Chapter 16 Answers
Ecosystems
Visual Understanding
Figure 16.5
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 blue-green
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.
Figure 16.7
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
The Energy in Ecosystems
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.
Materials Cycle Within Ecosystems
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.
How Weather Shapes Ecosystems
How is it that the amount of sunlight hitting a particular place on the earth affects the plants that
grow there in almost exactly the same way as the elevation of that place above sea level?
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.
Major Kinds of Ecosystems
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.