Practice Chapter 16



Homework Questions for Lecture 16

ES 1000

Use these questions to test your knowledge of Lecture 16.

A. Short answer:

1. Every ___________________ on Earth contains at least one extensive dry region.

2. A desert is an area in which potential ___________ is four times greater than precipitation.

3. An aridity index of 2.0 indicates potential annual evaporation is ________ the average annual precipitation.

4. Warm subtropical deserts that result from the descent of dried air occur at ____ degrees of latitude.

5. Desert landscapes consist mostly of __________ __________, not sand dunes.

6. Desert climates are always ______.

7. Deserts have an aridity index greater than _____.

B. Match the Terms.

1. Sahara _____ a. channel, usually dry

2. Aridity index____ c. lake evaporates quickly

3. Playa ____ d. in lee of mountains

4. Arroyo ____ e. potential evaporation/annual rainfall

5. Yardang _____ f. removes sand- and silt-sized particles

6. deflation _____ g. type mechanical weathering

7. Sandstorm ______ h. driest place on Earth

8. Rain shadow desert_____ i. black roller

9. Thermal expansion____ j. isolated rock, result of wind abrasion

10. Sand Dune _____ b. cross bedding; well sorted, frosted sand grains

C. True or False? Circle the correct answer.

1. Deserts that result from their location on the leeward side of mountain ranges are called rain shadow deserts. True or False?

2. Continental interior deserts are, in part, a result of their great distance from sources of moisture-laden winds. True or False?

3. Areas near cold ocean currents are often deserts because of onshore winds that intensify evaporation. True or False?

4. Polar Regions are considered deserts because they lack appreciable precipitation. True or False?

5. Mechanical weathering processes in deserts include thermal expansion, salt crystal growth and abrasion. True or False?

6. Colorful hues in some desert rocks are a result of chemical weathering. True or False?

7. Arroyos are desert stream channels that are usually dry. True or False?

8. Desert streams erode soil easily because there is little vegetation to stabilize it. True or False?

9. Flash floods occur because water is not readily absorbed by desert soils. True or False?

D. Multiple choice:

1) Which of the following statements concerning playas is NOT true?

a) Playas frequently drain into arroyos.

b) The bed of a playa consists of precipitated salts and fine-grained clastic sediment.

c) Playa lakes may exist only a few days or weeks at one time.

d) Playa lakes are fed by precipitation and not by groundwater.

2) Wind erodes solid rock by what process?

(a) Dislodging cemented grains directly from sedimentary rock.

(b) Quarrying crystalline bedrock composed of fractured igneous rock.

(c) Etching exposed rock with loose sand grains.

(d) Pressure lifting from surface winds.

3) Which of the following statements about deflation is NOT true?

a) The rate at which deflation lowers a landscape is usually on the order of only a few meters per thousand years.

b) Blowouts are surface depressions that form where surface vegetation has been disturbed.

c) Deflation basins are large blowouts.

d) Desert pavement results when deflation removes sand- and silt-sized particles, leaving behind pebbles and cobbles.

4) The bed-load of wind-transported sediment is moved by:

a) the process of traction.

b) the process of saltation.

c) the process of suspension.

d) the process of surface creep.

5) In which of the following locations would a small dune be most likely to form?

a) Where a blowout alters the flow of the wind.

b) Where a barren flat area provides a stable surface for sand accumulation.

c) Where a bush interrupts the flow of saltating sand.

6) Which of the following statements concerning dune migration is NOT true?

a) Dunes migrate when sand saltates up the windward side and is deposited on the leeward side.

b) Coastal dunes may migrate seaward rather than inland.

c) Large dunes migrate on the order of a few meters per year.

d) Small dunes migrate at about the same rate as larger dunes.

7) Transverse dunes are:

a) parallel ridges oriented perpendicular to the prevailing wind direction.

b) crescent-shaped dunes with horns in the upwind direction.

c) parallel ridges oriented parallel to the prevailing wind direction.

d) complex dunes that form when wind blows from three or more principal directions.

E. Short Answers. Your instructor did part 'f ' for you.

Discuss dry climates in general. Be sure to include:

a. What are the characteristics that define a desert?

b. Why does air rise near the equator?

c. Why is air dry when it falls at about 30 degrees of latitude?

d. What is this type of desert called?

e. Why do the deep interiors of continents have deserts?

f. Why do deserts form in the lee of mountains? (I'll do this one, parts c and g are similar: rising air expands, cools, water condenses to liquid droplets, it falls as rain, leaving the rising air very dry. As it falls on the lee side, it contracts and warms. Warm air can hold a lot of water, but this air is already dry. It removes any water in the soil on the lee of the mountain.)

g. Why do coastal deserts form inland from very cold currents? (Hint: cold air can’t hold much water, but if you warm it over the land, it can hold a lot).

F. Calculations related to Interval Data. Re-read slide 11 of Lecture 13 Observations

If we use means, data from any distribution quickly takes on a bell-shaped normal distribution. The statistical measures you used depend on this distribution; for example, the t statistic, where t = X1-X2 / SE x1-x2 was used for comparing mean potato beetle weights from two varieties of potato. A normal distribution is also a requirement for chi-square, which we used for tree soil moisture and for hurricane frequency.

Sometimes taking means doesn't make sense. For example, if you give the same test to two sections of students from the same course, and you want to see if the differences between sections are significant, you can't get meaningful results if each student takes that same test multiple times.

Counts can be especially troublesome if a result of zero is allowed. For example, before I began comparing the weights of Colorado Potato Beetle grubs, I counted them. The beetles were clumped: most plants had zero beetles, a few had as many as 47. This leads to a very asymmetrical distribution of the data.

Data that are not symmetrical are not normal. We can check if a sample data set is approximately normal by checking that the sample mean +/- the standard deviation contains about 70% of the observations. Today we’ll calculate the sample mean of interval (grouped) data.

Today's Example: Artificial Selection and the Survivorship of an Agricultural Pest.

One of the most interesting gene mutations ever found in the laboratory is Methuselah (mth), a variant in the gene that controls lifespan in the common fruit fly, Drosophila melanogaster, an introduced pest of fruit crops. Methuselah flies live much longer than wild flies, and mth flies are resistant to paraqaut, a chemical that generates free radicals and kills ordinary fruit flies. You can read about it here:

Lin, Y. J, Seroude, L., & Benzer, S. (1998). Extended life-span and stress resistance in

the Drosophila mutant methuselah. Science, 282, 943–946.



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You decide to try to find your own strain of long lived-flies. You set up a vial of food with one male and one female wild-type fly (+/+), strain Oregon R. All experimental vials are to be kept in identical conditions. After the flies mate and eggs are laid you remove the adults and begin counting as the new adults begin to emerge. Every day you transfer all adults to new vials with fresh food, one male and one female per vial. You also record the number of adults that die in each 3 day period. The progeny from the last surviving pairs are kept for the next generation.* Here are your results for the first generation:

[Here a month is exactly 30 days.] No flies died the first three days, 2 died from days 4-6, 27 from days 7-9, 65 died from days 10-12, 73 died from days 13-15, 44 from days 16-18, 27 from days 19-21, 15 from days 22-24, 17 from days 25-27, 5 from days 28-30. Then 7 died in days 31-33, 2 in days 34-36, 3 in days 37-39, 0 in days 40-42, 0 in days 43-45, 0 in days, 46-48, and finally 3 in days 49-51.

Time

Interval days |0-3 |4-6 |7-9 |10-12 |13-15 |16-18 |19-21 |22-24 |25-27 |28-30 |31-33 |34-36 |37-39 |40-42 |43-45 |46-48 |49-51 | |

Midpoint days |2 |5 |8 |11 |14 |17 |20 |23 |26 |29 |32 |35 |38 |41 |44 |47 |50 | |frequency |0 |2 |27 |65 |73 |44 |27 |15 |17 |5 |7 |2 |3 |0 |0 |0 |3 | |Time Midpoint x frequency |2 x 0 = 0 |5 x 2 = 10 |8 x 27 = 216 |11 x 65 = 715 |14 x 73 = 1022 |17 x44 = 748 | | | | | | | |0 |0 |0 | | |

1. Calculate the mean lifespan. For example, the day 4-6 interval has midpoint 5 with frequency 2, 5x2 =10. The day 7-9 interval has midpoint 8 with frequency 27, 8 x 27 = 216

I did a few. You do the rest. Then total them up and divide by the number of observations.

The number of observations, N, is 290, the total number of flies.

The mean lifespan is Σ midpoint x frequency = _______ / 290 = _______ days.

N

* If you continued this experiment to future generations, you would expect the mean lifespan to increase, because you are selecting the long-lived flies for breeding.

By the way, to compare asymmetrical data with other data sets, if means are not appropriate, we could also transform it to make it symmetrical, to approximate a normal distribution, and use the transformed numbers as our data. In the figures below, the data on the left were made more symmetrical by taking logs of the time, given in months.

Why would we bother to study flies? Because the basic genes that run all higher organisms, including us, are present in simple organisms. Drosophila are easy and cheap to breed, and they have very short generations, so we can do many experiments in a short time.

We know a great deal about how to locate and mark the genes in Drosophila. We also know how to turn the genes on and off. We can characterize the enzymes and polypeptides the genes code for. In this way we can learn how humans work, including our process of aging, and possibly slow it down.

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