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The Earth is a great web of interaction between various biotic
organisms and nonliving, abiotic factors that make up their
environment. The study of this web, and of the interactions that shape
both living organisms and the environment in which they live, is called

 Ecology is a critical component of biology; in some sense, it is the
place where everything we have learned up until now fits together and
functions in the real world. Up until this chapter we have studied
biology in an increasing hierarchy:
1.     Biochemistry
2.    The Cell
3.    Tissue
4.    Organ
5.    Organism

Ecology takes individuals and puts them into larger contexts:
1.    Population
2.    Community
3.    Ecosystem
4.    Biome
5.    Biosphere

Ecology is important on the SAT II Biology for another reason: it
makes up about 13 percent of the questions on the core of the test. In
addition, if you choose to take the Biology E—rather than the Biology
M—version of the SAT II Biology, then another 25 percent of the test
will have some relation to ecology. In other words, this chapter and
the material it covers are crucial.


Ecologists are interested in the interactions between organisms. Since
it takes more than one organism to have an interaction, the basic unit
of ecology is the population. A population is a group of individuals that
interbreed and share the same gene pool. While every individual in a
species has the capacity to interbreed with any other individual, a
population is a group of organisms that exist in the same specific
geographic locale and actually are interbreeding. All the killer whales in
the ocean make up a species, but only the killer whales that actually
live and migrate together—only the killer whales that actually
interbreed—make up a specific population.

 Populations are much more than the sum of their parts: a population
displays patterns and concerns that are not applicable to an individual
organism. Whereas an individual is concerned with living for as long as
possible and having as many offspring as it can, a population is
concerned with maintaining its number given the resources at hand.

Population Growth

A vital characteristic of a population is the rate at which it grows. The
rate of population growth depends upon a variety of factors, including
birth rate, death rate, initial population size, and resources. With
unlimited resources, a population can expand very rapidly. Two rabbits
that live in Rabbit Utopia and have five male and five female offspring
every four months will produce a population of 12 rabbits after four
months and 72 rabbits after eight months. Sounds like nothing, right?
After one year, the population will be 432 rabbits. After two years,
there will be 93,312 rabbits. And after three years, the population will
be more than 20 million rabbits. This rabbit population is following the
trend of exponential population growth, in which there is nothing to limit
the growth of a population, and that population correspondingly grows
by exponential factors. A graph of exponential growth looks like this:

Perhaps Rabbit Utopia can grow enough lettuce to support 20 million
rabbits, but normal nature cannot. In nature, when a population is
small, the resources surrounding it are relatively large and the
population will grow at near exponential levels. But as populations
grow larger they need more food and take up more space, and
resources become tight. Within the population, competition for food
and space grows fierce, predators move in to sample some of the
bounty, and disease increases. These factors slow the growth of the
population well before it reaches into stratospheric levels. Eventually,
the rate of population growth approaches zero, and the population
comes to rest at a maximum number of individuals that can be
maintained within a given environment. This value is the carrying
capacity of the population, the point at which birth and death rates are
 The carrying capacity of an environment will shift as an environment
changes. When there is a drought and less vegetation, the carrying
capacity of rabbits in a population will decrease since the environment
will not be able to produce enough food. When there is a lot of rain
and lush vegetation, the carrying capacity will increase.

Population Growth and Types of Reproduction

 Population growth is affected by species’ methods of reproduction.
The two most important types of reproduction are asexual and sexual
reproduction. Each type of reproduction has benefits and costs.

 Asexual reproduction—such as that found in plants that reproduce by
shoots or organisms that reproduce through parthenogenesis—requires
less energy than its sexual counterpart. Because it requires less time
and effort, asexual production allows a population to grow very
quickly. For example, parthenogenesis occurs when an unfertilized egg
develops offspring. Parthenogenesis creates female organisms that are
identical to their mothers; the eggs of these female organisms
undergo parthenogenesis and produce more females. By eliminating
the necessity of males from the reproductive equation,
parthenogenesis doubles the rate at which a population can grow.
However, by eliminating males and sexual reproduction, populations
that employ asexual reproduction limit their gene pool and the
resulting diversity among members. In times when an environment is
changing or competitive, the lack of variation damages these
populations’ ability to survive.

Sexual reproduction exhausts more energy and therefore progresses
slowly. A population that reproduces through sexual reproduction will
not grow as rapidly as an asexually reproducing population, but the
sexual population will maintain the diversity of its gene pool. A
sexually reproducing population is therefore more fit to survive in a
changing or competitive environment.

 Sexually reproducing organisms have two reproductive sub-strategies.
Organisms such as insects have many small offspring that receive very
little or no parental care, reach sexual maturity at a young age, and
reproduce only one or few times. In an environment with abundant
resources, this life-history strategy allows species to quickly reproduce
and exploit opportunities for population growth. The disadvantage of
this strategy is that it produces high mortality and great instability
when resources dwindle. The alternative strategy is to bear fewer and
larger offspring that receive intensive parental attention, mature
gradually, and reproduce several times. Humans employ this strategy
and are better suited to thrive in a competitive environment, exhibiting
lower mortality rates and longer life spans. The disadvantage here is
that the concerted investment of time and energy into a few
individuals makes it difficult for a population to surmount large
decreases in population size due to disasters or disease.


Just as individuals live within a population, populations exist within
communities. A community refers to all the populations that interact
with each other in a given environment and geographical area. The
specific role and way of life of each population is called a niche. When
populations have overlapping niches, a variety of types of interaction
may occur, including competition, symbiosis, predation, and other food
relationships. Communities are shaped over time by ecological

The Niche

Each population in a community plays a unique role in the community.
This role is referred to as a niche, and ranges from where the
members of a population live, what they eat, when they sleep, how
they reproduce, and every other characteristic that defines a
population’s lifestyle within a community. You can think of the niche as
a sort of node in the network of interactions that make up a
community. Wherever the niches of two populations overlap,
interaction follows.


When two populations share some aspect of a niche, such as a nesting
site or a food source, competition results. There are two basic
outcomes of competition between populations.

One population will be a more effective competitor.

The population that is more effective will eventually ―win‖ the
competition and drive the second, less effective population from the
niche. With the niche freed, the winning population will grow to the
carrying capacity of the niche.

The two populations will evolve into less competitive niches.
If two populations compete on even terms, it may be beneficial for
both populations to modify their niches so that the populations’ niches
overlap less, or not at all. In these cases, natural selection will favor
individuals in both populations that have non-overlapping niches, and
over time the two populations will evolve into different niches.


Symbiosis refers to an intimate association between organisms, called
symbionts. The symbiotic relationship may or may not be beneficial to
the organisms involved. There are three kinds of symbiosis: parasitism,
commensalism, and mutualism. Each type of symbiosis describes a
different relationship of benefits between the two symbionts. A
tapeworm is a parasite that lacks a digestive tract, and therefore
infects a host and steals predigested food; parasites benefit while their
hosts suffer. In commensalism, one species benefits and the other
remains unaffected. Barnacles and whales live in a commensal
relationship. Finally, in mutualism, both species benefit from the
presence of and interactions with each other. Lichens, which consist of
a fungus and alga that provide for each other, respectively, moisture
and food through photosynthesis, are a good example of a mutualist


Predation refers to one organism eating another. Predation does not
only refer to carnivores. Just as an eagle eating a rodent is a form of
predation, so is a rodent munching on some grass. In fact, predation
doesn’t always result in the death of the prey. An antelope that gets
eaten by a lion will die, but a tree that loses a few leaves to a hungry
giraffe will go right on living.

 Carrying capacity shifts in a periodic manner based on the cycles of
predation. When the population of rabbits increases, the population of
coyotes that eat the rabbits will also increase, since there’s more food
for the coyotes. However, at some point, there will be so many
coyotes eating so many rabbits that the rabbit population will fall in
number. The coyotes’ great success in eating rabbits has eliminated
their food source, and as the rabbit population declines, so will the
coyote population. But wait! As the coyote population dwindles, the
lack of predators allows the rabbit population to grow again, and so
Evolution Caused by Predation

The change in a population due to a shift in environment is one of the
engines of evolution. Imagine the rabbits and their predators, the
coyotes. As the coyotes increase in number, the rabbit population
ceases to grow, and many rabbits are caught and eaten. As the
coyotes increase in number, the carrying capacity of the rabbit
population shrinks. But it is important to notice that not all rabbits are
caught by the coyotes. The faster rabbits escape capture by the
coyotes far more often than the slower rabbits. Fast rabbits survive
and breed and have offspring, while slower rabbits get eaten. The next
generation of rabbits will therefore be faster, since they are descended
from faster parents—this is directional selection in action. The
population of increasingly fast rabbits means that the coyotes have to
be faster in order to catch the rabbits. More fast coyotes catch rabbits
and live to reproduce, creating a next generation of faster coyotes.
When two populations affect their mutual evolution in this manner it is
called coevolution.

 It is arguable that predation is actually helpful to the prey population.
Since predators want to capture prey with the least possible effort, the
weakest members of the prey population are usually targeted. In this
way, the predators often remove from the gene pool of a population
those prey animals that have the weakest and least fit alleles.

Food Relationships

Every organism needs food in order to live and has to get that food
from somewhere. Every organism can be classified by where they fit
into the food chain. Most broadly, all organisms fit into one of three
camps: producers, consumers, and decomposers.


Producers are able to produce carbohydrates from the energy of the
sun through photosynthesis, or, in some instances, from inorganic
molecules through chemosynthesis. Because they can produce their own
food, producers are also called autotrophs. Producers form the
foundation of every food chain because only they can transform
inorganic energy into energy that all other organisms can use. On
land, plants, mosses, and photosynthetic bacteria are producers. In
marine environments, green plants and algae are the main producers.
In deep water environments near geothermal vents, chemosynthetic
organisms are the main producers.

Consumers cannot produce the energy and organic molecules
necessary for life; instead, consumers must ingest other organisms in
order to get these materials. Consumers are also called heterotrophs
because they must consume other organisms in order to get the
energy necessary for life. There are three types of consumers; the
categories of consumers are based on which organisms a particular
consumer preys upon. Primary consumers, such as sheep, grasshoppers,
and rabbits, feed on producers. Since all producers are plants or
plantlike, all primary consumers are herbivores, which is the name for
a plant-eating animal. Secondary consumers eat primary consumers,
making them carnivores—animals that eat other animals. Foxes and
insect-eating birds are examples of secondary consumers. Tertiary
consumers eat secondary consumers and are therefore carnivores. Polar
bears that eat sea lions are tertiary consumers. Consumers that eat
both producers and other consumers are called omnivores.


Also called saprophytes, decomposers feed on waste or dead material.
Since they must ingest organic molecules in order to survive,
decomposers are heterotrophs. In the process of getting the energy
they need, decomposers break down complex organic molecules into
their inorganic parts—carbon dioxide, nitrogen, phosphorous, etc.

Food Chains and Food Webs

All predatory interactions between producers and consumers in a
community can be organized in food chains or more complex and
realistic food webs. A food chain imagines a strictly linear interaction
between the levels of producers and consumers we described above.
An abstract food chain appears below on the left, with examples of
animals that fit each category appearing on the right:

Each step in the food chain is referred to as a trophic level.

 Food chains are simple and help us to understand the predation
interactions between organisms, but because they are so simple, they
aren’t really accurate. For instance, while sparrows do eat insects, they
also eat grass. In addition, the food chain makes it seem as if there
are only four populations in a community, when most communities
contain far more. Most organisms in a community hunt more than one
kind of prey, and are hunted by more than one predator. These
numerous predation interactions are best shown by a food web.

In fact, the more diverse and complicated the food relationships are in
a community, the more stable that community will be. Imagine a
community that was correctly described by the food chain
grassinsectssparrowshawks. If some blight struck the grass
population, the insect population would be decimated, which would
destroy the sparrow population, etc. A more complex food web is able
to absorb and withstand such disasters. If something were to happen
to the grass in the food web, the primary consumers would all have
some other food source to tide them over until the grass recovered.

Food Webs and Energy Flow

Each trophic level in a food web consumes the lower trophic level in
order to obtain energy. But not all of the energy from one trophic level
is transferred to the next. At each trophic level, most of the energy is
used up in running body processes such as respiration. Typically, just
10 percent of the energy present in one trophic level is passed along to
the next. If the energy present in the producer trophic level of of a
food web is kcal, you could draw an energy pyramid to show the
transfer of energy from one trophic level to the next:

The energy lost between each trophic level affects the number of
organisms that can occupy each trophic level. If the secondary
consumer trophic level contains 10 percent of the energy present in the
primary consumer level, it follows that there can only be about 10
percent as many secondary consumers as there are primary
consumers. The energy pyramid is therefore also a biomass pyramid that
shows the number of individuals in each trophic level.

Biological Magnification

Because biomass drops so dramatically from one trophic level to the
next, any chemical present in a lower trophic level becomes heavily
concentrated in higher trophic levels. Beginning in the 1940s, a
pesticide called DDT was sprayed on crops to stop invading insects.
The concentration of DDT in any local area was enough to kill insects,
but not enough to hurt any of the larger organisms. But as each
predator ate its prey, the DDT became concentrated in successive
trophic levels. The small levels of DDT found in the insects became
much more concentrated as it was swallowed and digested by
predators. Eagles, sitting at the top of the food web, took in massive
amounts of DDT in the course of eating their prey. The DDT caused
the eagles to lay soft eggs that could not protect the developing
embryos inside.
Ecological Succession

Just as the people living in your neighborhood can come and go,
ecological communities change over time. One way a community can
change is if external conditions shift. If the weather in a certain
geographical area suddenly gets colder, certain populations will be
better off and will thrive, while others will shrink and disappear.

 However, change in communities is not always caused by external
factors: populations can change the environments in which they live
simply by living in them. The success of a particular population in a
particular area will change the environment to the advantage of other
populations. In fact, the originally successful population often changes
the environment to its own detriment. In this way, the populations
within a community change over time, often in quite predictable ways.
The change in a community caused by the affects of the populations
within it is called ecological succession.

 The first population to move into a geographical area is referred to as
a pioneer organism. If this pioneer population is successful in its new
location, it will change the environment in such a way that new
populations can move in. As populations are replaced, changing plant
forms bring with them different types of animals. Typically, as a
community moves through the stages of succession, it is characterized
by an increase in total biomass, a greater capacity to retain nutrients
within the system, increasing species diversity, and increasing size and
life spans of organisms. Eventually, the community will reach a point
where the mixture of populations creates no new changes in the
environment. At this point, the specific populations in the stable
community are said to make up a climax community. While individuals
within a climax community will come and go, the essential makeup of
the populations within the climax community will stay constant.

 Which species are dominant in a particular climax community is
determined by unique factors of that geographical area, such as
temperature, rainfall, and soil acidity. Since a climax community does
not change the environment, it also does not affect its own
dominance; a climax community will remain dominant unless
destroyed by a significant change in climate, or some catastrophic
event such as a fire or volcanic eruption.
Succession in Action

Ready for some action? Imagine a catastrophic event: a forest fire
rages through the Green Mountains of Vermont. The fires burn
everything and leave behind a barren, rocky expanse.

 The population of trees that once lived in this area can’t grow back
because the fire has changed the ground composition. Without tree
roots to act as anchors, rain washes away the soil and the ground
becomes rocky and barren. This rocky ground, however, proves ideal
to lichens, the pioneer population. The lichens colonize the rocks and
thrive. As part of their life process, lichens produce acids that break
down rock into soil. Lichens need solid places to survive: they are
victims of their own success. Mosses and other herbs are well suited to
living in the shallow soil environment created by the lichen, and they
replace the lichen as the dominant population.

The mosses and herbs continue to build up the soil. As the soil
deepens, the conditions favor plants with longer roots, such as
grasses. Eventually the land becomes suitable for shrubs, and then for
trees. The early dominant trees in the community will be species like
poplar, which thrive in bright, sunlit conditions. As more trees come to
live in the area, though, there is less sunlight for growing trees, and
the poplar do less well than trees such as maples that can grow in
shade. Eventually, the maples come to dominate the community, since
they do not change the soil and the shade they give off proves most
helpful to the growth of their own kind. The community has reached its
climax community, with maple as the dominant species. During all this
time, don’t forget, the changing vegetation has brought with it various
changes in animal populations.

 The SAT II Biology is most likely to test your knowledge of ecological
succession in an originally rocky area, as we just covered, or in a
pond. Succession in a pond follows a general pattern. Originally, the
pond will contain protozoa, some small fish, and algae. As individual
organisms die and water runs into the pond, sediment begins building
up at the bottom and the pond grows shallower. The shallower pond
becomes marshlike and fills with reeds and cattails. The standing
water eventually disappears, and the land is merely moist: grasses
and shrubs come to dominate. As the land grows even less moist, it
becomes woodland. And as trees come to dominate, the climax
community will arise from a species that can grow in the shade of its
Ecological Succession vs. Evolution

For the SAT II Biology, do not get confused between ecological
succession and evolution. In ecological succession, the populations
that make up a community change, but the characteristics of the
individuals within the population will not change over time. Ecological
succession is something that happens to communities, while evolution
happens to populations. Although succession has different rates, it is
much faster overall than evolution.


 The dominant species in a climax community interact with and depend
on nonliving (abiotic) factors in that environment. The most important
abiotic factors in an environment, and for the SAT II Biology, are the
chemical cycles, the availability of sunlight and oxygen, the character
of the soil, and the regulation of these various phenomena. Together,
the biotic and abiotic elements make up an ecosystem.

Chemical Cycles

Inorganic elements such as carbon, nitrogen, and water pass through
the environment in various forms. These elements are vital to life:
they are consumed, excreted, respired, and otherwise utilized by living
things. The passages of these elements between organisms and the
abiotic environment are called the chemical cycles.

The Carbon Cycle

The carbon cycle begins when plants use CO2 from the air to produce
glucose, which both animals and plants use in respiration and other life
processes. Animals consume some of these plants as a source of food.
Animals use what they can of the carbon matter and excrete the rest
as waste that decays into CO2. Plant and animal respiration releases
gaseous CO2. The carbon that plants and animals do use remains in
their bodies until death. After death, decay sends the organic
compounds back into the Earth and CO2 back into the atmosphere.

The Nitrogen Cycle

Nitrogen is a vital component of amino acids and nucleic acids, which
are the fundamental units of proteins and DNA. The nitrogen cycle
begins with inert atmospheric nitrogen (N2), which is generally
unusable by living organisms. Nitrogen-fixing bacteria in the soil or on
the roots of legumes transform the inert nitrogen into nitrates (NO3–)
and ammonium (NH4–). Plants take up these compounds, synthesize
the 20 amino acids found in nature, and transform them into plant
proteins; animals, typically only able to synthesize 8 of the 20 amino
acids, eat the plants and produce protein using the plant’s materials.
Plants and animals give off nitrogen waste and death products in the
form of ammonia (NH3). One of two things can happen to the
ammonia: 1) nitrifying bacteria transform the ammonia into nitrites
(NO2) and then to nitrates (NO3–), which re-enter the cycle when they
are taken up by plants; 2) Denitrifying bacteria break down the
ammonia to produce inert nitrogen (N2).

The cycling of water and phosphorus are also important, as these
substances are limited and vital to the life processes of most

The Water Cycle

The majority of the Earth’s water resides in the oceans and lakes,
which act as water storage depots. This water escapes into the
atmosphere through evaporation and condenses into clouds.
Precipitation in the form of rain, snow, hail, etc. returns water to the
ocean and lakes, and also brings water to dry land. Water on land may
either return to the oceans and lakes as runoff, or penetrate into the
soil and seep as groundwater.

Oxygen, Sunlight, and Competition

 Oxygen and sunlight are both vital to most forms of life. The relative
abundance or lack of oxygen in a particular geographic or physical
locale will create competition among organisms and drive evolution.
Oxygen is abundant in the atmosphere, and is therefore readily
available to terrestrial species. But in order to penetrate aquatic
environments, oxygen must be dissolved in water where it exists in
smaller concentration.

 Like oxygen, sunlight is necessary to life for most organisms. In
terrestrial species, competition for sunlight has pushed evolution of
plants, with some plants growing broader leaves and branching to
capture more rays. Sunlight cannot travel through water as easily as it
can travel through air, so at great ocean depths, light is scarce. At
these sorts of depths, autotrophic organisms have to find some way to
produce energy that does not use light, such as chemosynthesis.
The Soil

The nature of soil determines which populations can be sustained in a
given ecosystem. High acidity inhibits most plant growth, but may be
ideal for some plants that are better adapted to acidic soil. The texture
of the soil and amount of clay it contains affects its ability to retain
water, while the presence of minerals and decaying organic matter
influence the types of plant life that can be supported.


Different climatic conditions are produced by the geography and
uneven heating of the Earth. Plant and animal forms that are
characteristic of a particular geographic area with a common climate
constitute biomes. Each biome is characterized by specific climax
communities. All the biomes together form the biosphere.

Terrestrial Biomes

The various biotic and abiotic factors at play on Earth result in six
major terrestrial biomes. Terrestrial biomes are categorized according
to the types of plants they support. The fundamental characteristics of
each type are described in the list below.

Tropical rain forest.

Rain forests have the highest rainfall of all biomes (100–180 inches
per year), which results in the greatest animal and plant diversity.
Trees form canopies that block sunlight from reaching the ground.
Most animal species live in the canopy, while the forest floor is
inhabited predominantly by insects and saprophytes and consists of
soil low in nutrients. Decomposed products on the forest floor are
washed away or quickly reabsorbed by plants. Tropical rainforests can
be found in Central America, the Amazon basin in South America,
Central Africa, and Southeast Asia.


This biome is characterized by grassland with sparse trees, with
extended dry periods or droughts. Tropical savanna generally border
rain forests and receive a yearly total of 40–60 inches of rainfall. They
support large herbivores, such as antelope, zebra, elephants, and
giraffe. Most tropical savanna exist in Africa. Temperate savanna, such
as the Pampas in Argentina and the prairies east of the Rocky
Mountains in the United States, receive only about 10–30 inches of
rain a year. Grasses and shrubs dominate the landscape and support
insects, birds, smaller burrowing animals, and larger, hoofed animals
such as bison.


Deserts are the driest biome, receiving less than 10 inches of rain per
year. They exhibit radical temperature changes between day and
night. Animals of the desert such as lizards, snakes, birds, and insects
are typically small and have adapted to the dry, hot climate by being
nocturnally active. Plants, such as cactus, have evolved waxy cuticles,
fewer stomata, spiky leaves, and seeds capable of remaining dormant
until sufficient resources are available. Deserts exist in Asia, Africa,
and North America.

Temperate deciduous forest.

Rainfall in temperate deciduous forests is evenly distributed
throughout the year. The biome has distinct summer and winter
seasons. It has long growing seasons during the summer. In winter,
the deciduous plants drop their leaves and enter a period of dormancy.
Beech and maple dominate in colder variations of this biome, while
oak and hickory are more prevalent where temperatures are warmer.
Animals in deciduous forests are both herbivorous and carnivorous,
such as deer, fox, owl, and squirrel. The forest floor is fertile and
contains fungi and worms. Temperate deciduous forests exist mainly
on the east coast of North America and in central Europe.


The taiga is a forest biome, but is colder and receives less rainfall than
deciduous forests. Coniferous (cone-bearing) trees, especially spruce,
dominate the taiga. The trees also have needle-shaped leaves that
help conserve water. Taiga forests sustain birds, small mammals such
as squirrels, large herbivorous mammals such as moose and elk, and
large carnivorous mammals such as wolves and grizzly bears. Taiga
exist mainly in Russian and northern Canada.


This biome is located in the far north and is covered by ice sheets for
the majority of the year. The soil, down to a few feet, remains
permanently frozen, though in the summer, the topsoil can melt and
support a short growing season. Very few plants grow in the
northernmost parts of the Tundra, but lichens, mosses, and grasses
occupy some more southern areas. Animals must be well suited for
extreme cold or must migrate. The tundra supports large herbivores
such as reindeer and caribou, large predators such as bear, and some

Aquatic Biomes

Aquatic biomes account for 70 percent of the Earth’s surface and
contain the majority of plant and animal life. Aquatic biomes also
account for a vast portion of the photosynthesis, and therefore oxygen
production, that occurs on Earth. There are two types of aquatic
biomes, based on the type of water found in each: marine and


Marine biomes refer to the oceans that all connect to form a single,
great body of water. Since water has an immense capacity to absorb
heat with little temperature increase, conditions remain uniform over
these large aquatic bodies. Marine biomes are divided into three
zones: intertidal/littoral, neritic, and pelagic.

 The intertidal zone, also called the littoral zone, is the region where
land and water meet. It experiences periodic dryness with changing
tides and is inhabited by algae, sponges, various mollusks, starfish,
and crabs.

 The neritic zone extends to 600 feet beneath the water’s surface and
sits on the continental shelf, hundreds of miles from shores. Algae,
crustaceans, and numerous fish inhabit this region.

 The pelagic zone consists of a photic zone (reaching 600 feet below sea
level) and below that, an aphotic zone. Light penetrates the photic zone,
which is why it contains photosynthetic plankton. The photic zone also
is home to heterotrophs such as fish, sharks, and whales that prey on
these producers as well as on each other. No light penetrates the
aphotic zone, which is a kind of watery circus of the bizarre, where
extreme cold water, darkness, and high pressure have spurred strange
evolutionary paths. The region is home to some chemosynthetic
autotrophs. Other denizens of the deep are scavengers that feed upon
dead organic matter falling from the higher realms, and predators who
feed on each other.


Freshwater biomes include rivers, lakes, and marshes. Life here is
affected by temperature, salt concentration, light penetration, depth,
and availability of dissolved CO2 and O2. Freshwater biomes are much
smaller than marine biomes, so conditions are less stable. Organisms
that live in these regions must be able to handle the greater extremes.
The very nature of freshwater also demands special characteristics of
the organisms that live within it. In freshwater environments, the salt
concentration within the cell of an organism is higher than the salt
concentration in the water. A concentration exists between the interior
of cells and the exterior environment: water from the environment is
constantly diffusing into the organism. Organisms in freshwater need
homeostatic systems to maintain proper water balance.

Review Questions

 How is an ecosystem organized, from least to most comprehensive?

Individual, community, population, biome

Individual, population, community, biome

Individual, population, niche, community

Individual, niche, community, population

Individual, population, biome, niche

Why is energy lost as it moves from producers to primary consumers?

Saprophytic activity

Biomass decreases

Secondary consumers eat primary consumers

Respiration and metabolic activity

None of the above

 Which of the following organisms participate in the nitrogen cycle?

Denitrifying bacteria

Chemosynthetic bacteria


All of the above

None of the above

 When resources are abundant and the environment is stable, which reproductive strategy
is most effective?

Sexual reproduction, with intense parental care and slow development

Sexual reproduction, with slight parental care and quick development


All of the above
None of the above

Why might growth slow from an exponential rate in a population of sheep?


Lack of space


All of the above

None of the above

What is a relationship in which two organisms both benefit from their association?






 Which is true of ecological succession?
Pioneer species move into new communities first.

Climax communities have lower total biomass than preceding communities.

Species diversity is greatest in the early stages of succession.

Climax communities shift constantly.

All of the above

 Tundra is characterized by

trees such as beech, maple, and oak

high biodiversity


a short growing season following rainfall

none of the above

 Which of the following releases carbon dioxide into the atmosphere?

Animal consumption of producers


Bacterial decay

None of the above

 In a plains community, the population with greatest biomass would be



prairie dogs




1. (B)
Individual organisms occupy particular niches (geographical locations as well as roles).
Populations consist of individuals of an interbreeding species. Many coexisting
populations constitute a community, and many communities coexist within a biome.

 2. (D)
Only 10 percent of energy moves between trophic levels, because it is lost to sustain
respiration and metabolic processes. Saprophytic activity does not explain the loss of
energy as you move up a food pyramid. Biomass decreases because energy is lost, not the
other way around. While it is true that secondary consumers eat primary consumers, this
scenario does not effect the change in energy capacity between trophic levels.

3. (D)
All of the listed organisms are involved in the nitrogen cycle. Decaying (saprophytic),
nitrifying (chemosynthetic), denitrifying, and nitrogen-fixing bacteria all play roles in the
nitrogen cycle. Decaying bacteria produce ammonia (NH3), which is transformed into
nitrites (NO2) and nitrates (NO3-) by nitrifying bacteria. Denitrifying bacteria convert
ammonia into free N2 in the atmosphere.
 4. (C)
Asexual reproduction such as parthenogenesis takes greatest advantage of unlimited
space and resources in a stable environment. This mode of reproduction facilitates rapid
population growth. Although species diversity created through sexual reproduction is
sacrificed, it is not necessary in a noncompetitive atmosphere. Organisms (no matter how
similar) in an environment without limitations do not compete with one another.

 5. (D)
Depleted resources, competition for food and space, predation, and disease all slow
population growth. These factors shape carrying capacity for populations in any given

 6. (B)
The first answer is a bit of a trick: symbiosis refers to a number of different relationships
between organisms, including a mutually beneficial relationship, but it does not refer
specifically to that sort of relationship. Mutualism does refer to a relationship beneficial
to both organisms. Commensalism helps one organism and does not harm the other,
while parasitism benefits one organism and harms the other. Competition refers to a
battle for resources and survival between populations.

7. (A)
As an ecosystem moves through the stages of succession, it is characterized by an
increase in total biomass, a decrease in net productivity relative to biomass, a greater
capacity to retain nutrients within the system, increasing species diversity, increasing size
of organisms, increasing life spans, and complex life cycles. Climax communities will
not shift unless there is a cataclysmic event.

8. (C)
Beech, maple, and oak populate the temperate deciduous forest. High biodiversity is not a
characteristic of the barren tundra. The desert exhibits short growing seasons immediately
after precipitation.

 9. (D)
Carbon dioxide in the atmosphere is directly produced by bacterial decay of waste and
dead organic material. (Respiration also produces CO2, but it is not listed among the
answer choices.) Photosynthesis and the animal consumption of producers contribute to
the carbon cycle, but are not directly responsible for the production of CO2.
Chemosynthesis is not involved in the carbon cycle.

10. (D)
Biomass decreases from producers up through each level of consumers. Grasses, the only
producer in the group, must have the largest biomass.
Intraspecific Competition

Intraspecific competition is competition between members of the
same species.
 In the summer of 1980, much of southern New England was
struck by an infestation of the gypsy moth (Porthetria dispar). As
the summer wore on,
• the larvae (caterpillars) pupated;
• the hatched adults mated, and
• the females laid masses of eggs (each mass containing several
   hundred eggs) on virtually every tree in the region.
• In early May of 1981, the young caterpillars that hatched from
  these eggs began feeding and molting.
• The results were dramatic:
◦ In 72 hours, a 50-ft beech tree or a 25-ft white pine tree would
  be completely defoliated.
◦ Large patches of forest began to take on a winter appearance
  with their skeletons of bare branches.
• In fact the infestation was so heavy that many trees were
  completely defoliated before the caterpillars could complete
  their larval development. [View!]
• The result: a massive die-off of the animals; very few succeeded
  in completing metamorphosis.

Here, then, was a dramatic example of how competition among
members of one species for a finite resource — in this case, food
— caused a sharp drop in population.

The effect was clearly density-dependent. The lower population
densities of the previous summer had permitted most of the
animals to complete their life cycle.

The graph shows a similar population crash; in this case of
reindeer on two islands in the Bering Sea. Why the population on
St. Paul Island went through so much more severe a boom-and-
bust cycle than that on St. George Island is unknown.
 Many rodent populations (e.g., lemmings in the Arctic) go through
such boom-and-bust cycles.

Interspecific Competition
 All the ecological requirements of a species constitutes its
ecological niche. The dominant requirement is usually food, but
others, such as
• nesting sites
• a place in the sun (for plants)
 may be important as well.
When two species share overlapping ecological niches, they may
be forced into competition for the resource(s) of that niche. This
interspecific competition is another density-dependent check on
the growth of one or both populations.

Like so many factors in ecology, interspecific competition is more
easily studied in the laboratory than in the field. This graph (based
on the work of G. F. Gause) shows the effect of interspecific
competition on the population size of two species of paramecia,
Paramecium aurelia and Paramecium caudatum.

When either species was cultured alone — with fresh food added
regularly — the population grew exponentially at first and then
leveled off.

However, when the two species were cultured together, P.
caudatum proved to be the weaker competitor. After a brief phase
of exponential growth, its population began to decline and
ultimately it became extinct. The population of P. aurelia reached a
plateau, but so long as P. caudatum remained, this was below the
population density it achieved when grown alone.

The habitat of most natural populations is far more complex than a
culture vessel. In a natural habitat, the species at a competitive
advantage in one part of the habitat might be at a disadvantage in
another. In addition, the presence of predators and parasites would
limit population growth of the more successful as well as the less
successful species. So, in a natural setting, the less effective
competitor is usually not driven to extinction.
       Over time, interspecific competition can result in
evolutionary changes that reduce the intensity of competition — a
phenomenon called character displacement.

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