Download BIO 150

WEEKLY STUDY GUIDE FOR BIO 107
The following questions provide a study guide for what I want you to get out of the lectures.
Ideally, it is worth reading over the questions before I give the lecture because it will make it
easier for you focus on the essential concepts and questions. The sum total of all of the questions
are pretty much what you need to get out of the course, and thus serve as a nice overall summary
of the course material.
TIPS: Some questions are longer than they would be for an exam (i.e., more detail) and are just
packaged together to allow you to see what main points I wanted you to obtain from the lectures
on the topic. Read your notes carefully and don’t just study from the study questions. There may
be a few topics in the notes that did not make it onto the study questions. I have tried to make
these as comprehensive as possible, but reserve the right to add things the may not have made it
from the notes. Also, aspects of different topics may be put together in a way that asks you to link
together different concepts. To help prepare you for this, some of the same material is repeated in
more than one question. Work away at the questions on a weekly basis and do not leave
everything until the last minute before exams. There is too much here to digest quickly and by
leaving things till the last minute, you will not be able to take full advantage of the feedback we
can provide during office hours.
WEEK 1
EVOLUTION & DIVERSITY
1. Natural selection is often thought of as primarily an evolutionary process but it is also an
ecological process. There are several basic components to natural selection. What are
they? Which one is the ecological aspect? How does the process produce a match
between organisms and their environments (i.e. why is it that organisms are so darn good
at solving the ecological problems they face?)
2. A study of natural selection in Darwin’s finches illustrates the methods field biologists
use to study natural selection in wild populations. Describe the key aspects of the study,
both in terms of methods used and results the authors found. Be sure to indicate how their
findings add up to a complete demonstration of natural selection.
3. The Blue-headed Wrasse is a reef fish that spawns millions of gametes into the ocean
where they drift at the whim of currents. The entire world population of the Island Scrub
Jay occurs on the Channel Island of Santa Cruz. Which of these species would be more
suitable for a study of natural selection, and why?
4. Does natural selection always favor the same phenotypes in a given population? Answer
the question with reference to the effect of drought versus El Niño years on the Medium
Ground Finch (Geospiza fortis), a Darwin’s finch.
5. Design your own hypothetical field study of natural selection in your favorite organism,
making sure your favorite organism has features amenable to studying natural selection.
6. What is an adaptive radiation? What are examples of adaptive radiations? What do
adaptive radiations inform us about ecological specialization?
7. Darwin’s finches on the Galapagos Island archipelago are a classic example of an
adaptive radiation. However, one of the Darwin’s finches, the Cocos Finch occurs on an
isolated island (Cocos Island) near Costa Rica. This species provides a striking contrast to
the adaptive radiation of the finches on the archipelago. How so? What does this contrast
tell us about (i) about the role of speciation in the evolution of adaptive radiations and (ii)
how ecological selection pressures are resolved with, and without, speciation?
8. The most spectacular adaptive radiations occur in lakes or on islands. Why might this be
so?
9. Biogeography is the study of the geographic distributions of organisms. You observe
three species of an organism that look similar in morphology (maybe they are closely
related, and maybe they are not), but are each distributed on a different southern
continent; one is found in Australia, one in South Africa, and one in South America.
Describe three hypotheses that could account for this observed distribution. What
information would be needed for you to distinguish between the three hypotheses?
10. What is the significance of Wallace's Line?
WEEK 2
CLIMATE AND BIOMES
11. Climatic variation sets the stage for ecological diversity. The fact that the earth rotates on
a tilt relative to the angle of its orbital plane around the sun has profound importance for
the environmental problems faced by organisms. What are some of these consequences?
12. What is the significance of the (i) Arctic circle, (ii) summer solstice (June 21) and (iii) the
spring and fall equinox (Mar 21, Sept 21)?
13. Why is it colder at the poles than the equator?
14. Biomes provide a striking example of a match between organisms and their environment
on a broad geographic level. Biomes differ in both biological aspects and in biotic
aspects. What is a biome? What two abiotic factors are good predictors of the type of
biome that occurs in a given location? Biomes are distinguished on the basis of what type
of organisms? Are biomes examples of i adaptive radiations or ii convergence? Explain.
15. Name five biomes and describe one important climatic and biological feature of each.
16. By comparing attributes specific to each biome, come up with an interesting ecological
question that applies to each of the biomes.
17. If a physiological ecologist wanted to study a physiological generalist organisms (e.g. an
organism that can tolerate a wide range of conditions, say temperature), as opposed to a
specialist, should they plan an expedition to the arctic or the tropics. Why?
18. No organism can occur under all conditions. Illustrate this point graphically with a
tolerance or performance curve, pointing out how performance changes along some
ecological gradient. Point out where a species reproduces, grows and survives.
P erformanc e
19. The two curves below show the performance of one species of plant along an
environmental gradient for individuals grown in a (i) natural setting with other plants
(solid line; shows ecological optimum) and in a (ii) controlled outside plot without other
species present (dashed line; shows physiological optimum). Why do they differ?
Environmental gradient (e.g. soil pH)
PHYSIOLOGICAL ECOLOGY AND BODY SIZE:
20. What is the Island Rule regarding changes in mammalian body size on islands? (explain
in words and demonstrate with a graph).
21. What is Bergmann’s rule? Although I did not give an explanation for the pattern, can you
think of a hypothesis that might account for the pattern and how you might test your
hypothesis?
22. According to Robert Ricklefs, "size changes everything" in ecology. Body size correlates
with phenomena on a variety of scales. Give examples of correlates of body size on the
following scales: physiological, individual, population, geographic.
23. Sexual size dimorphism provides useful variation for examining the adaptive significance
of body size. Describe some of the patterns of sexual size dimorphism and what they
illustrate about the factors that affect the evolution of body size. What factor might select
on males favoring large size? What factor might select on females to favor large size?
24. Allometry is the study of how factors scale with body size – it is about proportional
changes in traits with changes in body size.
a)Allometric relationships are presented as the logarithm of body size and logarithm of
the factor of interest? Why logs? Show why converting measures of traits and body
size to logarithm turns an exponential curve into a straight line.
b)Why is the slope of the equation, the parameter 'b', so critical? What, specifically, is the
relationship between a trait of interest (Y axis) and body size (X axis) when b = 1?
When b > 1? When b < 1? When b < 0? (Hint: think about proportional changes in Y
as X changes). Give a biological example of each.
25. Small and large animals live their lives at very different paces. On the 'mouse to elephant
curve', the metabolic rates of mammals shows some striking relationships with body size.
How does metabolic rate (whole organism) scale with body size. Show with both a loglog scale and an arithmetic scale how mass-specific metabolic rate scales with body
mass in mammals. What is the significance of this pattern for the challenges that animals
face? Illustrate with a comparison of the differences between shrews and whales in annual
life cycles.
26. Hummingbirds eat nectar while ostriches each low quality browse. Explain, with
reference to mass specific metabolic rate, why this makes sense. A know-it-all person
tells you that they read in a book about a 3 gram hummingbird that eats nothing but grass.
Show how you could use allometric patterns to debunk this so-called know-it all.
27. Whales are the heaviest animals on earth. The way that metabolic rate and endurance
scales to body size in mammals has profound implications for the annual foraging
strategy of large whales. Describe this annual life cycle and show how the allometry of (i)
metabolic rate (ii) fat stores and (iii) endurance interact to make this lifestyle possible for
a very large animal. In other words, show how endurance scales with body size.
28. Gulliver’s Travels is a wonderful story about a giant visitor to a land of tiny people
(Gulliver is 100 times larger than the local residents). Nice story, but it raises some
interesting allometric problems. If you were going to rewrite the story to make it
allometrically correct, name of couple of descriptions you would add to the story.
WEEK 3
BEHAVIORAL ECOLOGY
29. Direct studies of natural selection and the optimality approach are different ways of
studying adapatation. Discuss these two approaches and show how they differ. Is the goal
of optimality studies to show that organisms are optimal?
30. With reference to the simple question, 'How many eggs should a bird lay in its nest?'
illustrate how the optimality approach is used to generate hypotheses and predictions that
can be tested by experiment.
31. A simple graphical model of optimal foraging can be used to predict the number of
worms a bird should collect in a foraging patch and carry back to its nest, given that it
seeks to maximize the rate of energy return to the nest. Draw this graphical model and
show how it predicts the optimal load size (worms/trip) for a bird. How does increasing
the distance between the nest and the patch change the prediction, all else being equal?
What kind of experiment could test this model? Kacelnik did a very elegant test of this
model, and found striking concordance between predicted and observed load sizes
(worms/trip). Do his results show that starlings are optimal? What, in general do
optimality models tell us about the world?
32. One of the most useful aspects of optimal foraging models is that they can be used to
explore what 'currency' is important to an animal making some foraging decision (e.g.
how many worms to carry, how big a territory to defend). The study of starlings by Alex
Kacelnik is a good illustration of this. Very briefly show why.
33. Risk-sensitive foraging studies show that animals sometimes 'gamble'. What is meant by
'risk-sensitive foraging'? Describe an experiment that would demonstrate whether animals
are ‘risk sensitive’ foragers (you can discuss the case history illustrated in class with
shrews or make up your own study). Whether small animals with high metabolic rates
avoid or preferred high risk rewards depends on their physiological state. Discuss this
pattern and why it makes sense. Are whales likely to show this sort of response? Why or
why not?
POPULATIONS
34. When populations are not restrained by competition for scarce resources like food or space,
they have the potential to increase geometrically (or exponentially) over time. Why do
growing populations show this pattern of growth rather than increasing linearly
(arithmetically)? Show why with reference to a population growth model that is based on
discrete growth (i.e. the  model). Illustrate with a specific numerical example where  = 2
(i.e. show the change in population size over a few time units, if the initial population size is
1).
35. If for a species, r = .1 per minute, what is r per hour?
36. What is the difference between ‘r’ and ‘’? Will populations with the following growth
parameters be stable, increasing or decreasing in size?
r = 1  = 0  = .5 r = 0  = 1
37. If  is the multiplier for a population with discrete growth, what is the general equation
for determining N at time t if the initial population size at time = 0 is No? What is the
equivalent equation for a population with continuous growth?
38. If one had a series of population estimates sampled over time, what simple graphical test
would allow you to see if (i) the population were undergoing geometric (i.e. exponential)
population increase and if it were, what that rate would be (r)?
39. Draw the corresponding population lines/curves on the graph on the right that correspond
to the graphs on the left.
40. What is the critical difference between stochastic population models and deterministic
population models?
41. What is the difference between demographic stochasticity and environmental
stochasticity? Which one is influenced by population size?
42. With a population that undergoes environmental stochasticity (e.g. good years versus bad
years), the population can go extinct even though the arithmetic mean value of  = 1.
Why?
WEEK 4
INTRASPECIFIC COMPETITION
43. Name two types of competition. How do they differ?
44. What is meant by 'density-dependence' and how can it help explain why populations do
not continue to grow forever? Draw a graph showing what density-dependence would
look like for density versus birth rate and density versus mortality rate. Combine these to
show how density-dependence can 'regulate' a population so that it will be stable at K, it’s
carrying capacity. (NOTE: think clearly about what a graph of density-dependence looks
like because some forms of the logistic model do not directly illustrate densitydependence).
45. What are some of the biological mechanisms that give rise to density-dependence?
46. Outline a general experiment you could perform to test for density-dependent effects on
some reproductive or survival parameter in your favorite organism.
47. What is the Allee effect? Why should conservation biologists be concerned with this form
of density-dependence?
48. Illustrate with a graph what is meant by under-compensating density-dependence and
over-compensating density-dependence. Why is this distinction important with respect to
population dynamics (i.e., whether the relation between N and time is stable (smooth) or
shows interesting and unstable dynamics)?
49. The logistic model is based on a very simple assumption about how per capita growth rate
(1/N dN/dt) changes with increases in density up to K? Illustrate this assumption with a
graph and show how one can convert this assumption into the equation for 1/N dN/dt that
is used in the logistic. What would a plot of N (y axis) versus time (x axis) look like?
50. In the logistic model of population growth, what is the significance of carrying capacity
(K)? What happens when N = K? When N < K? When N > K?
51. The continuous logistic and discrete logistic model result in very different population
dynamics. What is the difference and why specifically do these two models produce such
different outcomes?
52. Draw a graph showing population size (N) (y axis) versus time (x axis) for each of the
following population dynamics:
(a)
(b)
(c)
(d)
Chaos
Exponential growth
Logistic population growth
Stable limit cycles
53. What is the difference between chaotic population dynamics (chaos) and stochastic
population dynamics? Can you tell simply by looking at the pattern of population
dynamics? What is the signature of chaos? Draw it.
54. Various aspects of the social behavior (or antisocial behavior) of American coots reflect
the importance of density-dependent effects on reproductive success or survival. Describe
two such aspects and show specifically how the density-dependence is at work.
DISPERSAL AND POPULATION STRUCTURE
55. Dispersal is an important part of the life cycle of many organisms. What are the three
main hypothesized evolutionary benefits of dispersal?
56. Describe three mechanisms that organisms adopt to achieve dispersal (we discussed a big
list for plants and animals in class).
57. One possible benefit that favors the evolution of dispersal is unpredictability in habitat.
Provide evidence for this idea with specific reference to dispersal polymorphisms found
in some species, relating the degree to which habitats are short-lived versus permanent to
the frequency of dispersers and non-dispersers in populations. What evidence, based on
comparisons of disperser and non-disperser morphs, indicates that there is a cost to
dispersal?
58. The costs of inbreeding may favor the evolution of dispersal. What experiment and result
from a study of plants could you use to demonstrate this point, specifically using dispersal
distance as a proxy for inbreeding? Describe both the experiment and the specific result
that would demonstrate inbreeding depression. Be sure to identify any critical
assumptions that you make. What result would show an optimal dispersal distance? You
can use a graph to illustrate your answer but be sure you also fully explain in words all
important details.
59. In some mammals, male offspring disperse, while in others it is the females that disperse.
A comparative study showed that the gender of offspring that disperses makes sense in
terms of the mating system (tenure of male mating), and how the mating system affects
the risk of inbreeding. Describe the observed pattern and explain how it makes sense if
the evolution of dispersal has been shaped by the risk of inbreeding .
60. The Ideal Free Distribution is a theoretical model that predicts how organisms should
distribute themselves among habitats of different qualities, given that they are free to
settle where the choose and that that they choose habitat so as to maximize their success
(fitness). (a) Illustrate the logic of this model with a graph that shows the sequential
settlement of your favorite beastie in of two different habitats, good and bad. Label all
axes and lines, and explain the graph in words. What predictions does the model make?
What are the key assumptions? How could you experimentally test the model? (Outline
an experiment and specific result that would support the model).
61. What is meant by source and sink populations? What critical data would one require to
demonstrate source versus sink populations? The ideal free distribution suggests that
organisms should distribute themselves among habitats of different qualities so that
fitness (and therefore population growth) in different habitats should be equal. Why, then,
do source and sink habitats exist at all (i.e. what assumption of the ideal free is not met
where sources and sinks exist)?
62. The spatial distributions shown below represent:
(a)________random dispersion
(b)________uniform or spaced dispersion
(c)________clumped dispersion
B
C
A
Describe an ecological mechanism that could produce patterns A and B.
WEEK 5
LIFE TABLES
63. You have just completed your study of a cohort of your favorite organism,
the____________________ (give the beast a kinky name). You have compiled all of your
data and found that of the original 100 newborns you followed, your summaries show that
80 survived to year 1 where they had (on average) .5 babies each, 50 survived to year 2,
where they had 1 baby each, and none survived to year 3. Build a life table with these data
and show whether the population is stable, decreasing or increasing.
64. The following survival and birth data were obtained for an original cohort of 1000
individuals of a rare mouse, the Squishy Furball. Fill in the table and show whether this
population is declining, stable or increasing. Explain in words why the value of the
variable you have calculated, Ro, tells us if the population is growing, stable or
decreasing. That is, what exactly does Ro represent anyway, what do the terms lx and lxbx
mean and why does their summed value, Ro, tell us whether the population is changing or
not across generations? Finally, how does Ro differ from r or λ ?
Age
X
Number
alive
nx
0
1
2
1000
600
250
Babies/
female
bx
0
1
2
lx
lxbx
3
4
50
0
0
0
65. When generation time, G, is estimated, how can one convert the population growth rate
per generation, Ro, to the intrinsic rate of population change, r?
66. Data for survivorship curves can be extracted from life tables. When the logarithm of nx
is plotted as a function of age (time), ecologists note three basic types of survivorship
curves: I, II, and III. Draw the three types of survivorship curves, pointing out what each
curve means in terms of early versus late survivorship. Provide a real example of each.
67. You assemble life tables for two species, A and B. When you calculate Ro based on the
life table you find that both species have the same Ro. However, the populations are not
growing at the same rate. Explain with reference to generation time, G, why we see this
difference. What is generation time? Which population will have the faster growth rate
and why? (I have worked things out so that G will be a whole number –you shouldn’t
need a calculator).
Species A.
X
nx
bx
0
1000
0
1
500
3
2
250
0
X
nx
bx
0
1000
0
1
500
0
2
250
6
Species B.
68. An interesting property of a population that maintains the same life table parameters
across time (i.e. the same life table holds for cohorts born in different years) is that the
population will quickly achieve a stable age distribution. On the graph below, draw an
example of a population growing with a stable age distribution by showing how numbers
of each age class (N0 = newborns, N1 = 1 year olds, ...) will change through time when
plotted on a log scale. Then, in words, describe what this means in terms of (i) the slopes
of the lines you have drawn and (ii) in terms of what happens to the proportion of
individuals in each age class across time.
69. What type of survivorship curve (type I, II, or III) does the species with the following life
table have? Explain how you came to your conclusion.
X
0
1
2
3
nx
1000
100
10
1
WEEK 6
LIFE HISTORY EVOLUTION
70. Name three life history traits. Name three important life history trade-offs.
71. A simple model of the population growth rate (λ) of an annual versus perennial strategy
provides a clear illustration of one approach to studying life history evolution. This
approach assumes that natural selection favors the strategy that maximizes fitness
(measured by the λ of the strategy) in the face of extrinsic mortality (and trade-offs,
though we ignored them in this simplest case). The parameters of the model for the
annual and perennial strategies are:
For the annual:
• SO is the proportion of babies that survive to year 1 to breed
• BA babies are produced per adult
• because it is annual, the adult dies after breeding
For the perennial:
• SO is the proportion of babies that survive to year 1 to breed
• BP babies are produced per adult
• SP is the proportion of adults that survive each year
With this information:
a) Show the equations for λ for the annual and perennial strategies.
b) Use these two equations to show the conditions under which being an annual is a
better life history strategy than being a perennial (i.e., λ annual > λ perennial).
c) Rearrange the equations to show that the life history trade-offs between annual
and perennial reproduction entails two things: (i) the differences between the two
strategies in the number of babies produced and (ii) the survival of adults relative
to babies.
72. The evolution of clutch size in birds played a central role in the development of modern
life history theory, largely due David Lack’s ideas about adaptive clutch size (number of
eggs a bird lays in its nest). What was Lack’s idea? Propose an experiment you could
perform to test this idea. Describe or show graphically what result would support Lack’s
hypothesis, and what would reject it. If your experiment rejected Lack’s hypothesis, as
many experiments have done, two important life history trade-offs could be at play. What
are these important trade-offs? What additional data would you need to collect from your
proposed experiment (i.e., new factors to measure that Lack ignored) to test for each of
these trade-offs?
73. The two life tables below are for two different species. Based on life history theory —
specifically the evolutionary explanation for aging — explain which species will have an
earlier onset of aging (senescence). Explain specifically why it is expected to have an
earlier onset of aging. Hint: you don’t need to do any calculations, you can figure it out
just looking at the table.
SPECIES A
Ag
e
Num
ber
alive
X
nx
Babie
s/
femal
e
bx
0
1
2
3
4
1000
500
250
100
0
0
0
2
5
0
SPECIES B
Ag
e
Babies/
female
X
Num
ber
alive
nx
0
1
2
3
4
1000
400
100
50
0
0
2
1
2
0
bx
74. Timing of maturity (i.e. age at first reproduction) is an important life history variable that
varies enormously among species. According to life history theory, adaptive timing of
maturity entails a trade-off between beginning reproduction too early and too late —
waiting to begin breeding thus has costs and benefits. (i) Name one general cost (or risk:
think life table) and one benefit to delaying maturity. (ii) What comparative pattern
among species suggests that differences in either the cost or benefit affect the evolution of
optimal timing of maturity? (iii) What experiment could one conduct to show that natural
selection can change the timing of maturity (refer to the guppy paper or design your own
experiment).
75. Evolutionary biology tells us that there is no fountain of youth. Evolutionary explanations
for senescence (aging) focus on patterns of ‘extrinsic’ mortality and the likelihood that
any individual will live to an old age. (‘extrinsic’ is mortality due to external factors, not
due trade-offs intrinsic to the organism). Explain the GENERAL evolutionary hypothesis
for aging in the context of life history theory, specifically with reference to survivorship.
Then provide two more specific explanations in terms of (i) accumulation of mutations
that act late in life as opposed to early in life and (ii) trade-offs between genes that are
expressed early and late in life. Two lines of evidence, one experimental and one
observational, support the idea that the onset of aging is shaped by natural selection.
Briefly describe this support and explain why it indicates that aging is shaped by natural
selection.
76. The theory of 'r-and-K selection' is a habitat-based view of life history, where ‘habitat’
refers to the ecological factors that affect whether or not species are at their carrying
capacity. Name some life history traits associated with an r-selected species and with a Kselected species. What is the basic difference between an r-selected and K-selected life
history in terms of causal mechanism (hint: think about the two types of population
models for unlimited growth versus density-dependent growth). One of the problems with
r-and-K selection is that the assumed causal mechanisms are never demonstrated.
Describe an experiment that would allow you to test for the proposed causal mechanism
in a K-selected species.
77. You observe two species with quite different life history patterns. Species A lives a short
time, has early maturity and has many small babies. Species B lives considerably longer,
has later maturation and has far fewer babies. Use these two species to contrast the
difference between the old r-and-K selection approach to life history evolution and the
more recent optimal demography approach. Thus, explain in general terms how each of
the two approaches would account for the differences between Species A and B.
WEEK 7
INTERSPECIFIC INTERACTIONS
78. The nature and consequences of interspecific interactions is a central focus of ecology.
Name three types or categories of interspecific interactions between interacting pairs of
species; these can be distinguished on the basis of how each species affects the other (0,
+, -). Discuss one ecological consequence (e.g. population process) and one evolutionary
consequence (e.g. adaptive response) that would be expected from each type of
interaction.
COMPETITION
79. Explain fully, in words, the basis of the Lotka-Volterra competition models (i.e., the basic
recipe as outlined in class). Point out what simple population model serves as the starting
point and what modifications are added to examine the population consequences of two
competing species. How does one go from the models to isoclines and how do these
isoclines tell us about the outcome of competition? What possible GENERAL outcomes
are predicted by this model?
NOTE: since I provided lots of information in that lecture, I will provide you with the
answer below, but try the question on your own first.
ANSWER
(i) Two species, each have their own logistic population model dN/dt = rN(1-N/K).
(ii) The two species compete (i.e. consume shared resources; hog part of each
other’s K).
(iii) Competition coefficients indicate the impact of the species on each other and
allow us to convert numbers of one species to numbers of the other.
(iv) Therefore the carrying capacity (K) of each species is reached by mixture of
individuals of both species; the isocline for a species is the equation (and line
on a graph) that represents all possible combinations of densities of the two
species for which the population of the focal species is stable (i.e. dN/dt = 0).
(v) We plot both isoclines on state space graph (population size of species 1 (N1)
on the X axis, of species 2 (N2) on the Y axis.
(vi) We visually inspect the regions of the graph and look at the joint movement of
both populations in each area to assess the outcome of competition.
(vii) The two general outcomes are stable coexistence or competitive exclusion.
80. The magnitude of the competition coefficients used in the Lotka-Volterra competition
models tell us about the relative strength of two types of competition: intraspecific and
interspecific competition. ɑ is the effect of species 2 on species 1, β is the effect of
species 1 on species 2. Describe the relative strength of intraspecific versus interspecific
competition when:
ɑ=1
β>1
ɑ<1
β=0
What does it mean if ɑ > β?
81. What are three possible outcomes of competition in the real world (2 ecological, 1
evolutionary). What patterns would each outcome be expected to produce if you were
looking for evidence. For the ecological outcomes, describe a general experiment and
predicted results that would support each outcome.
82. Below is a typical Lotka-Volterra interspecific competition model shown graphically as
the isoclines of two species plotted as a function of the density of species 1 (N1) and the
density of species 2 (N2). Show with the use of arrows and vectors the joint movement of
the both populations for each region of the graph (each region is defined with reference to
the isoclines). Then describe, with reference to the isoclines, why the joint movement in
each region is the way it is (why the arrows point the way they do). State what the
outcome of competition in this system (name the outcome and also show on the graph
where the population will eventually stabilize). Illustrate, by drawing lines from the
equilibrium point to each axis, the equilibrium population sizes of N1 and N2.
83. Repeat the above exercise with the following graph:
84. Gause said that complete competitors cannot coexist. For complete competitors,
intraspecific and interspecific competition are equal (i.e. ɑ = β = 1). Illustrate Gause’s
claim by showing the basic conditions that must be met for stable coexistence of two
species; i.e. the inequalities based on the parameters e. ɑ, β, K1 and K2). (Recall, you can
figure out these conditions by comparing, for each isocline, the values where they cross
the Y axis and the X axis on the graph in the case where stable coexistence is the
outcome; e.g. question 81). Then, show that the inequalities are impossible when ɑ = β =
1.
85. In class, we modeled competition where both species have a negative impact on the other
(i.e. both ɑand β > 0). However, experiments have shown that competition can be
asymmetrical, with one species affecting the other, but not vice versa. In this case, one of
the competition coefficients would be 0. What would the isoclines look like for a
situation where we have stable coexistence, where species 2 affects species 1 (ɑ > 0) but
where species 1 does not affect species 2 (β = 0)? Redraw the stable-coexistence case
(shown in question 4) to illustrate this new case. You can figure out the answer for this
question from first principles (i.e., work through the equations for the isoclines and see
what happens when (β = 0).
86. The Lotka-Volterra models tell us what happens when competing species are able to
reach equilibrium (both K’s are reached). However, there are reasons why equilibrium
might not be reached, so that the model does not apply. What are two of these reasons.
Can you provide an example of each type?
87. You conduct the standard removal experiment with two species of plants to assess the
importance of interspecific interactions. When you remove species A from plots, the
density of species B increases, relative to control plots with both species present.
Similarly, when you remove species B from plots, the density of species A increases,
relative to control plots. Although this type of evidence suggests the plants may be
competing, there is another explanation. What is it called and how does it work? How
could you distinguish between these two explanations?
88. Distinguish between ‘interference competition’ and ‘exploitative competition’ and give a
biological example of each. (This was covered earlier in intraspecific competition and
density-dependence).
Predation
89. What evidence is there that predators affect prey density?
90. Are predators always bad for potential prey populations? (By potential prey, I mean
things the predator could eat but does not necessarily eat. Hint: the seastar removal
experiment provides an answer)
91. What is an ‘isocline’? Draw and identify prey and predator isoclines on a phase diagram
of prey and predator population size for the basic Lotka-Volterra predation model (i.e.
NOT competition). Identify the joint equilibrium. Any nudge of the populations will
produce stable limit cycles. For such a situation, show using thin arrows for the change in
each species numbers on each side of their isocline. Use fat arrows to show the joint
movement of predator and prey numbers.
92. What is the most important basic conclusion from the simple models like Lotka-Volterra
and the one you examined in the lab with respect to the influence of predation on simple
predator-prey systems? Do laboratory studies support or refute this conclusion?
93. Distinguish between a functional response and a numerical response by predators.
94. Modeling has also taught us that the behavior of predators can stabilize population
dynamics, particularly with respect to the efficiency with which individuals capture
individual prey and how this changes with prey density. This relation between rate of prey
capture per predator and prey density is called the functional response. With graphs,
illustrate the three types of functional responses (I, II, and III; in predator-prey context).
Show the corresponding relationships when these are graphed in terms of proportion of
the prey population consumed, instead of the number of prey consumed per predator.
With these second graphs, explain with reference to density dependence why a type III
functional graph can stabilize prey populations (i.e. why predators can impose densitydependent mortality that brings the prey population to an equilibrium. Describe two
mechanisms that can produce a type III functional response.
95. Geographic variation in the pattern of vole population dynamics within Fennoscandia
provide support for the idea that simple one-predator one-prey systems are unstable and,
moreover, that prey switching behavior by predators can stabilize prey population
dynamics? Describe the patterns that lead to this conclusion.
96. On your trip to Alaska you discover a lemming that shows dramatic and regular
population fluctuations. Two different mechanisms could account for these fluctuations,
one involving interactions among species, one involving interactions within species.
Describe these explanations and what you could do to distinguish between them.
WEEK 8
PREDATION CONTINUED
97. There are many ways that animals can potentially reduce the risk of predation (as
demonstrated in the slide show). List four of these possible mechanisms. For one of
these mechanisms, outline an experiment you could conduct to test whether the
mechanism is actually operating in a species, and clearly identify the result of your
experiment that would support the hypothesized mechanism.
98. Toxic snakes like coral snakes are provide good examples of (i) aposomatic coloration,
(ii) Batesian mimicy and (iii) Mullerian mimicry. What do each of these terms refer to?
Illustrate these terms using coral snakes and their mimics. Describe an experiment you
could conduct to determine if coral snake aposomatic warning colors actually deter
predation. Describe an experiment you could conduct to determine how a bird predator
could know to avoid coral snake warning colors (i.e., is the knowledge of the warning
colors innate or it learned)?
99. Mullerian and Batesian mimicry have different sorts of dynamics in terms of whether we
expect a long-term stable signal to evolve or a less stable dynamics where selection favors
one species to move away from the other. Explain. Related to this, explain why
frequency-dependent selection (i.e. when fitness depends on the frequency of different
types) is particularly important in systems with Batesian mimicry.
DISEASE/PARASITISM
100.Ecological and evolutionary thinking has advanced our understanding of the spread and
population dynamics of infectious disease and parasites. (a) The spread of disease can be
examined with a simple model: Rp = SβL. What do each of the four terms represent?
Explain what is meant by the 'transmission threshold' and show how it is represented by
rearrangement of the model and with a graph? What are the implications for human
density and the persistence of diseases? How can this explain the observation that
diseases like measles show population cycles? What is 'herd immunity' and how is it
explained by the concept of transmission threshold?
101. As victims of infectious disease and parasites we often focus on the symptoms from our
own perspective (i.e. the bad consequences of the disease to us or other organisms).
However, many symptoms of disease make sense in terms of adaptations of the parasite
or disease to increase its rate of spread from host to host. Provide an example of this and
explain how the symptom might work to increase the fitness of the disease.
102. It was originally thought that diseases would always coevolve with their hosts to become
progressively more benign. However, recent evolutionary insights show that this is not
always true. Discuss the evolutionary theory of disease virulence and the evidence that
supports this theory. You should consider the two phases of the life cycle of the disease
and how the presence or absence of a ‘vector’ affects the trade-offs between the two
phases.
103. Parasitic cuckoos in Europe and the songbirds they parasitize provide convincing
evidence that co-evolution has shaped traits and behaviors seen in the cuckoos and their
hosts.
a) What is co-evolution?
b) Outline the hypothesized co-evolutionary process that has taken place between
cuckoos and their hosts. For each of the two participants — cuckoos versus hosts —
indicate specifically which trait or behavior has negatively impacted the other participant
and how this has caused natural selection for a response by the harmed party.
c) Outline the evidence for each of the proposed steps of the co-evolutionary process.
104. The observation that songbirds parasitized by European cuckoos will reject cuckoo eggs
but not cuckoo chicks has long puzzled biologists, because cuckoo chicks are so different
from host chicks. One potential explanation for this paradox comes from understanding
how birds in general are able to recognize eggs or offspring — parents learn to recognize
their chicks or eggs by learning (imprinting) the visual characteristics the first time they
nest. With this in mind, explain why natural selection would not favor a rejector host who
used the following learning rule: 'learn the characteristics of the chick(s) in my first nest
because they are likely to be mine, and then for the rest of my life, reject any chicks that
are different from those first chicks.'
HERBIVORY
105. Evidence that herbivory has negative fitness consequences for plants comes from the
ways that plants defend themselves against herbivores. What are three general defenses
plants can use to reduce the risk of herbivory?
106. The antagonistic coevolution between Heliconius butterflies and Passiflora vines
(passionfruit) has lead to some very interesting defenses that the plants use to reduce the
risk herbivory by the butterflies. What are these defenses and how do they work?
WEEK 9
MUTUALISMS
107. Describe three examples of mutualisms. For each example show how each participant
benefits.
108. Species cannot be described as purely mutualistic or purely harmful because the nature
of their interactions with other species depends on which species they are interacting
with. This point is illustrated by the biology of the honeyguide, particularly a comparison
of the breeding biology (what chicks do) and the foraging ecology (how the parents get
food). Explain.
109. At times, it can be difficult to distinguish predation (and other harmful interactions) from
mutualism. Show that this is so by describing predation on conifer seeds by birds that
cache the seeds for the future (i.e., smart birds like jays bury the seeds and return to find
them at a later date).
110. Many plants have clear mutualistic relationships with ants. Dan Janzen’s work with antacacia trees in Costa Rica shows this nicely. How do the ants benefit from their
association with the plants? What benefit do the plants gain from having an association
with the ants?
111. It is thought that mutualisms can evolve from interactions that start out as purely
parasitic relationships. What evidence from yucca moths and their relatives provides
support for this idea? Your answer should include a brief description of the biology of
yucca moths and their relatives.
METAPOPULATIONS
112. What is meant by metapopulation? Metapopulation theory predicts that under certain
conditions, an equilibrium can be reached where a stable fraction of patches (P) is
occupied. In theory, what two factors does P depend on?
113. By altering habitat and decreasing the number of patches available to a species with
metapopulation dynamics, humans increase the risk of extinction for the species. Why?
In terms of the simple metapopulation model, what critical thing happens that causes
global extinction (i.e., P falls to zero) and why?
114. What evidence would you need to collect from a field study to show that the population
dynamics of a species is described by the metapopulation concept. You can illustrate your
answer with reference to a study of the Granville Fratillary, an endangered butterfly.
115. Why should ecologists and conservation biologists care about metapopulations? Discuss
three implications of metapopulations to ecologists and conservation biologists.
SUCCESSION
116. Three different mechanisms of succession differ in whether species interactions affect
the outcome of succession, and whether all species can establish after a disturbance.
Outline the key differences between Facilitation, Tolerance, and Inhibition mechanisms.
117. Describe an example of succession that fits the Facilitation model.
118. Describe an example of succession that fits the Tolerance model, and explain why we
see a sequential change in the plants found in the community after a disturbance.
119. The graph below provides a specific mechanism for why we would see a change in the
plants species that occur in succession (e.g., Tolerance model). Explain, with reference to
the graph, why would see a change in plants throughout succession (with the assumption
that relative photosynthetic rate or growth rate also indicates relative competitive
differences between the plants at different light intensities). Also, earlier in the course, we
discussed whether competitive differences between species are fixed species level
properties, or depend on the environment. With respect to this question, what does this
graph show?
WEEK 10
FOOD WEBS
120. A critical question of ecology is whether populations are limited by what they eat or by
what eats them. Ecologists have found that the answer to this question depends on the
number of trophic levels in a food web. Show why this is so. What evidence from field
studies of sea otters provides elegant support for these ideas and why? Do these patterns
have any implications for conservation biology? How?
ISLAND BIOGEOGRAPHY& BIODIVERSITY
121. What is meant by a ‘species-area’ relationship (or species-area curve; if logged then it
becomes a straight line). Provide an example of an equation for a species area curve,
pointing out what each variable means. Then, using a graph, show how species-area
relationships for islands versus mainland areas demonstrate that both the size of the area
assessed and its insularity (island-ness or degree of isolation) affect the number of species
found in an area.
122. The equilibrium theory of island biogeography can explain why both island size and
isolation affects the number of species found on an island. Using graphs, show the basic
idea behind this model. Explain clearly in words the reason why extinction and
colonization rates show the relationships they do with number of species on an island (i.e.
why they show negative or positive slopes). Also explain fully why it is an equilibrium
model of diversity (i.e., what happens if the number of species is above or below the
equilibrium number of species. For an island that is at the equilibrium number of species
for a long period of time and we examined the specific species present, would our list of
species remain constant across visits? Why or why not?
123. What evidence is there in support of the theory of island biogeography? Include both
observational patterns (species area relationships etc.) and an experiment.
124. What is the major limitation of the theory of island biogeography in terms of why it
might not apply to the mainland or to large islands?
125. Island biogeography has been used to think about how we should design nature reserves,
because such reserves are often islands of pristine habitat in a human altered landscape.
This lead to the S.L.O.S.S. debate: for a given size of area to be preserved, is it better to
make a Single Large Or Several Small reserves? Discuss some of the factors that will
influence this decision.
126. In many taxa, species richness (i.e., the number of species in area) shows a striking
relationship with latitude. What is this pattern? Describe four hypotheses that can
potentially explain why species richness increases as one heads towards the tropics from
the poles. Explain specifically the logic of each hypothesis – i.e. specifically how does
hypothesized mechanism explain the change in species richness with latitude.
WHEW!