Midterm Exam 2 Notes

Since there are no answer keys posted for the exams, here is a space to create our own pool of key points for each question. Hopefully this will help in studying for the final.

1. a) Draw diagrams showing i) stabilizing selection, ii) directional selection, iii) disruptive selection
b) Provide example of each

a) (Image from Ricklefs)


b) Stabilizing: giraffe’s neck. Long necks are better for getting a mate, short necks are better for drinking water. Medium-sized necks have been selected for.

Directional: horse size. Environmental change – forests turned into open savannahs, and food changed from leafy plants to grasses. Molars grew larger, and horses grew taller.

Disruptive: finch beak size. Different beak sizes useful for different seed sizes. When environmental changes occur that affect different seed abundances, beak sizes that were previously ideal may no longer be viable.

2. Population growth with dN/dt = rN, r > 0.
a) Draw a graph
b) Name this growth pattern

Population growth with dN/dt = rN(1-N/K)
a) Why apply this equation rather than dN/dt = rN?
b) What is K?
c) Draw graph and label K
d) Name this growth pattern

dN/dt = rN
a) (Image from lecture slides)


b) Exponential growth

dN/dt = rN(1-N/K)
a) if limiting factor or maximum population limit exists
b) K = carrying capacity
c) (Image from lecture slides)


d) logistic growth

3. Starting cohort: 50 individuals. After 1 year, 30 remain and have 2 offspring each. After another year (year 2), 10 are alive and have 4 offspring each. None survive in 3rd year.
a) fill in table

Age (x) Survival (lx) Age-specific survival (sx) Fecundity (mx) or (bx) lxbx xlxbx
0 1.00 0.60 0 0.00 0.00
1 0.60 0.33 2 1.20 1.20
2 0.20 0.00 4 0.80 1.60
3 0.00 0.00 0 0.00 0.00
Ro (sum of lxbx) 2.00
Expected number of births weighted by age (sum of xlxbx) 2.80

**b) Calculate Ro (net reproductive rate of a single individual in her lifetime)

Ro is the sum of the product of the survival rates * birth (maternity) rates:
In this case, Ro is 2.00

c) Calculate generation time, T

Generation time, T is the sum of expected births weighted by age / sum of survival rates * birth rates:
In this case, T = 2.80 / 2.00 = 1.40

d) Calculate lambda**

Lambda = Ro1/T:
In this case, Lambda = 2.00(1/1.4) = 1.641

4. a) Draw a resource utilization spectrum for 2 competing species (A & B) that partially overlap in resource use. What can we conclude from this?
b) Draw resource utilization spectra for species A and B when they occur alone in the absence of the other, so that this information will help interpret the role of competition in part a.



From this we can conclude either
i) competition is insignificant: A and B specialize in resource use with different preferences
ii) competition is significant and has led to niche partitioning: perhaps one species outcompetes the other and displacement has occurred



-Species A has same resource utilization when alone and when with B.
-Species B has much wider spectrum of use when alone.
-When occurring together, A is superior competitor and outcompetes Bin their shared resource preferences
-A and B coexist because B has wider tolerance and lives in conditions that A cannot live in.

5. In lab experiments, predators typically extinguish prey and then starve to death, therefore making predator-prey systems unstable. Name and describe 3 mechanisms that allow predator and prey to coexist and persist in natural ecosystems.

1. Environmental variation - environmental conditions and habitat vary over space, so it is difficult that preditors would locate all potential prey.

2. Prey-switching - if a predator's preferred resource population becomes low, they may begin consuming a different prey population. This decreased pressure can allow the preferred prey population to grow again in numbers.

3. Prey defenses - prey species have evolved many mechanisms to avoid be consumed. These include simple escape or hiding, warning coloration (aposomatic coloration), mimicry (palatable species have come to resemble unpalatable species that predators know to avoid), chemical defense (release of chemicals to deter predators or maintenance of chemical compounds that deter them), noise-making, and others.

6. Explain “trophic cascade” with a 3-trophic level example. What changes might occur if a top predator is added?

Trophic cascade: top-down or bottom-up control of trophic levels (the idea that a change in the number of individuals at one trophic level can indirectly change numbers in a trophic level beyond that which they directly affect.)
3-trophic level example: fish eat zooplankton, which eat phytoplankton. If fish population increases, predation on zooplankton increases, zooplankton population decreases, predation on phytoplankton decreases, phytoplankton population increases.
4-trophic level examples: add bear population. Bear decreases fish population, which reduces predation on zooplankton and increases zooplankton population, which in turn increases predation on phytoplankton and decreases phytoplankton population

7. Describe the vertical profile of temperature and oxygen in a dimictic, eutrophic lake over 4 seasons. Include graphs of vertical profile of temperature and oxygen in the summer. Explain dimictic and eutrophic in terms of generating the oxygen profile.

8. Explain the consequences of straightening a meandering river with Lane’s law.

Qs * D50 = Qw* S
Qs = sediment flow
D50 = 50th percentile sediment diameter
Qw = water flow
S = stream slope

With straightening,
-stream slope gets steeper, water flow remains the same
-throws stream out of equilibrium
-stream gains power and degrades stream bed
-balance the equation with the left side – increase sediment load
-disrupt aquatic ecosystems and habitats

9. Compare hydrologic pathways using the terms infiltration, evaporation, transpiration, and runoff in Watershed A (recent deforestation) and B (suburban development with houses and roads) relative to the reference watershed (mostly forested, high water absorption in soil, stream).

Watershed A relative to reference:
-less transpiration (less vegetation)
-more runoff - more peak runoff causing flashier flows
-less evaporation (less water available on site due to runoff)
-less infiltration (less vegetation requiring water in soil, faster saturation)

Watershed B relative to reference:
-less infiltration, more runoff (impervious surfaces) - more peak runoff causing flashier flows
-some transpiration, but much less (trees and plants in yards)
-less evaporation (less water available on site due to runoff)

Unless otherwise stated, the content of this page is licensed under Creative Commons Attribution-ShareAlike 3.0 License