Chapter 53:
Population
Ecology
Essential Knowledge
2.a.1 – All living systems require constant
input of free energy (53.3 & 53.4).
 2.d.1 – All biological systems from cells and
organisms to populations, communities, and
ecosystems are affected by complex biotic
and abiotic interactions involving exchange
of matter and free energy (53.1 – 53.5).
 4.a.5 – Communities are composed of
populations of organisms that interact in
complex ways (53.1-53.3, 53.5 & 53.6).

Population Ecology
Study of the factors that affect population
size and composition.
 Population:

 Def: Individuals of a single species that occupy
the same geographic area
 Ex: Humans living in Indianapolis, IN
Important Characteristics
1. Density
2. Dispersion
Density
 Number of individuals per unit area or
volume.
 Ex:
 Diatoms - 5 million/m3
 Trees - 5,000/km2
 Deer - 4/km2
Dispersion
 Pattern of spacing among individuals.
 Types:
1. Clumped
2. Uniform/Even
3. Random
Clumped Dispersion
May result form a patchy environment.
 May increase chances for survival.
 Ex:

 Schooling behavior
 Flocks of birds
Uniform Dispersion
 Often the result of antagonistic interactions
between individuals.
 Known as even or regular
dispersion
 Ex:
 Territories
 Spacing between desert plants
Random Dispersion
Often the result of the absence of strong
attractions or repulsions between
individuals.
 Not a common pattern.

Demography
The study of the vital statistics that affect
population size.
 Ex: Birth and Death rates
 Factors of Demography:

 Age structure of population
 Birth and death rates
 Generation time
 Sex ratio and reproductive behavior
Life Tables
Mortality summary for a cohort of
individuals.
 First developed from life insurance studies.
 What do they show us?

 Mortality rate per year
 Life span of the organism
 Fecundity (birth rate)
Survivorship Curve
Plot of the numbers of a cohort still alive
over time.
 Curve Types:

 Type I
 Type II
 Type III
Type I
Low early deaths.
 High late deaths.
 Ex:

 Humans
 Other large
mammals
Type II
Constant death rate.
 Ex:

 Annual plants
 Many invertebrates
Type III
High early deaths.
 Low late deaths.
 Ex:

 Trees
 Oysters
Comment
Curve type may change between young and
adults.
 Ex: Nestlings - Type III
Adult Birds- Type II

Life History Strategies
1. "r" or Opportunistic species
2. "k" or Equilibrial species
"r" Species
Increase fitness by producing as many
offspring as possible.
 Do this by:

 Early maturation
 Many reproductive events
 Many offspring
Result
Maximize reproduction so that at least a
few offspring survive to the next generation.
 Most offspring die (Type III curve).

"k" Species
Increase fitness by having most offspring
survive.
 Do this by:





High parental care
Late maturation
Few reproduction events
Few offspring.
Result
Maximize survivorship of each offspring.
 Few offspring, but most survive (Type I
curve).

What is the strategy?
For a weed?
 For an endangered species?
 For Garden Pests?

Population Growth
DN/Dt = b - d
 Where:

 N= population size
 t = time
 b = birth rate
 d = death rate
Rate of Increase
r = difference between birth rate and death
rate.
 r=b-d

Equation with “r”:
DN/Dt = rN
 N = population size
 t = time
 r = rate of increase

From Calculus

The equation DN/Dt = rN becomes:
 dN/dt = rmax N
 rmax = intrinsic rate of increase
Exponential Growth
dN/dt = rmax N
 Characteristic of "r" species.
 Produces a “J-shaped” growth curve.
 Only holds for ideal conditions and unlimited
resources.

Logistic Growth
dN/dt = rmax N K-N
K
 K = carrying capacity
 Result of logistic growth?

 “S-shaped” growth curve
 Characteristic of “k” species
 Common when resources are limited
Comment
K is not a constant value.
 Populations often oscillate around “K” as
the environment changes.

Additional Comments
Populations often overshoot “K”, then drop
back to or below “K”.
 AP Exam rarely asks you to work the
equations, but you should be able to give
them.

Regulation of Population
Size
1. Density- Dependent Factors
2. Density- Independent Factors
Density-Dependent
Affect is related to N
 As N increases, mortality increases
 Ex: Food, nesting space, disease

Density-Independent
Affect is not related to N
 Mortality not related to population size
 Ex: Weather and climate

Population Cycles
Cyclic changes in N over time
 Often seen in predator/prey cycles
 Ex: Snowshoe Hare – Lynx
 Causes?

 Density dependent factors
 Chemical cycles
 Saturation strategy to confuse predators
Age Structure Diagrams
Show the percent of a population in
different age categories
 Method to get data similar to a Life Table,
but at one point in time

Importance

Can be used to predict future population
growth trends, especially for long lived
species.
Exponential Growth

Produces age structures that are a triangle
or pyramid shape
Logistic Growth

Produces age structures that have even
sizes between most age categories
Declining Populations

Produce age structures with a narrow base
and wider middles
Summary







Identify the difference between population
density and dispersion.
Recognize the types of dispersion patterns and the
interactions that lead to them.
Identify the types of survivorship curves.
Recognize the characteristics of "r" and "k" life
history strategies.
Identify the types of population growth models.
Identify factors that regulate population size.
Recognize how age-structure diagrams relate to
population growth.