General Ecology 2006: Lectures 4&5
Properties of Populations
I.
Properties of populations (overview)
A.
Definition of a population: A group of organisms of the same species occupying a
particular space at a particular time.
1.
Should ideally define that “space” and “time” in terms that are of ecological
relevance to the organism and/or your particular study.
a)
Examples:

Laysan’s albatross on Midway Island during breeding season

Overwintering monarch butterflies in mountains of central MX
2.
Populations with highly mobile organisms are the most difficult to define.
a)
Example: tuna
3.
Populations are sometimes identified by artificial, human-based boundaries in order
to manage them (i.e. the wolf population of Yellowstone Park)
4.
Populations as genetic units
a)
Gene pool: sum of all genetic information within a population
5.
Populations as evolutionary units
a)
Evolution = change in the genetic make-up of a population over time.
II.
Characteristics of populations
A.
Basic characteristic: density (#/area; #/depth-integrated area, #/volume)
B.
Four population parameters that change density: natality (reproductive output),
mortality, immigration, emigration
C.
Secondary characteristics: age distribution, genetic composition, etc...
III. Quantifying populations: Unitary vs. modular organisms
A.
Unitary: Individuals easy to identify; separate genetic individuals, or “genets”
1.
Examples: All vertebrates, insects, individual snails, etc…
B.
Modular: Clones that, while they may have individual structures, are genetically identical
and derived from a single zygote
1.
These asexually produced units are called “ramets”, and may be connected to their
parent organism or separate from them.
a)
Examples: Aspen trees, coral colonies (images)
b)
The genetically identical ramets, taken together, are a genet.
2.
Problem of counting (What is “one”? Can’t see clone boundaries…)
a)
Ramets may be considered individuals when they appear distinct
b)
Percentage cover (area) may be quantified rather than the number of
individuals.

Example: quadrat sampling
c)
Ask: What is ecologically relevant? Are you interested in the resources
used? Genetic diversity?
IV.
Metapopulations [Fig. 10.3]
A.
Definition: Relatively isolated populations, often isolated by topography or habitat
fragmentation (natural or human-caused), can have gene flow via immigration and
emigration. The metapopulation is the sum of all these more isolated populations.
B.
Examples: The 5 diagrams show different patterns of immigration and emigration.
Arrows indicate direction in which gene flow occurs.
1.
Example (diagram a): Queen conchs (Honduras  Belize  Florida)
2.
Example 2 (diagram b): Mainland  Island populations
3.
Example 3 (diagram c): Patch reefs
4.
Example 4 (diagram d): Galapagos Islands
5.
Combination of a-d.
V. Density and dispersion
A.
Most basic characteristic of populations: How many are there within the region of
interest at a particular time?
a)
Crude density vs. Ecological density

Example: sea urchins in low intertidal)
B.
Spatial patterns of distribution [Fig. 10.4]
1.
Depends upon distribution of resources as well as intraspecific competition
a)
Uniform: Might occur in cases of territoriality.

Example: Golden eagle territories [Fig. 10.5]
b)
Clumped: Most common, based on clumped distribution of desirable
conditions or resources.

Example: Plantago, key host plant for Edith’s checkerspot,
Euphydryas editha, requires serpentine soil that has a limited
distribution on Jasper Ridge

Animal distribution may be based on particular physical conditions
or on distribution of their food

Example 1: Human populations often near water

Example 2: Edith’s checkerspot distribution then follows the
clumped distribution of Plantago (its food)
c)
Random: Rare

Example 1: Spiders on the forest floor

Food and habitat is randomly distributed

Example 2: Desert plants (such as creosote bushes)

Severe competition for water keeps them spaced apart
C.
Temporal patterns of dispersion
1.
Daily, tidal, seasonal, interannual, wet vs. dry seasons, etc…
2.
Emigration/immigration: one-way travel
a)
Dispersal of seeds/larvae/young is common
b)
Dispersal of adults

Often for breeding: will seek out new habitat away from natal
territory; competition from conspecifics is hopefully reduced

Several insect species may or may not disperse in this way,
depending upon relative crowding, food availability, suitable habitat
for laying eggs, etc…
3.
Migration: round trip made. An individual may make a single round trip (salmon)
or several (gray whales, Canadian geese), or may be part of a circuit that is not
completed in its own lifetime (monarch butterflies)
a)
Often involves alternating between ideal feeding and breeding grounds.

Example: California gray whales [Fig. 10.9, #3]

Feeding is best in cold, food-rich northern waters during the
boreal summer (feed on mud-dwelling organisms)

Breeding aggregates form in warm waters, an environment
more thermally favorable for young whales

See also other examples in the text!
b)
Longest migration: arctic ternsfollow favorable temperature and food
(surface zooplankton) conditions, “endless summer”.
VI.
Age structure
A.
Stable age distribution: Theoretically, continuously breeding populations should tend
towards an age distribution where the ratio of age classes remains constant, even if the
population is growing
B.
Stationary age distribution: Deaths balance births, so that the population is a constant
size through time. Also will have a stable age distribution.
1.
NOTE: Stationary populations are stable, but stable populations aren’t always
stationary (see “a” above)
C.
Theoretical age pyramids most applicable to long-lived animals [Fig. 10.10]
1.
Be able to recognize a growing population compared to a shrinking one.
a)
When might a shrinking population show a high proportion of young?
D.
Effects of failed breeding in a particular year will be seen in the age distributions of
successive years
1.
Example: Cactus ground finch [Fig. 10.11].
a)
Focus on missing cohorts. Why are they missing?
b)
Focus on large cohorts. Why are they so large?
c)
NOTE: Think about progression through time/time lag
2.
Missing cohorts: under what circumstances might this lead to population
extinctions?
E.
Problematic to calculate for plants, due to
1.
Modular structure of individuals
2.
Asexual reproduction
VII. Sex ratio
A.
Tends toward 1:1
B.
Often different between conception and birth
1.
1:1 at conception slightly favors males at birth Slightly favors females at
sexual maturity
a)
In general, male mortality is higher between birth and maturity. Possible
reasons include…

Males are hemizygous (why would this matter?)

Male behavior may be more stressful, especially prior to and during
breeding season (male-male competition…)

Males of many species more prone to risky behavior
VIII. Mortality and natality
A.
Survivorship curves [Fig. 10.13]
1.
General patterns
a)
Type I: High degree of survival throughout lifespan, followed by high
mortality at old age

Many mammals, especially those with parental care

Look carefully—usually an initial drop due to infant mortality
b)
Type II: Linear, with constant mortality rates

Similar risks throughout life; moderate predation on all life stages.
Many small birds and reptiles show this type of pattern.
c)
Type III: High mortality in early life, with a low proportion making it to
reproductive age

Marine organisms with larval dispersal
d)
NOTE: Curve is based on the number of individuals initially in the cohort.
Estimate can be off if initial cohort is undercounted.
2.
Variability of survivorship pattern within a population:
a)
Summer annual garden rocket: diff. curves for diff. cohorts [Fig. 10.19]
Why?
B.
Mortality curves [Fig. 10.20]
1.
Plotted with the same life table data for red deer populations
a)
Steep drop in # of survivors corresponds with high mortality
b)
NOTE: Mortality curve not based on initial # in cohort.
C.
Fecundity curve for red deer [Fig. 10.21]
1.
Shows average reproductive potential of the cohort at each age
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.
What makes counting modular organisms problematic? What are some of the options for
quantifying modular organisms? On what basis would you choose your method? Provide at least
one example of a method chosen and why you chose it (real example or from your imagination).
5.
Why are populations of marine organisms often difficult to quantify (2 reasons)?
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 is a metapopulation? Be able to explain Figure 3, using real or theoretical examples to
illustrate your answer (and know the real examples given). How have humans changed some
continuous populations of plants and animals into metapopulations?
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.
Why do gray whales undergo their long migration along the west coast of North America?
13.
Which animal has the longest north-south migration route?
14.
What is meant by a stable age distribution? A stationary age distribution? Must a stable age
distribution be stationary? Must a stationary age distribution be stable?
15.
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?
16.
Understand and be able to explain Fig. 10.11. Be especially sure you understand how the bottom
and top graphs are related.
17.
What factors might cause a “missing cohort”, as in Fig. 10.11 for the cactus ground finch, to
occur? What factors might cause a larger than average cohort? What are the consequences of this
missing cohort? Under what circumstances might a missing cohort be especially deleterious to the
population?
18.
Name two factors that 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?
21.
What is plotted on the x-axis of a survivorship curve? On the y-axis (be clear as to the scale used
here)?
22.
Sketch the three different types of survivorship curves, and provide an example for each. Know
which type is which!
23.
Explain the differences in survivorship among the three curve types (I, II, and III) for each life
stage (i.e. dependency, reproductive, post-reproductive)
24.
Does a given species always show the same characteristic pattern of survivorship? Use an
example to explain your answer.
25.
Compare a mortality curve to a survivorship curve (i.e. what does high mortality look like on a
survivorship curve compared to a mortality curve)? Note that Fig. 10.15 and 10.20 are plotted
from the same data!
26.
What is a fecundity curve, and what does it take into account?