Workshop on Population ecology
by Blase Maffia and Mike Gaines
I. Pre-workshop
A. Introduction.
Ecology is the study of the relationship between organisms and their environment. Population
ecology is the study of the changes in growth and composition of single groups of species
(populations). This workshop will help you gain an understanding of the numerous factors that
may bring about population changes. For example, species vary in the timing of their growth,
reproductive age, and death. Species compete with other species for resources, species are eaten,
and species are limited by the conditions found in their environment. These factors can influence
the distribution and abundance of a species.
B.
1.
2.
3.
Background knowledge.
Be able to graph an example of arithmetic (linear) and exponential (J-shaped) growth.
Students should have read Chapter 52 of Campbell et al., 5th ed. prior to the workshop.
Review/define the following terms:
a. population - members of the same species living in a defined geographic area
b. density - number of individuals per unit area (or volume)
c. dispersion - spatial pattern of distribution of individuals in the population
d. fecundity - power of a species to multiply rapidly
e. cohort - members of a population that are the same age, usually in years or
generations
f. intrinsic rate of increase (r) - measure of the (maximum) rate of increase of a
population under contrrolled conditions
g. age structure - pattern of age distribution in a population; the number of individuals
in each age class in a given population.
h. generation time - the time between the birth of a parent and the birth of its offspring
i. intraspecific competition - competition between members of the same species for
resources of any kind
j. exponential population growth - the steepest phase in a growth curve, that in which
the curve is described by an equation containing a mathematical exponent
k. logistic population growth - a model of population growth described by a symmetrical
S-shaped curve with an upper asymptote (representing K, carrying capacity).
l. carrying capacity (K) - the amount of animal or plant life that can be supported
indefinitely on available resources; the number of individuals that the resources of a
habitat can support
m. density dependent - having an influence on individuals that varies with the number
of individuals per unit area in the population.
n. density independent - having an influence on individuals that does not vary with the
number of individuals per unit area in the population.
o. life table - table presenting complete data on the mortality schedule of a population
p. survivorship curve - a plot of the number of individuals in a cohort that are still
alive at each age; used to represent age-specific mortality
q. K-selected species - species in which the chief determinant of life history is a low
reproductive rate with a high rate of offspring survival
r. r-selected species - species in which the chief determinant of life history is a high
reproductive rate
C. Benchmarks.
The objectives of this workshop are to enable the student to
1. understand the dynamic nature of population growth
2. graph and interpret age pyramid diagrams
3. understand what factors affect the growth and decline of populations
II. The workshop
A. Complete the following concept map
B. Age Pyramids
1. Using age pyramids (see p. 1103), sketch and label pyramids for a population that is
increasing:
decreasing:
stable:
2. Select one of the above pyramids. Project what would happen if a famine or economic
prosperity occurred. Sketch the pyramid.
3. Write the equation for exponential growth. Label and explain the terms.
dN/dt = rmaxN
dN = change in the population size
dt = change in time (i.e., time interval)
dN/dt = actual growth rate of the population over time
rmax = maximum intrinsic rate of increase of the population (births - deaths over time
interval)
N = population size
This essentially demonstrates the intrinsic growth rate of a population unchecked by
environmental resistance
4. Write the equation for logistic growth. Label and explain the terms.
dN/dt = rmaxN [K-N]
K
dN and dt = same as above
rmax = same as above
K = maximum population size that a particular environment can support with no net
increase or decrease over a relatively long period of time.
This demonstrates the growth rate of a population when checked by environmental
resistance.
5. Fecundity, mortality, age at first reproduction, clutch size, and parental investment are usually
interrelated. On the following graphs, sketch the relationship you would predict between the two
variables.
6. There are numerous ways in which a population biologists examines his/her population of
study organisms. Descriptions of population dynamics include numerous characteristics/
attributes. The next series of questions asks you to use terms to describe/ categorize
population(s).
a. One may see clumped dispersion, random dispersion, or uniform dispersion of populations.
Describe conditions which may result in these patterns.
random - no obvious pattern of distribution
no competition or other factors affecting closeness of conspecifics to each other
clumped - individuals occur in groups
most common pattern in nature, especially in animals; found in animals that live
in herds, or where environmental resources are concentrated, or where
environmental factors favor survival, germination, etc.
uniform - individuals are evenly spaced
results from direct interaction between individuals: aggression/territoriality in
animals, allelopathy (secretion of poisons into the soil) by plants
b. There are three types of survivorship curves (Type I, Type II, and Type III). Describe the
characteristics of populations which exhibit these curves. Name an animal that falls into one of
each of these categories.
Type I - High survival of juveniles and middle age classes; death rate steeply slopes at
higher age classes (e.g., Homo sapiens)
Type II - Likelihood of death is approximately the same in all age classes, and curve is
linear from birth to maximum age (e.g., many small "prey type" animals)
Type III: - Very high juvenile mortality, but stable survival rate once the critical juvenile
period has passed. (e.g., animals with free-swimming, free-living larvae; plants with very
large numbers of endosperm-poor seeds, spore plants, etc.)
c. Define density- dependence. Explain how such factors affect population growth.
A density dependent environmental factor is one that intensifies as population density
increases. For example, if a population grows so large that food becomes scarce, then
competition for that resource intensifies as the population continues to grow. Such factors
often determine K, the carrying capacity of the environment.
d. Describe how amount of rainfall and sunlight can function as density-independent factors in
controlling population growth.
These factors affect all individuals in the population, regardless of the number of
individuals in the population. If there is too little rainfall or sunlight, all individuals in the
population will suffer, even if the population is far below the environment's carrying
capacity.
e. List the three major characteristics of a life history and explain how each affects the:
1. birth
2. reproduction
3. death
i. Number of offspring produced by an individual - All other factors being equal, the
longer the time interval between birth and death, the higher the reproduction.
ii. Population's growth
This can lead to a discussion of life tables and population changes.
f. Distinguish between r-selected populations and K-selected populations.
An r-selected species produces large numbers of offspring, each of which has a
relatively high risk of death at a very early age.
A K-selected species produces relatively few offspring, but invests relatively more
energy in each offspring, thus reducing the likelihood of its death at an early age.
g. Explain how predation can affect life history through natural selection.
Discuss predation at each of the three events in the life history, and its effect on the
individual's fitness and the population's structure.
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Mycorrhizae are plant root-fungus symbioses. They can have a profound impact on the
growth and survival of individual plants and therefore plant populations. This can then lead to
influencing plant community composition and succession of an area. Look at the following graph
(modified from Maffia 1997). By final harvest, Sunflower plants grown at high density without
mycorrhizas are twice the size of those grown with mycorrhizas. The total number of Sunflower
plants grown without mycorrhizas are, however, only half that of the Sunflowers grown with
mycorrhizas. Based on these data, describe a scenario where it is advantageous to have
mycorrhizas; disadvantageous to have mycorrhizas. Speculate as to why Sunflowers continue to
associate with mycorrhizas.
Sunflower mean biomass at four harvests
Biomass (g)
control
0.8
mycorrhizas
0.6
0.4
0.2
0
0
1
2
3
4
Harvest number
Discuss "situation dependence." In some cases, the environment determines whether
having mycorrhizae is a benefit or not.
Without: fewer, bigger
With: more, smaller
The gist of this is that with mycorrhizae, more plants germinate and survive, possibly
meaning that the presence of mycorrhizae allows sunflowers to survive who would not have
been viable without mycorrhizae.
In the pots without mycorrhizae, only the most robust sunflowers have survived. In a
mycorrhizae-free environment, these would have a tremendous selective advantage, if this
survivorship is genetically based.
Why are they smaller with mycorrhizae? Now you get the density-dependent factor
rearing its head: with more sunflowers competing for the same resources, each one is
smaller.
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III. Post-workshop review and practice
1. Explain how age structure, generation time, and sex structure of populations can affect
population growth (hint: look over the age pyramid diagrams).
See Chapter 52, page 1103)
2. List several examples of how a "stressful" environment may alter a population's structure.
(Use the example of mycorrhizae, and then branch out to other examples.)