1. CHAPTER 15 “TAKE-HOME MESSAGE” CHALLENGE
QUESTIONS
15.1 An ecosystem is made of two components: the biotic environment, or community,
consisting of the living organisms within an area, and the physical environment, or the
habitat in which these organisms live. A habitat consists of its chemical resources of the
soil, water, and air as well as its physical conditions. List some of the aspects that make
up the physical conditions of a habitat.
Answer: Temperature, salinity, moisture, humidity, and energy sources.
15.2 Terrestrial biomes are determined by the temperature and precipitation amounts as
well as whether those factors are constant or vary by season. By contrast, how are aquatic
biomes determined?
Answer: Aquatic biomes are defined by physical features such salinity, water
movement, and depth.
15.3 Global weather patterns are mainly determined by the amount of solar energy
falling on an area. The amount of solar energy that falls on an area largely depends on the
angle at which this light hits the earth. Explain what this means.
Answer: Because the earth is round, solar energy hitting at a Pole is spread out over a
wider area compared to an equal amount of solar energy hitting the earth at the equator.
15.4 A rain shadow is a dry region on the landward side of a mountain range. How is
it formed?
Answer: As moist air blows against this mountain, it rises, cools, and loses its moisture
as precipitation. As this drier air passes across the mountain range, it warms and does not
lose its moisture as precipitation, and perhaps even pulls moisture from the soil.
15.5 Why is most of Europe significantly warmer than regions in Canada at the same
latitude?
Answer: The Gulf Stream brings warm water from near the equator to this area,
warming it significantly.
15.6 What is the difference between a food chain and a food web?
Answer: A food chain indicates the trophic level to which each organism belongs:
primary producers, primary consumers, secondary consumers, or tertiary consumers. A
food web acknowledges that many organisms operate at multiple trophic levels. For
example, a human eating a hamburger is both a primary consumer and a secondary
consumer.
15.7 Only about 10% of the biomass of an organism at one trophic level is converted
into biomass of an organism at the next trophic level. List a consequence of this
inefficiency.
Answer: 1) There are far more plants than animals. 2) The top carnivores are relatively
rare.
2. 15.8 How is the ocean to a thirsty shipwreck victim like nitrogen gas in the atmosphere
to most organisms?
Answer: Both are abundant and both are unusable. More than 78% of the atmosphere is
nitrogen gas, but for most organisms nitrogen is only usable after its bonds are broken
down in the soil by nitrogen-fixing bacteria.
15.9 What is coevolution?
Answer: Coevolution is a situation in which the evolution of one species significantly
affects the evolution of a second species, which in turn again affects the evolution of the
first species, and so on.
15.10 What factors are encompassed by an organism’s niche?
Answer: An organism’s niche encompasses the space it requires, the type and amount of
food it consumes, the timing of its reproduction, its temperature and moisture
requirements, and virtually every other aspect that describes the way the organism uses
its environment.
15.11 Use the sporting competition terms of “win,” “lose,” and “tie” along with
biological terms from this section to describe possible outcomes when two related species
have completely overlapping niches?
Answer: With competitive exclusion, one species will win while the other species will
lose by being driven to extinction in that location. In resource partitioning, the two
species can tie by altering their use of the niche, dividing the resources.
15.12 Behavioral adaptations exhibited by organisms in order to reduce predation
include hiding or escaping (as exhibited by many long-legged mammals), and alarm
calling and fighting back (as exhibited by Belding’s ground squirrels). List two physical
adaptations exhibited by organisms in order to reduce predation.
Answer: Physical adaptations exhibited by prey include mechanical defenses such as the
quills of a porcupine, chemical defenses such as the toxic strychnine produced by certain
plants, warning coloration such as that found on monarch butterflies, and camouflage
such as that found in many leaf- or twig-resembling insects.
15.13 Ectoparasites and Endoparasites are both involved in parasitism, a symbiotic
relationship between two species in which one organism (the parasite) benefits, while the
other organism (the host) is harmed. How do ectoparacites and endoparasites differ? Give
at least one example of each.
Answer: Ectoparasites such as lice, leeches, and ticks, live outside their hosts.
Endoparasites such as viruses live inside their hosts.
15.14 Mutualism is a symbiotic relationship between two species in which both
organisms benefit. Describe two examples.
Answer: Photosynthetic algae live inside coral and providing the coral with energy
through photosynthesis. Cellulose-digesting microbes live inside the digestive systems of
termites and allow them to digest wood. Pollinators and the plants from which they
obtain nectar are another example of mutualism.
3. 15.15 What is the key reason why succession so rarely leads to climax communities?
Answer: Succession does lead to climax communities, but only in the absence of
disturbance. However, disturbance is a fundamental part of most ecosystems.
15.16 In architecture, a keystone is the large stone at the top of an arch, which keeps the
two sides of the arch in place. How is a keystone species like a keystone in an arch?
Answer: Both the keystone and the keystone species have a large influence on the
composition of their “structures” (the arch and the community respectively). A keystone
literally holds together an arch. The presence of the keystone species keeps together
species diversity in a community.
