Ecology, Concepts And Applications
5th Canadian Edition
by Manuel C. Molles
Answers to Review Questions
,Answers to Review Questions
Chapter 2 – Life on Land
1. Draw a soil profile for the area around your university. Indicate the principal layers, or
horizons. Describe the characteristics of each layer.
This exercise will require a good shovel and a place to dig that is both safely away from any
buried services (gas, electric, etc.), and will not raise the ire of property owners! Dig down about
a metre, if you can, and notice the changes in soil texture and colours. Drawings should reflect
figure 2.5 and discussions under concept 2.2, as well as soil descriptions for the applicable biome
(concept 2.3). You can consult with your instructor for further help in identifying things you see
in your pit.
2. 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?
Use figure 2.2 and the associated discussion as a guide. High precipitation in the tropics is
produced by high rates of evaporation and subsequent condensation of water vapor in ascending
air masses. The clouds formed in this way produce the heavy precipitation associated with the
tropics. High precipitation at temperate latitudes is produced when warm, moisture-bearing
subtropical air meets cold polar air, which forces condensation of the water vapor in the
subtropical air mass. Many tropical environments experience drought during periods of El Niño
(see chapter 23).
3. Use what you know about atmospheric circulation and seasonal changes in the sun’s
orientation to earth to explain the highly seasonal precipitation in temperate grasslands.
(Hint: Why does most precipitation in this biome come during the summer months?)
The seasonal rainfall of these biomes is produced by shifts in the latitude at which the sun is
directly overhead. Figure 2.2a, which shows the sun directly overhead at the equator (generating
storms through the processes of evaporation, condensation and precipitation of water), is really a
snapshot of a dynamic system. The latitude at which the sun is directly overhead shifts between
23.5°N and 23.5°S, as the earth orbits the sun (fig. 2.1). This shifting latitude is sometimes called
the "solar equator,” and acts as a generator of precipitation. In temperate grasslands, the sun is
never directly overhead, but in summer, the sun is more intense, driving higher rates of
evaporation, and precipitation. The drier seasons come when solar input is more oblique.
4. 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. Yet temperate grasslands and temperate
forests at similar latitudes differ in precipitation and, therefore, vegetation types. What
other geographic variable might affect the distribution of temperature and precipitation
and, therefore, of biomes?
, Other major factors are altitude, distance from the oceans (the interiors of large continents are
often dry), and the presence of mountains which can force release of precipitation in rising air
masses, forming a "rain shadow" in the leeward (downwind) side.
5. 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?
At very high latitudes it is cold much of the year with a brief cool growing season. In tropical
climates the dominant seasonal change is from wet season to dry season. In other words,
seasonal terms associated with midlatitudes do not fit the annual variations that occur in other
parts of the earth. It would be interesting to catalog the traditional terms for seasonal changes
used by native peoples living in different climatic zones. For example, Costa Ricans have
categorized seasonal changes more thoroughly than is customary in temperate climates: in the
middle of the wet season, for instance, there is a period of reduced rainfall which Costa Ricans
call veranillo, meaning "little summer.”
6. Biologists have observed much more similarity in species composition among boreal
forests and among areas of tundra in Eurasia and North America than among temperate
forests or temperate grasslands around the globe. Can you offer an explanation of this
contrast based on the global distributions of these biomes?
There has been much greater connection and exchange across the tundra and boreal forests which
form nearly continuous bands in the northern hemisphere. In contrast, the temperate forests and
grasslands are more fragmented with fewer opportunities for biotic exchange over long periods
of evolutionary history.
7. The use of fire by humans to maintain and extend savanna biomes was our first foray
into controlling nature. How was this experiment similar to and different from our foray
into agriculture in terms of impacts on natural biomes?
In both, the use of fire to maintain savanna and the development of agriculture, we keep
ecosystems in an early successional state (to be discussed in later chapters), with less diverse
plant and animal communities. In each case, this influences ecosystem function. The
maintenance of savanna’s for grazing largely replaced wild herbivorous animals with a set of
domesticated grazers, maintain some of the functionality of the natural system it replaced.
Agriculture has had more dramatic effects on changing natural communities, particularly where a
forested ecosystem is replaced by agricultural systems. The vegetational communities are very
different between the agricultural system and the natural system. Agriculture is also an extractive
enterprise, and it can contribute to removal of nutrients and carbon from soil, reducing soil
fertility over time.
6. How is the physical environment on mountains at mid-latitudes similar to that in tropical
alpine zones? How do these environments differ?
