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Chapter 20 Answers
Ecosystems
Visual Understanding
1. Figure 20.5
A. One thousand calories of energy is produced by a certain amount of algae. Explain
the probable efficiency of having humans eat, respectively, (1) the algae itself, (2) the
heterotrophs, (3) the smelt, or (4) the trout. B. How many more people could be fed if
we all ate algae? C. Why can’t we survive just eating algae?
A. 1) If we eat the algae we would capture about 15% of the energy in the algae, or 150
calories. 2) If we eat the number heterotrophs that it takes to eat the 1000 calories of
algae we would obtain 30 calories. 3) If we eat the smelt that eat the heterotrophs that
eat the 1000 calories of algae we would obtain about 6 calories. 4) If we eat the amount
of trout that would be nourished by eating the smelt that eat the heterotrophs that eat the
1000 calories of algae we would obtain about 1.2 calories (obviously it is a tiny little bite
of a piece of trout!).
B. Eating the algae itself would yield 150 calories to a human, and eating the equivalent
amount of trout (that was nourished by the smelt that ate the heterotrophs that ate the
original 1000 calories of algae) yields about 1.2 calories to a human. So you could feed
125 times as many people if we all ate algae.
C. Many people do eat large amounts of algae, from the health food spirulina (a bluegreen algae) to nori (a seaweed type of algae used for sushi). They are a large part of
many diets around the world. The reality, however, is that in developed countries people
want the meat and ice cream and other products that come from organisms higher on the
food chain.
2. Figure 20.7
Imagine that you are a molecule of water. Create a journey for yourself, starting with
falling as part of a drop of rain onto the earth, and ending in a cloud ready to fall once
again. Take a trip through a plant along the way.
Answers will vary. Imagine falling on a bajada (slope) on a mountainside one summer
afternoon and running downhill to a small creek. On your way you are stopped at the
side of the creek by some plant roots and taken up into a large cottonwood tree. You
travel up through the tree into its leaves where some of your companion water molecules
help the leaves make nutrients through the process of photosynthesis. It is a hot, sunny
morning, however, and you are one of the water molecules lost, through transpiration,
out of the leaves and into the air. You rise up through the air to join with millions of
other water molecules to form large thunderhead-type clouds (cumulonimbus) and in a
few hours you will then fall to earth in another afternoon monsoon thunderstorm.
Challenge Questions
1. Given the amount of sunlight that hits the plants on our planet, and the ability of plants
for rapid growth and reproduction, how come we aren’t all hip deep in dead plants?
Obviously something happens to all those dead plants. Detritivores are small creatures
that eat the leaves and wood; this might include pillbugs and termites, crickets,
earthworms, and many kinds of beetles and other small arthropods. Many of these same
organisms, and others such as flies, also act as detritivores breaking down dead animals
and animal wastes. Decomposers are generally considered to be the bacteria and fungi
that attack dead organic material and break it down. Scavengers are a similar category,
usually considered to be animals that eat dead animals; vultures, eagles, and hyenas are
in this category.
2. Plants need both carbon (C) and nitrogen (N) in large amounts. Compare and contrast
how plants obtain these two important elements.
Plants obtain the carbon that they need by opening the stoma on the undersides of leaves.
Air enters the leaf through the stoma, and the carbon dioxide in the air is taken up by the
plant and used for cellular respiration, making glucose from carbon dioxide and water.
Plants obtain the nitrogen needed to make amino acids by taking in certain nitrogenous
compounds (NHx) through their roots. These compounds are only made available to
plants through the action of certain types of bacteria that are found in the soil or in direct
association with plant roots.
3. Why do increasing latitude and increasing elevation affect, in the same way, which
plant species grow at a place?
Depending on your location on the planet you receive the most sunlight possible per unit
area (near the equator) or some smaller amount as you move closer to the poles. The
closer you are to the poles, the sun is very low in the sky even at noon; this means that
you receive much smaller amounts of sunlight per unit area. This means more heat in the
summer, and warmer winter temperatures, at the equator than at the poles.
As you go up a mountain, starting at sea level, it becomes cooler the higher you go.
Temperature is strongly related to the types of plants that are able to grow in a particular
location. Therefore plants that grow in northern latitudes as a rule can also grow in
southern latitudes at the tops of tall mountains.
4. Pick two very different land ecosystems. Compare and contrast the sunshine, rainfall,
major temperature features, and something about the plants and animals.
Answers will vary.