect –
respond to depressed resource levels).
– Contest (interference) competition involves
social or chemical interactions that limit a
competitor’s access to resources (direct interaction).
Density-Independence
• Density-independent
factors limit populations
regardless of their density
– Examples: climate,
weather, floods, fires,
pesticide use, pollutants,
and overhunting.
Density-Independence
Density-Independence (DI)
No association
with population
density – they act
on a population
independent of
density
Per Capita
Birth Rate
Population Density
- e.g., weather
(floods, hurricanes)
fires, earthquakes,
volcanoes
Chapter #11 – Population Regulation
(pg. 223 – 237)
11.4 –
Intraspecific Competition Affects Growth and Development.
 As populations density increases toward a level of insufficiency
(per unit for basal metabolism), individuals can reduce food intake.
 Reduced food intake slows individual growth.
 Reduce growth can lead to reduced reproductive output and
success.
 Thus, there is an inverse relationship between density and rate of
body growth.
Chapter #11 – Population Regulation
(pg. 223 – 237)
11.4 –
Intraspecific Competition Affects Growth and Development.
Chapter #11 –
Population Regulation
(pg. 223 – 237)
11.5 –
Intraspecific Competition Can
Reduce Reproduction.
Chapter #11 – Population Regulation
(pg. 223 – 237)
11.6 –
High Density Is Stressful to Individuals.
 As population density increases, space becomes restricted.
Aggressive behavior (competition for space) becomes more
frequent.
 Increased crowding and social contact can cause stress, triggering
hormonal changes that restrict reproduction and growth.
 Increased stress may also lead to immune system suppression and
increased frequency of disease.
 Thus, there may be reduced births and increased deaths.
Chapter #11 – Population Regulation
The Chitty Hypothesis
11.6,7 –
The Chitty hypothesis depends upon having two types of individuals in the population. One is tolerant of
having other individuals close by (ie, tolerant of high densities) and has a low reproductive rate. The second
type is intolerant and tends to disperse from the population under conditions of high density. They also have
a higher reproductive rate than the tolerant individuals.
Increasing phase
Population size increases.
Peak phase
Competition between animals increases (for
food, mates, shelter...)
Tolerant individuals stay but they have a low
reproductive rate compared to
dispersers.
Declining phase
Population size starts to decrease because of
emigration of dispersers and low reproductive
rate of tolerant individuals that
remain.
Low density phase
Animals space themselves out
Competition between animals decreases
Increasing phase
Dispersers remaining in the population (or born
into the population) increase in relative
frequency because of their higher reproductive
rate
Population size increases (back to step #1)
Intolerant individuals (dispersers) leave and
colonize neighboring (often marginal) areas.
Chapter #11 – Population Regulation
(pg. 223 – 237)
11.8,9 – Social Behavior May Function to Limit Populations.
Territoriality Can Function to Regulate Growth.
Home Range: an area in which an animal normally
lives and is not necessarily associated with any type
of aggressive behavior. Dominance hierarchy may
exist among individuals with overlapping home
ranges.
Territory: a defended, more or less fixed and
exclusive area maintained by an individual or social
unit occupying it.
Chapter #11 – Population Regulation
(pg. 223 – 237)
11.8,9 – Social Behavior May Function to Limit Populations.
Territoriality Can Function to Regulate Growth (limit access).
Chapter #11 –
Population Regulation
(pg. 223 – 237)
11.10 – Plants Preempt Space and
Resources – Are They
“Territorial”?
 Preventing root invasion can
be a form of competition.
Big Sagebrush
(Artemisa tridentata)
Chapter #11 – Population Regulation
(pg. 223 – 237)
11.11 – Density-Independent Factors Can Influence Population Growth.
Chapter #11 – Population Regulation
(pg. 223 – 237)
11.11 – Density-Independent Factors Can Influence Population Growth.
Ultimately . . . .
• The abundance of a population at any
given time is the result of complex
interactions between densityindependent and density
-dependent forms of
environmental resistance.
Next Time . . . .
Interspecific Competition