Bio 150: Ecology
Study Questions WEEK 6
LIFE HISTORY EVOLUTION
1. Name three life history traits. Name three important life history trade-offs.
2. 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.
3. The evolution of clutch size in birds played a central role in the development of
modern life history theory ideas about adaptive clutch size (number of eggs a bird lays in
its nest). Ecologist David Lack proposed there is an optimal clutch size that will
maximize mothers’ fitness (reproductive success). Propose an experiment you could
perform to test this idea. Describe or show graphically what result would support this
hypothesis, and what would reject it. If your experiment rejected this 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?
4. 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.
5. 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. (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).
6. 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.
7. 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
K-selected 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.
8. 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.
INTERSPECIFIC INTERACTIONS
9. 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
10. 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.
(viii) The two general outcomes are stable coexistence or competitive exclusion.
11. 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. α 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:
α = 1,
β > 1,
α < 1, β = 0,
What does it mean if α > β?
12. 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.
13. 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.
K1/"
Species 1
Species 2
N 2 K2
K1
K2/!
N1
14. Repeat the above exercise with the following graph:
15. Gause said that complete competitors cannot coexist. For complete competitors,
intraspecific and interspecific competition are equal (i.e. α = 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. α, 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 α = β = 1.
16. In class, we modeled competition where both species have a negative impact on
the other (i.e. both α 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 (α > 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).
17. 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?
18. 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?
19. Distinguish between ‘interference competition’ and ‘exploitative competition’ and
give a biological example of each. (This was covered earlier in intraspecific
competition and density-dependence).