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ECOSYSTEM RESPONSES
reflect
There is a saying, “No man is an island,” which means
that people need one another in order to survive.
Everyone on Earth is interconnected in some way.
This is not only true of human beings, but of all living
and many nonliving things on Earth. In an ecosystem,
individual organisms, populations, and entire
communities interact with each other and with their
environments. In fact, much of how living things behave
is in response to changes in their environment. What are some of these environmental factors
that cause change? How do living things respond to these changes?
Ecosystems
Earth is made up of several important parts, such as
air, soil, and living creatures. These parts are called
spheres, such as the atmosphere or the lithosphere.
The sphere containing life is called the biosphere
because bio means “life.” The biosphere contains
all the living things on Earth in various ecosystems.
Ecosystems are composed of all the biotic (living)
and abiotic (nonliving) factors within a particular
environment.
The organisms in an ecosystem are organized into
levels based on the number of individuals present and
how they interact. The first and smallest level of an
ecosystem is an individual organism. One human is
an organism. A lone wolf is also an organism. In a prairie
ecosystem, a single buffalo is an organism.
What are the abiotic factors of
this ecosystem? What are the
biotic factors?
Organisms are rarely found alone. More often than not, several organisms live together.
More than one organism of the same species in an area is called a population, which is the
second level of organization in an ecosystem. A population is made up of organisms of the
same species that are able to interact and mate with one another. All of the buffalo living
together in the prairie are an example of a population.
Populations are not isolated from other groups of organisms. In the prairie ecosystem,
buffalo live among populations of birds, insects, grasses, and trees—to name a few. All of
these populations living together in the same area make up a community, which is the third
level of organization in an ecosystem.
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ECOSYSTEM RESPONSES
The abiotic factors of an ecosystem include climate, water, living space, inorganic nutrients,
and food. These abiotic factors often serve as limiting factors to biotic components of the
ecosystem. When a limiting factor, such as water, is in short supply, then the productivity of
the ecosystem, including the organism’s ability to survive, is limited.
External Factors That Affect Organisms, Populations, and Communities
The abiotic and biotic factors in an ecosystem are interconnected. When biotic and abiotic
factors change, they affect the other parts of the ecosystem. The response to change made
by an organism affects the population, which in turn, affects the whole community.
Organisms: Changes to an ecosystem, especially changes that cause a decrease in the
supply of a limiting factor, affect an organism’s ability to survive. Thus, organisms respond
in a way that attempts to counteract the effect of the external factor. An organism’s goal is
always survival. It responds to external factors in its environment
in order to maintain homeostasis. Humans do this all the time
homeostasis:
without thinking about it. If a person gets too cold, he or she puts
a steady, stable
on a jacket or wraps up in a blanket. If a person’s body becomes
internal state
dehydrated, signals are sent to the brain to indicate thirst. In
response, a person gets a drink of water.
Temperature is an external factor that affects all
organisms. Responses vary among different species, but
maintaining homeostasis, including a suitable internal
body temperature, is a common goal among all organisms.
In cold temperatures, some animals may begin to shiver
involuntarily. Shivering is characterized by small muscles
contractions in the core of an organism’s body. These tiny
contractions use up energy in the form of ATP. When the
molecular bonds in ATP are broken, heat is generated. The
result is an increase in body temperature. Ectotherms, or
animals that use external sources to regulate their body
temperature, respond to decreasing temperatures in a
variety of ways. They may bask in the Sun or on a hot
rock, burrow into an insulated nest, or curl up to minimize
the amount of surface area that is exposed to cold.
This lizard is basking on
hot, sunlit rocks in order
to keep its body warm.
Even plants respond to temperature changes. Many plants enter a dormant, or resting,
state in winter where they do not grow or produce food. In preparation, they lose their
leaves and their stems become weaker. This allows the plants to conserve energy and
survive colder temperatures.
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ECOSYSTEM RESPONSES
what do you think?
In an effort to survive, organisms respond to the various external factors that affect their
ecosystems. Look at the examples of external changes in the table below. For each one,
list a possible response that an organism might have to maintain homeostasis.
External change
Temperature increase
Drought
New predator
New competition for food
Organism response
Populations: Populations respond to external factors in ways that reflect the efforts of the
individual members to survive and reproduce. Populations can change in size, distribution,
and range based on the responses of its organisms. For example, consider a population
of deer living in a forest. Suppose a disease affects the plants that the deer use as a food
source. In response to this external factor, the population may shrink as some members
die from starvation. Birth and death rates also determine the size of a population. When
birth rate exceeds death rate, the population grows and vice versa. Birth and death rates
change in response to external factors. Low predation, abundant food and water, ample
living space, and low competition
for resources contribute to high
birth rates and population growth.
The abundance or scarcity of
nutrients not only affect population
size, but its distribution and range
in the ecosystem. Organisms will
go where resources are available.
This can cause populations to
travel together (migrate) to new
Zebra migrate in groups in order to find an
areas or spread out in smaller
ample supply of food.
groups to decrease competition.
look out!
Even in ideal living conditions, the size of a population can reach a limit. Once a population
becomes too large, it outgrows the availability of food, water, and living space. Resources
are not unlimited, and can serve as limiting factors to population size. As the population
grows, competition for resources increases. Those organisms that cannot successfully
compete for resources will die.
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ECOSYSTEM RESPONSES
The death rate begins to equal the birth rate and instead of experiencing growth, the
population becomes stable. The population may still experience short periods of growth,
but they will be balanced out with short periods of decline. At this point, the environment
has reached its carrying capacity for that population. The carrying capacity is the maximum
number of members of a population that an ecosystem can support.
Communities: Every species has a unique
way of life called a niche. A niche includes a
set of abiotic and biotic factors a species needs
to survive and reproduce. This can include the
type of food it eats, light and water needs, ideal
climate conditions, living space requirements,
and survival and reproduction strategies.
Species in a community typically occupy
different niches. For example, in a seashore
community, snails and barnacles are able to live
in the high tide zone. During low tide, this area
is not covered with water. In contrast, algae and
sea stars could not survive here for long without
drying out. Although these species live in the
same ecosystem, they do not share the same
niches.
Fish, coral, seaweed, sea turtles,
and many other aquatic species
occupy a variety of niches in a reef
community.
Interactions between species often revolve around the different niches they occupy,
which can result in changes to the community as a whole. Communities respond to
changes in biotic and abiotic factors in ways that reflect the responses of the individual
organisms and populations they contain. When confronted with a changing environment,
an individual organism responds in a way that enables it to survive. These responses affect
the population the organism belongs to. Cumulative responses of a population affect the
structure and biodiversity of the community as a whole.
Predation is an example of a factor that can affect the structure of a community. For
example, in a forest community, a population of hawks feeds on rabbits, squirrels, and other
small mammals. The relationship regulates the population size of the rabbits and squirrels.
However, if an external factor such as disease or competition kills individual hawks and
decreases the hawk population, then the rabbit and squirrel populations will grow because
of decreased predation. The external factor was a detriment to the hawk population, but
benefited the rabbits and squirrels. As a result, the structure and species distribution of the
community changed. A community may also change when a new species immigrates into
the community or one emigrates out.
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ECOSYSTEM RESPONSES
what do you think?
Clownfish and sea anemones have a mutualistic
relationship. The clownfish are protected from
predators by the sea anemone’s stinging tentacles.
In return, the anemone benefits from scraps of food
dropped by the clownfish. Predict what role external
factors affecting the sea anemone population will play
in the clownfish population and the reef community as
a whole.
Looking to the Future: Human Activity and Habitat Destruction
An organism’s habitat not only provides shelter, but
also the food resources it needs to obtain nutrients.
Habitat is particularly important to the giant pandas of
China. These large bears survive almost exclusively on
the bamboo plant, which makes up 99% of their diet.
Bamboo is not rich in nutrients, so the pandas must
eat large quantities of the plant to adequately nourish
themselves. Pandas spend about 12 hours a day
eating up to 38 kg (84 lb) of bamboo. Unfortunately,
this food source is becoming more difficult to find in
their habitat.
Too often, human activity is the external factor affecting populations of plants and animals
in their natural environment. The pandas’ bamboo forest habitat is being destroyed as the
demands of the farming and timber industries grow. Although bamboo is a fast-growing
plant, it only grows at specific altitudes and pandas require a lot of bamboo to survive. Two
breeding pandas need approximately 30 square kilometers to support them, which is equal
to about 15,000 football fields combined.
The destruction of the bamboo forests decreases the shelter and food resources available
to the panda population in China. As a result, the panda population is dwindling. Scientists
estimate that the wild panda population is fewer than 2,500 individuals, which makes this
unique creature an endangered species.
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ECOSYSTEM RESPONSES
What Do You Know?
Response to changes in an ecosystem
can vary among different organisms in the
same community. An external factor that is
advantageous to one population may be
harmful to another. This affects the diversity
and structure of the community that the
populations live in.
Each scenario below describes a change
in an ecosystem. Read each scenario
and predict the most likely outcome of the
change by filling in each blank with either
“increases” or “decreases.”
1. The birth rate of field mice
exceeds death rate. This
____________________
the mouse population and
____________________ the grass
population on which the mice feed.
2. A population of bluebirds emigrates
from a forest ecosystem. This
____________________ the size of
the community within the ecosystem
and ____________________
competition for resources among
other bird populations that eat the
same foods as the bluebirds.
3. A grassland ecosystem
experiences a drought. This
____________________ competition
for water among all organisms and
____________________ plant
populations in the grassland.
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ECOSYSTEM RESPONSES
connecting with your child
Response to the Ecosystem
To help them learn more about ecosystem
responses, have students monitor their own
responses to external factors over several
days. Encourage students to pay attention
to things they naturally do without realizing
that they are responses to the ecosystem.
For example, students could observe their
responses to changes in the temperature,
noise level, presence of food, amount of
light, and presence of other people in their
surroundings. Have students record how
they responded to each situation and why
they made their choice. Most often, it will
be because the response benefited them
in some way. Have them explain how the
benefit could relate to survival in the wild.
Next, have students take care of a small
houseplant, aquarium, or terrarium over
several weeks. Direct them to introduce
changes to the external conditions in order
to observe how the organisms respond.
Suggestions include varying the amount of
food, water, light, and heat available to the
houseplant, aquarium, or terrarium. Have
students record their observations and
encourage them to make predictions each
time they introduce a new external factor.
Ask students to record how each organism’s
response affects other members and parts
of the community. Be sure that students
make every effort to treat the organisms with
respect. They should not introduce external
factors that unnecessarily cause distress or
harm to the organisms.
Here are some questions to discuss with
students:
• Why must organisms respond to
changes in their ecosystem?
• How do you think these responses affect
the long-term survival of species?
• How do you think responses of a
community affect an ecosystem?
Biome? Planet?
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