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Transcript
17
Ecosystem Change and Resiliency
T
hroughout this unit ,
you have
mainly learned about human
activities that cause ecosystems to
change and some that allow them to
resist change. These include overfishing, the introduction of invasive species, and runoff of chemicals, and more
responsible interventions, such as sustainable fishery management and
aquaculture.
However, a single event, whether natural, such as a volcanic eruption or
forest fire, or human error, such as a
large oil spill, can also bring longlasting changes to ecosystems.
Challenge
00 What determines if an ecosystem can recover from a major event?
Both natural p
­ henomena
such as volcanic eruptions
(above) and human-caused
phenom­ena such as oil spills
(left) cause change in the
­ecosystems where they
occur.
134
ecosystem change and resiliency • Activity 17
MATERIALS
For each student
5 sticky notes
Procedure
1. Use the “Read, Think, and Take Note” strategy as you complete the following
reading.
2. When you have completed the reading, share your “Read, Think, and Take
Note” responses with your partner. If possible, answer any questions your
partner had about the reading. Note any similarities in your responses.
Choose one response to share with the class.
3. Share your response to the reading in a class discussion, as directed by
your teacher.
Reading
Ecosystem Change and Resiliency
In today’s world there are few, if any, ecosystems left untouched by people in some
way. In some cases this has resulted in major changes, and in others there have
been very minor changes. Humans have influenced ecosystems for thousands of
years, but current technology and large population size have intensified human
impact. Yet while scientists previously thought that ecosystems would basically
stay the same in the absence of human intervention, decades of research have
shown that ecosystems are constantly
changing and shifting.
any things cause disturbances in an ecoM
system. Ecologists use the term disturbance
to refer to an abrupt event in an ecosystem
that suddenly and significantly changes the
resources available, the number or type of
organisms, or the kinds of species present in
an ecosystem. Natural disturbances include
volcanic eruptions and fires. Two examples of
disturbances caused by humans are oil spills
and clear-­cutting of forests. The response of
an ecosystem to a disturbance depends in part
on how major the disturbance is and the
extent of the damage done.
An area like this Hawaiian coastline where there has been a recent lava
flow has no soil and few, if any, living organisms present.
135
Science & Global Issues/Biology • ECOLOGY
This area has undergone
­primary succession after a
­volcanic eruption.
In responding to a disturbance, an ecosystem undergoes what is referred to as ecological succession. Ecological succession is the natural process in which a disturbed area is gradually taken over by a species or groups of species that were not
there before. For example, if a volcano erupted and the molten lava smothered a
large area of land, eventually that area would undergo succession. In Hawaii and
other volcanic locations, barren expanses of hard lava rock have turned into thick
forest ecosystems in fewer than 150 years.
Primary succession is ecological succession that starts in an area where there are
essentially no living organisms. In the lava-covered area, the first life you would
see would be small organisms, such as spiders, that can live without soil. Eventually, dust and leaves from surrounding areas would collect in the cracks and crevices of the lava, and some of the more hardy plants, introduced by wind-blown
seeds or birds, would begin to grow there. As the decades passed, more of the
surface would become covered in plants, as leaves and other debris decayed and
formed soil. The level of biodiversity would rise as more soil developed.
When an ecosystem undergoes a disturbance and the soil remains, secondary
­succession develops instead of primary succession. This might occur after a wildfire, or in a now-unused field where generations of ranchers had grazed cattle.
Often there are still plants that survived the disturbance. If there are no further
disturbances, those areas tend to return to their previous states. In Yellowstone
Park after the huge wildfires in 1988, some of the first plants to grow back were
grasses, small flowering bushes, and a bush called fireweed (named for its ability
to grow well after fires). By 2000, small pine trees were beginning to replace the
trees that had burned down, and today there are sections of burn areas in Yellowstone where it is difficult to tell that such a large fire ever occurred.
136
ecosystem change and resiliency • Activity 17
An ecosystem’s ability to return to a stable state after a disturbance is a measure of
its resilience. If an ecosystem has a high level of resilience, it will be much more
likely to recover from a major disturbance. The level of resilience depends on several important factors, one of which is the native biodiversity within and surrounding the disturbed area. Because different species occupy different habitats
and perform different roles within an ecosystem, the more species that survive the
event the more likely those habitats and roles will be quickly filled again after a
major disturbance. Another critical element for ecosystem resilience is the presence of species in nearby areas that might repopulate the disturbed area. For
example, if a coral reef has been severely damaged by a large storm, fish and other
organisms in nearby areas might help to repopulate the reef. Major disturbances,
however, also increase the chances that an area will be occupied by invasive species, because there is so much available habitat. The resistance of an ecosystem is
one factor that can help prevent this. The resistance of an ecosystem results from
the natural factors within an ecosystem that help it to withstand external pressures
and maintain normal functions. For example, if an invasive species of fish were
released in a lake where there were native turtles that preferred to prey on those
fish, the resistance of that lake to the invasive fish is high.
As ecologists learn more about ecosystem disturbance, resilience, and resistance,
they have found that while ecosystems are often able to recover from major disturbances, they also have a point after which they cannot. One concern is that evidence suggests that many of the world’s fisheries are at or beyond this point.
Fisheries biologists and ecologists are working with many of the world’s commercial fisheries to try to determine what, if anything, can be done to prevent our
ocean ecosystems from being fished past the point of resilience. As you will see in
the next activity, there are several possible solutions that may help to repair commercial fisheries around the world.
After wildfires in 1988, areas of
Yellowstone National Park
underwent secondary
succession.
137
Science & Global Issues/Biology • ECOLOGY
Analysis
1. What is the difference between primary and secondary successions?
2. How does the native biodiversity of an ecosystem affect its resilience?
3. How is the resilience of an ecosystem different from its resistance to
disturbance?
4. What factors would you investigate to help you decide whether an ecosystem
is nearing the point where it is no longer resilient and, therefore, unable to
recover from a disturbance? Explain why you chose those factors.
Key vocabulary
disturbance
resilience
ecological succession
resistance
invasive species
secondary succession
primary succession
138