General Ecology: Lecture 4
October 3, 2005
I.
Properties of populations
A.
Definition of a population: A group of organisms of the same species occupying a
particular space at a particular time.
1.
Ideal: define “space” and “time” in an ecologically relevant way
a)
Example 1: Laysan’s albatross on Midway Island during breeding season
b)
Example 2: Overwintering monarch butterflies, mountains of central Mexico
2.
Populations with highly mobile organisms are the most difficult to define.
a)
Examples…
3.
Populations are sometimes identified by artificial, human-based boundaries
a)
Example…
B.
Characteristics of populations
1.
Basic characteristic: density
2.
Four population parameters that change density: natality (reproductive output),
mortality, immigration, emigration
3.
Secondary characteristics: age distribution, genetic composition, patterns of
distribution, etc…
C.
Quantifying populations: Unitary vs. modular organisms
1.
Unitary: Individuals easy to identify; separate genetic individuals, or “genets”
a)
Examples
2.
Modular: Clones that are genetically identical and derived from a single zygote
a)
ramets

Examples: Grasses, aspen trees [Fig. 10.2], coral colonies
b)
Problem of counting (What is “one”? Can’t see clone boundaries…)

What do scientists do in this case?
D.
Metapopulations [Fig. 10.3]
1.
What distinguishes metapopulations from populations?
2.
Example 1: Queen conchs: pattern of movement (see also 10.3a)
3.
Example 2: Island populations (see also 10.3b)
E.
Populations as genetic units.
1.
What is the gene pool?
F.
Populations as evolutionary units
1.
How is evolution defined in terms of populations and genes?
G.
Density and dispersion
1.
Most basic characteristic of populations. How many are there within the region of
interest at a particular time?
2.
Crude density vs. ecological density
a)
Define each
b)
What is the difference?
3.
Spatial patterns of distribution [Fig. 10.4]
a)
Depends upon distribution of resources as well as intraspecific competition

Uniform: Might occur in cases of territoriality. For example,

Example 1 (text): Golden eagle territories [Fig. 10.5]

Example 2: Damselfish

Clumped: Most common. Why?

Plant distribution and microclimate

Which aspects of microclimate might be important?

Examples: Plantago, key host plant for Euphydryas,
requires serpentine soil.

H.
I.
Consumer distribution and food resources

Euphydryas distribution then follows the clumped
distribution of its food source.

Random: Rare. Why? Examples?
4.
Temporal patterns of dispersion
a)
Types of patterns?
b)
Emigration/immigration: one-way travel

Dispersal of seeds/larvae/young is common

Examples

Dispersal of adults

Why might adults disperse?

Examples of adult dispersers
c)
Migration: round trip made.

Three types of round trips (including examples)

Often involves alternating between ideal feeding and breeding
grounds.

Example: California gray whales [Fig. 10.9]

Why do they migrate between Alaska and Mexico?

Other examples on same figure

Longest migration: arctic terns

Why do they migrate this long distance?
Age structure
1.
Stable age distribution defined
2.
Stationary age distribution defined
3.
Theoretical age pyramids are especially applicable to mammals and birds with
longer lifespans [Fig. 10.10]
a)
Growing populations will have higher proportions of young
b)
Aging/shrinking populations will have higher proportions of older
individuals
4.
Effects of failed breeding in a particular year will be seen in the age distributions of
successive years [Fig. 10.11]
a)
How would population be affected overall? Compare short-lived to longlived species
5.
Why is it problematic to calculate age structures for plants?
Sex ratio
1.
Patterns from conception to birth
2.
Patterns between birth and maturity
a)
Why is male mortality higher during this time period? Possible explanations
include:

Genetics: Hemizygous nature of males

Various aspects of male behavior
Study questions
1.
Be able to define “population” and discuss what factors you might consider when attempting to
define a population for ecological studies. Provide examples of ecologically relevant
populations—both those given in lecture as well as your own examples.
2.
What is the most basic characteristic of a population? Also, name the four key parameters of a
population that can change this basic characteristic.
3.
Define the following and provide examples:
a.
Unitary organism
c. genet
b.
Modular organism
d. ramet
4.
Why are populations of marine organisms often difficult to identify (2 reasons)?
5.
What is meant by a metapopulation? How does it differ from a continuous population with
pockets of clumped individuals? How have humans changed some continuous populations of
plants and animals into metapopulations?
6.
What is meant when we refer to a population as a genetic unit? What is a “gene pool”?
7.
Define “evolution” in terms of populations and genes.
8.
What are some ways of counting modular organisms in ecological studies?
9.
What is the difference between crude density and ecological density?
10.
List/describe the three possible patterns of spatial distribution, and describe environmental
conditions that would lead towards each pattern. Also provide examples for each pattern, briefly
describing what has led towards that pattern.
11.
Describe several temporal patterns of dispersion in organisms, including examples of each.
12.
Describe three different patterns of migration, and provide an example of each.
13.
Why do gray whales undergo their long migration along the west coast of North America?
14.
Which animal has the longest north-south migration route?
15.
What is meant by a stable age distribution? A stationary age distribution? Must a stable age
distribution be stationary? Explain.
16.
Be able to recognize the age distribution of a population that is likely to be growing rapidly vs.
one that is aging or going extinct. Under what circumstances might you see a high proportion of
young to adults in a population that is declining rather than growing?
17.
How might a “missing cohort”, as in Fig. 10.11 for the cactus ground finch, occur? What are the
consequences of this missing cohort? Under what circumstances might a missing cohort be
especially deleterious to the population?
18.
What factors make determining the age structure of a plant population problematic?
19.
What is the “normal” sex ratio of most populations of organisms? Describe the typical changes in
sex ratio from conception to birth to maturity in this type of population.
20.
What are the two alternate explanations given for why male mortality in many populations is
higher than female mortality between birth and maturity?