Quiz 2: Life on Land

Geology/Geography/Oceanography/Atmospheric Sciences

Daniel Janzen (1981a, 1981b) proposed that the seeds of the guanacaste tree were once dispersed by several species of large mammals that became extinct following the end of the Pleistocene about 10,000 years ago. There may have been other plant species with a similar relationship with large herbivorous mammals. How do you think the distributions of these plant species may have changed from the time of the extinctions of Pleistocene mammals until the introduction of other large herbivores such as horses? How might the introduction of horses about 500 years ago have affected the distribution of these species? How could you test your ideas?

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Daniel Jansen's study of the guanacaste tree led him to the conclusion that the dispersion of the seeds of the tree was carried out by the large herbivorous animals including ground sloths, camels and horses following the end of Pleistocene about 10000 years ago. However, these animals became extinct probably due to over hunting by humans. As a result, for thousands of years, the seeds produced by the guanacaste tree were not dispersed effectively in the absence of the dispersers. However, it continued to produce a large number of fruits. This led to the significant reduction in the number of Guanacaste trees in the region. There would have been numerous other plants which were dependant on these large herbivorous animals for the dispersal of their seeds. After the extinction of these animal species, these trees too would have suffered low dispersal leading to an adverse effect on the plant population size. Some of them may also have got extinct. However, many of these plant species survived, like the guanacaste tree. This fact suggests that some alternative mechanisms of seed dispersal may have existed or alternative dispersers may have done the job of seed dispersal for these trees. Alternative means of the seed dispersal could be through multiple different vectors such as rodents and the other small animals. Also, the plants would have had to depend more on abiotic factors such as wind and rain for seed dispersal. Some trees are known to have evolved special structures on their fruits. Some examples of such structures are long and curved outgrowths which can easily cling on to animal feet and body parts. Hence, they get dispersed with the moving animals.

Q 2

If you measured the heights of 100 seedlings randomly drawn from the hypothetical population, instead of the 11 measured in the example, would the sample mean be likely to be exactly 5.7 cm?

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Q 3

Draw a typical soil profile, indicating the principal layers, or horizons. Describe the characteristics of each layer.

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A soil profile is the vertical arrangement of different layers of the soil from top soil to bedrock. It is composed of four distinct horizons as shown and described below: O horizon : It is the topmost layer that is primarily composed of organic matter. The upper layer of O horizon contains fallen plant litter like leaves, flowers, fruits, twigs, and dead organisms. The lower layer of O horizon contains fragmented and partially decomposed organic matter. The decomposition is carried out by soil microbes, nematodes, mites, and other small burrowing animals. A horizon : It is composed of a mixture of the inorganic minerals and organic matter. It is rich in the organic material and biological activity. The burrowing animals like earthworms mix minerals and organic matter from the O horizon into the A horizon. Clay, iron, aluminum, silicates and soluble organic matter are slowly leached from A horizon. B horizon : It is mainly composed of deposits of leachates coming in from A horizon. The deposits may form distinct colors and banding patterns in this horizon. C horizon : It forms the deepest layer of the soil. It is composed of the weathered parent rock material. Sand, silt, and clay are produced by weathering of rocks due to action of water, frost and plant roots. It also contains the rock fragments. Beneath the C horizon is the bedrock.

Q 4

Would the mean height of a sample of 100 seedlings likely be closer to the true population mean than the mean of a sample of 11?

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Q 5

Describe global patterns of atmospheric heating and circulation. What mechanisms produce high precipitation in the tropics? What mechanisms produce high precipitation at temperate latitudes? What mechanisms produce low precipitation in the tropics?

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Q 6

Use what you know about atmospheric circulation and seasonal changes in the sun's orientation to earth to explain the highly seasonal rainfall in the tropical dry forest and tropical savanna biomes. (Hint: Why does the rainy season in these biomes come during the warmer months?)

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Q 7

We focused much of our discussion of biomes on their latitudinal distribution. The reasonably predictable relationship between latitude and temperature and precipitation provides a link between latitude and biomes. What other geographic variable might affect the distribution of temperature and precipitation and, therefore, of biomes?

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Q 8

You probably suggested altitude in response to question 5 because of its important influence on climate. Some of the earliest studies of the geographic distribution of vegetation suggested a direct correspondence between latitudinal and altitudinal variation in climate, and our discussion in this chapter stressed the similarities in climatic changes with altitude and latitude. Now, what are some major climatic differences between high altitude at midlatitudes and high altitude at high latitudes?

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Q 9

How is the physical environment on mountains at midlatitudes similar to that in tropical alpine zones? How do these environments differ?

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Q 10

English and other European languages have terms for four seasons: spring, summer, autumn, and winter. This vocabulary summarizes much of the annual climatic variation at midlatitudes in temperate regions. Are these four seasons useful for summarizing annual climatic changes across the rest of the globe? Look back at the climate diagrams presented in this chapter. How many seasons would you propose for each of these environments? What would you call these seasons?

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Q 11

Biologists have observed much more similarity in species composition among boreal forests and among areas of tundra in Eurasia and North America than among tropical rain forests or among Mediterranean woodlands around the globe. Can you offer an explanation of this contrast based on the global distributions of these biomes shown in figures 2.10, 2.22, 2.31, and 2.34?

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Q 12

To date, which biomes have been the most heavily affected by humans? Which seem to be the most lightly affected? How would you assess human impact? How might these patterns change during this century? (You may need to consult the discussion of human population growth in the Applications section of chapter 11).

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