Unit D: Changes in Living Systems

Chapter and Unit Review Suggested Answers

Chapter 1 Review Questions

Knowledge

1.	a.	population	b.	trophic level	c.	mutualism	d.	transpiration
	e.	producer	f.	habitat	g.	competition

2.	a.	Biodiversity refers to the number of species. Biomass refers to the dry mass of living tissue.
	b.	Abiotic refers to non-living factors, such as temperature, soil type, climate, and pH. Biotic refers to the living organisms in an environment, such as producers and decomposers.
	c.	A food chain has a single path, whereas a food web has multiple paths for the flow of energy through feeding relationships.
	d.	Producers make their own food, usually through the process of photosynthesis. Consumers eat other organisms as their source of food.
	e.	Habitat destruction means the habitat is destroyed. Habitat fragmentation refers to the dividing up of what was once continuous habitat.

3.	Sample answers may include the following. Abiotic factors of a forest ecosystem include wind speed, soil pH, air temperature, and the amount of precipitation. Biotic factors include trees, birds, squirrels, ants, earthworms, and fungi.
4.	A food web is more realistic than a food chain, since living things will likely eat a variety of nutrient sources. A food chain only shows one example of the feeding pattern in an ecosystem.
5.	Sunlight provides the input energy for nearly all ecosystems. And sunlight enables producers to make organic compounds that become the energy source for other living things in the ecosystem.
6.	A biogeochemical cycle is a diagram that represents the movement of elements, chemical compounds, and other forms of matter from the abiotic and biotic factors of an ecosystem.
7.	Only about 10% of the energy and matter is passed from one trophic level to the next level because the organisms at each level need some of the energy to complete their life functions. Eventually, all of the energy is lost as heat to the environment.

Applying Concepts

Note that answers may vary in the Example column of the completed table.

INTERACTIONS BETWEEN SPECIES

Type of Interaction	Effect on Species #1	Effect on Species #2	Description	Example
mutualism	+	+	form of symbiosis in which both organisms benefit	butterflies and purple coneflowers
commensalism	+	0	form of symbiosis in which one organism benefits while the other is neither helped nor harmed	cowbirds and bison
parasitism	+	–	form of symbiosis in which one organism—the parasite—benefits while the other organism—the host—is harmed	tapeworms and bison
predation	+	–	interaction in which one organism—the predator—benefits by killing and eating the other organism—the prey	red-tailed hawks and black-tailed prairie dogs
competition	–	–	interaction where two or more organisms compete for limited resources	lynx and great horned owls

9.	This simplified energy pyramid outlines an Arctic ecosystem.

10.	This food chain represents the Arctic ecosystem.

11.	There are no tertiary consumers for this ecosystem because the total energy available at each level is quite small. The first trophic level has a small amount of energy available for the next level because the Arctic growing season is so short that there is little opportunity for producers to build significant quantities of organic molecules for primary consumers. It follows that there is less energy at the level of the primary consumers to pass on to the secondary consumers. At this point, the secondary consumers have to hunt over such a large range to sustain themselves that the system cannot support a level of tertiary consumers.

12.	a.	The graph showing the annual population changes for Arctic foxes living in this same area would likely show peaks in the years following the peaks in the lemming population. This means that the Arctic fox population would likely peak in 1994, 1998, and 2002. The reason for this is that the four-year cycle shown in the lemming population would be shadowed by a similar four-year cycle in the Arctic fox population. This is because the availability of lemmings is closely tied to the population of Arctic foxes, since the lemming is the primary food source for the Arctic fox. In the years that the lemming population peaks, the Arctic foxes take advantage of the plentiful food source and produce many offspring that show up in the following year. The crash in the lemming population triggers a corresponding crash in the Arctic fox population. Therefore, the twinning of the two cycles continues.
	b.	The years in which the lemming population peaks would be very stressful for the lemmings’ favourite vegetation. In the following year, the vegetation may not be able to produce much foliage due to the stress it was subjected to in the previous year. The result is that there would be much less vegetation available in the year following a lemming population peak. This would cause the lemming population to crash. During the years of reduced lemming numbers, the vegetation would have a period of recovery, only to be followed by the next boom in the lemming population.

13.

An example of mutualism is the lichen, which is the symbiotic relationship between a fungi and an alga. Competition is demonstrated by individual lemmings competing for food and by snowy owls and Arctic foxes competing for lemmings. Mosquitoes and black flies are parasites that take a blood meal from a host species.

14.	a.	Since the ground is cold and low in oxygen because it is saturated with water, decomposition occurs very slowly. Since decomposition is a process that releases carbon dioxide back into the atmosphere, the peat bogs and muskeg act as sinks for carbon if this process is impeded.
	b.	If global warming occurs, then the rise in temperature will accelerate the process of decomposition. The peat bogs and muskeg could then become sources of carbon instead of sinks.
15.	a.	The person who eats the vegetable salad has the role of a primary consumer or herbivore, while the person eating chicken has the role of a secondary consumer or a carnivore. Since tuna is a high-level predator in the ocean ecosystem, a person who eats tuna is a tertiary consumer or a top predator.
	b.	The salad eater is in the second trophic level, the person eating the chicken is in the third trophic level, and the tuna eater is in the fourth or fifth trophic level.
	c.	Most people would be classified as omnivores because they eat food from both plant and animal sources. A typical lunch for many people includes both plant and animal products—a glass of fruit juice, a tuna sandwich, and a piece of fruit.
	d.	It is impossible for a person to be a producer because humans are incapable of photosynthesis.

16.	As energy moves from lower trophic levels to higher levels, there is less and less energy available because only about 10% of the energy at one level is passed on to the next level. By the time the energy has been passed to the sixth trophic level, there is such a small amount of energy available that it is unlikely to be able to support life.
17.	Producers are organisms that assemble complex organic compounds from elements and simple compounds. Organisms like spruce trees and freshwater algae do this through photosynthesis.
18.	An organism that feeds on other living organisms as its source of essential elements and compounds is called a consumer. Primary consumers or herbivores feed on producers. Black-tailed prairie dogs, grasshoppers, and mosquito larvae are all herbivores. Secondary consumers or predators feed on primary consumers. This group includes the swift fox, the yellow warbler, and dragonfly larvae. The final group of consumers are the tertiary consumers—these carnivores feed on other carnivores. These top predators are represented by the red-tailed hawk, the peregrine falcon, and the northern pike.
19.	Bacteria and fungi are two types of micro-organisms that obtain their essential nutrients and compounds from feeding on dead and decaying matter. These organisms are also called decomposers.
20.	The decomposers release simple inorganic molecules used by producers in the process of photosynthesis. Prairie grasses and freshwater algae are examples of producers that use these simple inorganic compounds.
21.	The carbon cycle could be described in the following way.

	step 1:	Terrestrial plants and marine plants remove carbon dioxide from the air and combine this with water to produce glucose and other organic compounds. In this process—called photosynthesis—oxygen is released as a waste product.
	step 2:	As primary consumers feed on the plants, the complex organic compounds are transferred. Compounds are transferred again as primary consumers become prey of secondary consumers, and as secondary consumers are fed upon by tertiary consumers.
	step 3:	As the tissues of dead producers and consumers are fed upon by the decomposers, complex organic molecules are broken down into simpler inorganic forms. In this case, carbon dioxide is released into the atmosphere.
	step 4:	Atmospheric carbon dioxide is used by the producers and the whole cycle begins again.

22.	a.	Forest fragmentation creates more open spaces and edge habitats, which cowbirds prefer over dense forests. As roads and farms push north into forests, cowbirds could follow these roads as extensions of the open spaces they are accustomed to on open fields and grasslands.
	b.	Although cowbirds originally followed herds of bison and ate insects that the bison stirred up, in agricultural areas cowbirds will follow cattle in the same way. Cowbirds are classified as brood parasites because they will lay their eggs in other bird’s nests and let the other birds raise the cowbird chicks. Songbirds that prefer edge habitats are prime candidates for brood parasitism by the cowbirds. It follows that as farmland adds to forest fragmentation, cowbirds that follow agricultural expansion soon begin to reduce the numbers of area songbirds.

23.	When excess nitrogen compounds leach into local streams and lakes, a harmful algal bloom can develop where the population of algae explodes to create thick floating mats of green ooze. Eventually, the algae population cannot sustain itself and the algae die in significant numbers. As bacteria feast on masses of algae, the oxygen concentration in the water drops dramatically. This disrupts the balance within the ecosystem so much that fish begin to die from a lack of oxygen in the water.
24.	Parasitism is a long-term relationship in which the parasitic organism benefits while the host organism is harmed. It is not in the interest of the parasite to kill the host because then its source of nutrients is gone. Predation is also an interaction in which one organism benefits and another is harmed. However, in predation the prey is killed and the interaction only lasts as long as it takes the predator to kill the prey.
25.	If more people switched to vegetarian diets, there would be fewer energy losses. As animals are raised for food, the animals use energy to maintain their body functions, move around, and keep warm. Using the idea that only 10% of available energy at one trophic level can be passed to the next level, the simplified arithmetic looks like this: Consider the amount of vegetation it takes to raise enough meat to feed 100 people. This same amount of vegetation could feed about 1000 people if they ate it directly instead of using it for animal feed. This is oversimplified. However, a more efficient use of food is generated by producers if humans eat as first-order consumers (herbivores) and not as second-order consumers (carnivores).
26.	A raptor is a bird of prey. These birds have sharp talons and hooked beaks for tearing meat from the animals that they hunt for food. Examples of raptors include owls, falcons, hawks, and eagles.
27.	DDT was a pesticide used to control insect populations. Small amounts of this compound were absorbed into the bodies of insects. When secondary consumers, such as fish and small birds, fed on huge numbers of insects, the concentration of DDT increased in their bodies so that it exceeded the concentration in the insects. Further magnification of the concentration of DDT occurred when raptors, such as eagles and peregrine falcons, fed on the secondary consumers. In the case of peregrine falcons, the concentration of DDT was sufficient to make the shells of their eggs very thin and susceptible to damage. This caused the population of these birds to decline.
28.	If the population of raptors within an ecosystem begins to decline, then the population of the animals that the raptors prey upon will likely increase. For example, if the number of owls in an area declines, the population of mice that the owls feed upon will increase. This, in turn, can have a negative effect on the plant community as the increased numbers of mice eat more vegetation.
29.	According to data from 2002, the ecological footprint of the average Albertan was 10.7 ha and for the average Canadian it was 7.7 ha. The difference is that in other parts of Canada, hydroelectric power is more common than it is in Alberta. Hydroelectric power has a smaller ecological footprint than burning coal, which is more common in Alberta. Albertans also have 20% more vehicles registered per capita than the Canadian average. A larger percentage of these vehicles are light trucks and SUVs, which consume more fossil fuels.
30.	The following are two sample calculations: number of Earths needed to support the human race if everyone on the planet lived like an Albertan number of Earths needed to support the human race if everyone on the planet lived like a Canadian
31.	For most people, significant changes have to do with food eaten and transportation used. If you switched to a diet containing less meat, your footprints would be smaller because of the efficiencies associated with eating plants directly instead of feeding plants to animals.

Chapter 2 Review Questions

Knowledge

1.	Primary succession is the process of changing—in successive stages—an environment from an area of bare rock and few species to a complex community.
2.	Logging, forest fires, tilling soil, or any other event that removes the vegetation without removing the soil could all cause the process of secondary succession to begin.
3.	Secondary succession occurs where soil already exists. It is a much faster process than primary succession.
4.	Open populations are affected by the number of births, deaths, immigration, and emigration, while closed populations do not have any immigration or emigration to affect their numbers. In many cases, closed populations are artificial.
5.	Carrying capacity is the maximum number of individuals that can be sustained in a given environment for an indefinite period.
6.	This graph shows the exponential growth of a population.

7.	The carrying capacity of a population is outlined in this graph.

8.	a.	The ability to fly allows an owl the advantage of being able to see from a great vantage point and attack from the air.
	b.	A sharp beak and talons are good for killing and tearing apart prey.
	c.	Huge eyes allow for good vision even in low light or dark conditions.
	d.	Feathers that muffle the sound of flying make the owl a stealthier hunter. The prey does not even hear the owl approaching for a kill.
9.	The three main concepts of Darwin’s theory of natural selection are the following: Organisms usually produce more offspring than can survive. There is variation among individuals in a population. Organisms compete for limited resources.
10.	Fitness in biological terms is the ability of an organism to survive and then successfully reproduce. It is not solely a measure of an individual’s strength, speed, or health, although this may influence its fitness.
11.	A mutation is a change to the genes. The genes provide instructions to the body and the change in instructions may be harmful—this is demonstrated by cancer. A mutation may provide an advantage, for example, a changed beak shape for the Galapagos finch for a more varied diet. Eye colour for a cat is an example of a neutral mutation.
12.	Acceptance of Darwin’s theory is dependent upon acknowledging the evidence preserved in the fossil record that organisms have changed over time. People who are reluctant to accept this evidence have difficulty with Darwin’s theory.
13.	Darwin suggested that the variety of finches on the islands all came from a common seed-eating finch ancestor. Within the population of offspring there was a variation in many traits, including some finches born with slightly different beaks. This enabled them to eat different types of food that, in turn, provided them with the advantage of less competition for that food source. Those finches best suited to the new food source survived to breed and demonstrated their superior fitness to other members of the population—these finches were able to pass on their adaptation of being able to utilize a new food source to their offspring. Over many generations, several new species of finches developed that were well adapted to eating different foods than their ancestors did.
14.	Asexual reproduction does not require a mate for reproduction. Time and energy is saved by simply budding or sending a runner out to create a new plant. Asexual reproduction is fast (as in bacteria). A drawback to asexual reproduction is that there is a lack of variation, which limits natural selection and evolutionary change.

Applying Concepts

15.

The conditions that allowed the exponential growth of the beavers on Tierra del Fuego include a vast food supply of old-growth forests and no natural predators.

16.

Similarities between these two situations would be the exponential growth of both populations after they are introduced by humans to a new habitat where there is a lack of predators and competition. In both cases, the herbivores are causing environmental destruction and are conflicting with the interests of the people who introduced them.

17.

A difference between the two situations is the fact that the beavers are aquatic animals and the potential for them to spread further is a risk. The reasons for introduction also differ between the two situations—the beavers were introduced for fur and the rabbits were introduced for food and sport.

18.

Note the following polar-bear advantages.

Adaptation	Reasons for Advantage
slightly webbed toes	improves swimming after prey
white fur	helps polar bear sneak up on its prey by blending in with its surroundings
a thick layer of fat and black skin underneath fur	thick fat acts as insulation and black skin helps absorb heat
female polar bears hibernate in dens with cubs until spring	saves on energy required for moving and hunting—can conserve energy and use it for producing milk for young
ability to stand up on hind legs	allows bear to see prey over snowdrifts or get a better scent of prey
covers its black nose when stalking a seal	increases its camouflage

19.

The conifers preserve their place as the dominant species by keeping their environment most favourable for conifers.

Species such as grasses and shrubs are not able to grow as well in a climax forest—this creates stability in the community. The process of succession has ended until the climax community is removed.

20.

The disturbed area around the volcano is going through the process of primary succession. The soil has been removed and the graph shows a large number of pioneer species, such as lichens and mosses.

21.

Answers will vary somewhat, but the three bar graphs should look similar to these.

22.

It would be unlikely for someone to see an equal amount of each type of organism in the sample area. This is because one type of organism creates conditions unsuitable for the growth of another type of organism. The process of succession replaces one dominant community with another. For example, tall coniferous trees would create too much shade for grasses or shrubs to thrive.

23.

The graph should look similar to this one.

24.

The insect population in terrarium #1 is displaying S-curve growth. The population reaches its carrying capacity and stays there. The insect population in terrarium #2 is displaying J-curve growth and goes through population explosions and crashes.

25.

If both insect populations were placed in the same terrarium, the insects that reproduce exponentially would probably outcompete the slower reproducing insects. The rapidly rising population of the insects that were in terrarium #2 would eat all the food before the other population had a chance to reach its carrying capacity.

26.

The sickled cells would have a harder time carrying oxygen because of their shape. Individuals with sickle cell anemia would constantly be short of oxygen.

27.

Although the sickle cell anemia mutation makes individuals sick, it provides resistance to malaria. Illness from sickle cell anemia is a better alternative than death from malaria, so the gene persists in the population.

28.

The removal of malaria from the population would probably result in a reduction in the number of individuals who carry the sickle cell anemia mutation over time, because carrying the mutation would no longer be an advantage.

29.

Darwin would explain the development of the prehensile tail of the New World monkeys as follows.

At one time, most New World monkeys had non-prehensile tails, but because of natural variation (mutation) a few had tails that were more flexible. The more flexible tails provided an advantage for moving through the trees, and these individuals were able to find food, mates, and/or escape predators better because of this adaptation. The flexible-tail monkeys were better able to survive to create more offspring, and soon flexible tails were found in much of the population. Over time, more and more flexible tails were selected until a true prehensile tail had developed.

30.

Weismann’s experiment did not support Lamarck’s idea of acquired inheritance of characteristics because there was no change to the genes or instructions in the mice to produce shorter tails. Lamarck’s theory was not accepted because there was no evidence to support the theory.

31.

It is still important to study Lamarck because he caused people to think about the idea that populations of organisms change over time. This was a revolutionary idea for his time, and he helped pave the way for Darwin’s ideas.

Unit D Review Questions

The following table compares the processes of primary and secondary succession.

Description	Primary Succession	Secondary Succession
relative speed of process	slower	faster
starting conditions	no soil present	soil present
events causing process to begin	new volcanic islands from lava flow hardening, glaciers receding	clearcut logging, forest fires, tilling of soil
resulting community	climax community	climax community

2.	a.	population
	b.	J-curve
	c.	doubling time
	d.	open population
	e.	exponential growth
	f.	carrying capacity
	g.	closed population
	h.	S-curve

Note the following adaptations table. Other examples are possible.

Type of Adaptation	How It Makes Organism More Successful	Example of Organism with Adaptation
speed	easier to catch prey and escape predators	cheetah or pronghorn
camouflage	makes it more difficult to see organism and, therefore, harder to find	walking stick insect or flounder
bright colours	attractive to mates	peacocks or parrotfish
sharp spines	undesirable to eat	cactus or porcupine
produces poison	undesirable to eat	poison arrow frog or rhubarb leaves
thick fur or hair	warmth in winter	polar bear or deer
horns or antlers	protection or to fight for mates	moose or deer

Note the following contributions to the field of biology.

Researcher	Scientific Contribution
Malthus	Malthus hypothesized that populations grow exponentially but resource production does not—this causes war and famine. He provided an analysis and awareness of how human population change is affected.
Lamarck	Lamack proposed a theory of evolution that was unable to be proved, but he caused people to think about how organisms might have changed over time.
Darwin	Darwin proposed a theory of evolution by natural selection that is supported by many pieces of evidence. His controversial theory went against the common belief of the time that organisms were fixed in their current form and were unchanged since Earth was created.

5.	a.	theory of natural selection
	b.	theory of evolution
	c.	selective breeding
	d.	asexual reproduction
	e.	fitness
	f.	mutation
	g.	gene

Your sketches of the strip mine should be similar to this one if you live in a boreal forest. If you live in the grasslands, trees would not be found at the centre nor at the edges.

	a.	In the centre, you should expect to find trees.
	b.	You should find lichens, mosses, and ferns at the outer edges of the mine.
	c.	Trees would be found in the surrounding area that was not moved.

The predators kept the elk population at a sustainable level (carrying capacity).

The elk population grew exponentially after 1925.

The elk may have overeaten their food source, causing mass starvation. Overcrowding may have caused disease to spread and reduce the population.

10.

It appears that the biologists re-introduced a predator or began culling the herd to keep numbers at a more manageable level.

11.

Note the following camel adaptations.

Adaptation	Reasons for Advantage
two sets of long eyelashes	The long eyelashes help keep both bright sunlight and blowing sand out of the eyes.
wide, padded feet	The padded feet help keep the camel from sinking in the sand as it walks.
nostrils that can open and close	Closed nostrils prevent sand from blowing up the nose during dust storms.
a hump located on its back that is a reservoir for fat	The fat can be broken down to create water during dry times.
thick, tan-coloured fur	This helps the camel blend into its surroundings. A light colour reflects the heat.
a tough mouth and strong teeth	A tough mouth is needed for eating sharp and spiny (cacti) desert plants.

12.

Since these colonizing plants grow in open meadows, they would be very accessible to grazing herbivores. Defence mechanisms allow them to grow well in the open, yet not be overeaten.

13.

Some of the concerns over shipping water to cities in the United States could include the following:

There might not be enough water left for Alberta’s own growing population.
The cost of water for Albertans will increase.
Once the pipeline is opened, can it ever be closed without huge repercussions in trade with the United States?
More water to those thirsty cities means more growth in those cities and an increase in the demand for water.

14.

Abiotic factors affecting survival on the island are wind, temperature, water, and light.

15.	Arctic foxes, ptarmigans, shrubs, lichens, grasses, and insects are the island’s biotic factors.
16.	A likely relationship is predation of the fox on the ptarmigan.
17.	grasses, shrubs, and lichens ptarmigans and insects Arctic foxes
18.
19.	Camouflage is an advantage for making stealthy attacks on its prey.
20.	The ptarmigan would increase in numbers. This could reduce the amount of vegetation because of overgrazing.

21.	a.	Secondary succession would take place because soil is already present.
	b.	Fire increases the amount of carbon dioxide in the air.
22.	A reduction in the hare population would result in a reduction of the lynx’s food source and, therefore, a reduction in the lynx numbers.

23.	A reduction in the hare population would cause the plant numbers to increase, other herbivores would increase because of less competition, and other predators (such as the wolf) may be reduced in numbers.
24.	Other factors could include disease, competition, other predators, and a shortage of food.
25.

26.	a.	Changing its fur colour creates two types of camouflage to help the hare hide in more than one season. Brown fur blends with summer trees and bushes, while white fur blends in with snow.
	b.	At one time, most members of the population likely had only one fur colour, but a few members of the population had two colours because of a mutation. Over time, the members that had two fur colours were better able to escape predators, so they survived to pass on their genes. The single-coloured hares stood out against one of their habitat backgrounds, so they were easier targets for predators and were killed off. Eventually, only hares with two fur colours were left in the population.
27.	Forest fires, the increased burning of fossil fuels, and the cellular respiration of animals add carbon dioxide to the atmosphere.
28.	a.	The process of secondary succession has occurred to create grasses and shrubs for hares to browse. More hares mean more lynxes can be supported by the same area.
	b.	The prevention of forest fires means that there will be less recycling of nutrients, no succession will take place, and biodiversity will be reduced.
29.	Abiotic tests would include light intensity, wind speed, soil pH, soil temperature, the amount of precipitation, and soil quality.

30.	Answers will vary.
	a.	For example, you can choose soil pH for the manipulated variable and weed growth for the responding variable. The controlled variable is the weed species and its location of growth.
	b.	In this case, you can set up the experiment where different areas supporting the growth of the weed have different values for soil pH.
	c.	Predictions would be that the weeds could likely tolerate a wide range of soil pH values because weeds are found in a variety of different locations, all with varying values for soil pH.

31.	a.	The fifth section acted as a control.
	b.	The manipulated variable is the type of herbicide.
	c.	The responding variable is the growth of the weed.
	d.	Controlled variables for this experiment would include the size of the crop section, the amount of water, the amount of sunlight, the time of day, as well as the frequency of recording observations.

32.

Mosquitoes play an important role in aquatic food chains. As was demonstrated in the description of freshwater ecosystems in Chapter 1, mosquito larvae are first-order consumers that feed on algae. The mosquito larvae then become an important food source for frogs, dragonfly larvae, and small fish like the stickleback. If this link in the food web is completely removed, it affects the whole aquatic ecosystem.

33.

Answers will vary. You should include at least one producer and one decomposer. The most efficient arrangement would be to have the humans serve the role of the primary consumers. If humans are herbivores, it dramatically reduces the energy losses that occur as humans are placed in higher trophic levels on the energy pyramid.

34.

Answers will vary. Again, it is essential that producers and decomposers both be present. It is unrealistic to think that this self-contained environment could support many secondary consumers and unlikely that it could support any tertiary consumers. This food web illustrates an approach where the humans have a vegetarian lifestyle.

35.	Answers should address energy flow and matter cycling. It is critical that a continual source of energy be utilized because energy will be continually leaving the system in the form of heat. Therefore, answers may include energy-creating and energy-saving devices like solar panels, access to sunlight for growing plants, and attempts to minimize energy losses. If the habitat is to be truly self-contained, then it must be sealed so that no matter can escape. All essential chemical compounds must be recycled with systems that attempt to mimic Earth’s biogeochemical cycles. As an example, the production of oxygen for human inhabitants exhaling carbon dioxide must be considered. Water is another important substance to consider, given its central role in supporting life.
36.	Sample answers for ensuring that these compounds were maintained at appropriate levels are as follows: Carbon cycle—ensure enough producers are available to absorb carbon dioxide, and limit the combustion of carbon materials. Nitrogen cycle—the presence of nitrifying and denitrifying bacteria is essential, as is the ability to supplement nitrogen levels by adding bacteria. Water cycle—collect and filter waste water, and set up the collection of naturally purified water from evaporation and transpiration. Wastes—compost organic wastes, and have decomposers to help with the recycling of matter.
37.	Less variation would mean that the populations of the various life forms could be more susceptible to diseases and to the effects of harmful mutations. Since this is a closed population, similarities exist to other closed populations that evolved in isolation on Earth. The Galapagos Islands come to mind as one example. If this environment could sustain itself for thousands or millions of years, new species could eventually develop.
38.	a.	This system is similar to Earth because Earth is a spaceship that relies upon energy from the Sun, but is otherwise self-contained. Energy flows through the food webs of the biosphere and matter is cycled.
	b.	The most obvious difference is that the Earth system is so much larger. Subtle differences include the fact that Earth is incredibly complex and intricate. It is difficult to conceive of an artificial system designed by humans that could be as sophisticated. Part of the reason for the challenge in designing systems for supporting life as sophisticated as the biosphere is that the fossil record suggests that the Earth system has been slowly developing over billions of years. Even if it took one human lifetime to design a self-contained habitat, this time falls far short of the time that it took to develop the biosphere.
	c.	Earth is currently the only self-contained habitat that humans have. People better look after it, because it is the only life-support system available.