Download Workshop on Population Ecology I. Pre

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 53 of Campbell (9th edition) prior to the workshop meeting.
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 controlled 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. Age Pyramids
1. Using age pyramids (see p. 1189 of Campbell, 9th edition (Figure 53.24)), sketch and label
pyramids for a population that is
a. growing rapidly
b. growing slowly
c. not growing/decreasing
2. Select one of the above pyramids. Project what would happen if a famine or economic prosperity
occurred. Sketch the pyramid.
For any of the pyramids: famine takes the highest toll on the elderly and the very young. The
bottom and top of the pyramid should narrow. Economic prosperity would likely increase
resources available for reproduction, so the bottom of the pyramid would widen. Prosperity also
means more resources and higher survival at all levels, so the upper levels of the pyramid should
also widen.
Have students sketch the results for all three and discuss.
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. Sketch the relationships expected in the following:
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.
Recall the story of the guppies and their varying size and/or time to maturation in the presence
of different predators.
Discuss predation at each of the three events in the life history (birth, reproduction, death),
and its effect on the individual's fitness and the population's structure. Remember: once you’ve
had successful reproduction, you’re already in the game as far as natural selection is concerned.
(Does predation on the elderly result in natural selection?)
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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.
Discuss "situation dependence." In some cases, the environment determines whether having
mycorrhizae is a benefit or not.
• Without mycorrhizae: fewer, bigger plants
• With mycorrhizae: more, smaller plants
•
The gist of this is that with mycorrhizae, more plants germinate and survive. This may mean that
mycorrhizae allow some sunflowers that would not have been viable without mycorrhizae to have
a better chance of germinating and surviving. This will result in greater competition later, since
there are more plants, albeit smaller (since they have to compete for limited resources, a densitydependent factor).
In the pots without mycorrhizae, only the most robust sunflowers germinated and survived. In
a mycorrhizae-free environment, the genetically robust individuals would have a selective
advantage. With less competition, these individuals grow to a greater size.
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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 in Campbell).
See Chapter 53, page 1189 of Campbell)
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.)