Download ecological succession

ECOLOGICAL SUCCESSION
reflect
What would happen if a city park or a garden were left
unattended, with no mowing, weeding, watering, or any other
human interference? At first, weeds would grow and replace the
cultivated plants that may have been present. What would this
area look like after a month? What about after a year or after a
decade?
Ecological Succession
The observed process of change in the species composition
of an ecosystem over time is called ecological succession.
In a park that is no longer tended, weeds would replace
cultivated plants. Other plants may eventually replace these weeds. This process is called
succession.
When the process of succession begins in an area that is entirely uninhabited by living
things, it is called primary succession. Primary succession is a slow process because the
area in which it is occurring is barren. The first organisms that are able to grow in such a
location, called pioneer species, must be able to grow on bare rock.
Lichens are an example of a pioneer species.
They result from a mutualistic relationship
between a fungus and algae or bacteria.
The algae or bacteria are photosynthetic
and provide the fungus with a source of
food. The fungus absorbs other necessary
nutrients from the environment and provides
a structure in which the algae or bacteria can
live. Lichens break down bare rock into small
particles as they grow on it. This results in
soil production and provides a foothold for
Lichens and mosses (shown above)
the next group of organisms that moves in. The
are common pioneer species.
lichens change the physical environment in
such a way that it is easier for other organisms
to live there. In this stage, and each subsequent stage of succession, the organisms that
are best adapted to the current conditions will thrive and reproduce. They will be the most
abundant organisms in the environment until conditions change, and another set of organisms
move in that is more suited for the new conditions.
Mosses use the products released by lichens and begin growing on pockets in the rock.
Mosses continue the soil production begun by lichens and provide the necessary materials
for small plant seeds to germinate.
© 2013-2014 Accelerate Learning - All Rights Reserved
1
ECOLOGICAL SUCCESSION
The plants that first take root in these areas must be adapted to full sunlight and little soil.
Generally, small annual plants are best adapted to this environment. These annual plants
are gradually replaced by perennial plants and grasses, which grow more slowly. These
small plants provide the base of a food chain consisting of mostly small animals. In each
of these stages, the organisms present add to the soil and alter the environment. These
changes in the environmental conditions allow the next group of plants and other organisms
to flourish.
The soil and organic matter
produced by smaller plants allow
larger shrubs to begin growing. The
shade from these shrubs will inhibit
the growth of some of the small
plants underneath them, though
some small plants will continue
to grow in the spaces between
shrubs. Soon, the area will contain
more shrubs. The shrubs will
continue adding organic matter to
the soil and will provide shade for
germinating tree seeds.
The soil, organic matter, and shade will allow larger trees to begin germinating and
colonizing the area. Softwood trees such as pines often colonize an area first, followed by
hardwood trees such as oaks and maples. These trees shade large areas below them. This
alters the types of plants that grow in the area, as shade-tolerant plants replace the plants
from earlier stages of succession. The larger plants provide food and habitats for larger
animals. The interactions of all the organisms at each stage in this process change the
environment and pave the way for the next group of organisms.
© 2013-2014 Accelerate Learning - All Rights Reserved
2
ECOLOGICAL SUCCESSION
look out!
Succession is a predictable shift in species from
early stages to late stages. However, the plants
present at each stage of succession vary based on
the climate of the area. Not all areas can support
large trees, and those that do may support different
species of trees. In some areas the largest plants
that can be supported are shrubs; in others, the
environment limits the organisms to lichens and
small plants. The organisms present at the peak
of succession are part of the climax community for
that area. A climax community occurs when the diverse species within the community can
each maintain relatively stable populations over time. Temperature, water availability, food
availability, human activity, and many other factors determine the organisms present in
the climax community. Factors in the environment that limit the growth of plants and other
organisms are called limiting factors. For example, a desert has very little water and does
not allow for the growth of many large plants. In the desert, water is often a limiting factor
and the climax community consists of drought-tolerant plants such as cacti.
Secondary Succession
Secondary succession occurs in locations that were previously
inhabited but experienced a natural disaster such as a forest
fire, volcanic eruption, or flood. Secondary succession can also
occur in the wake of human activity, such as clear-cutting a
forest for timber.
natural disaster: a
force of nature that
results in catastrophic
consequences
These events may kill many, but not all, of the organisms in an area. After the catastrophic
event, a process similar to primary succession occurs. The main difference between
primary and secondary succession is that in secondary succession the process begins with
soil and some organisms already present, usually
microscopic soil organisms. The catastrophic event
generally results in a loss of large, shade-producing
plants. The sudden loss of shade following a
catastrophic event allows fast-growing annuals to
thrive because these plants generally prefer full
sunlight. Eventually, larger and slower-growing
plants replace these smaller plants. Because
secondary succession begins with soil and plants
already present, it occurs much more quickly than
primary succession.
© 2013-2014 Accelerate Learning - All Rights Reserved
3
ECOLOGICAL SUCCESSION
Some natural disasters, such as forest fires, occur regularly in certain areas. Plants in
these areas have adapted to these fires and some even require fires to thrive. For example,
some plant seeds will remain dormant in the soil until they are scorched by fire. Only then
will they germinate. This adaptation allows these plants to succeed in areas that experience
fires on a regular basis. The ecosystems in these areas are even dependent on fires to
maintain the health of the community. Inhibiting fires in these areas can cause lasting harm
to the ecosystem. Naturally occurring fires tend to burn quickly and will mainly kill small
plants and shrubs. These fires clear the dead leaves and other matter from the forest floor.
If humans extinguish these naturally occurring fires, the matter on the forest floor builds
up over time. Then, when a fire finally occurs that cannot be extinguished by humans, it
burns hotter and longer. This type of fire causes much more harm to an area and it will take
longer to restore the climax community. Managing forest fires, especially around areas of
human inhabitation, is a challenging and important job that involves cooperation between
many different local, state, and federal agencies.
The images above show three stages of secondary succession after a fire
destroyed the forest.
what do you think?
The image on the right shows lava from a volcanic
eruption flowing through a forest. Will the area
affected by the lava flow likely undergo primary or
secondary succession? Describe the steps that will
likely occur.
© 2013-2014 Accelerate Learning - All Rights Reserved
4
ECOLOGICAL SUCCESSION
Looking to the Future: The Importance of Biodiversity
Later stages of succession tend to have increased biodiversity. Biological diversity, or
biodiversity, is a measure of the variety of species and intricate interactions that support
one another within a geographical region. Biodiversity includes the full variety of genes,
species, communities, and ecosystems within a given space. For example, rainforests
typically have high biodiversity. Many different species of organisms make their home in the
rainforest, such as tree frogs, snakes, cats, trees, vines, fungi, and monkeys. In contrast,
an industrial farm would be considered to have low biodiversity. Only a few species of
plants are grown on the farm, such as wheat or corn. High biodiversity tends to add to the
stability of an ecosystem.
Biological diversity is impacted by
catastrophic events, during which many
species may be killed. The process of
ecological succession can restore the
biodiversity of an ecosystem. An area with
high biodiversity is more likely to recover
quickly from a catastrophic event. Human
activities can lead to habitat destruction
for organisms living in the ecosystem. The
loss of habitat can decrease biodiversity in
Habitat destruction decreases the
an area by making resources more scarce.
biodiversity of an ecosystem.
Competition for dwindling resources usually
results in fewer organisms. In some cases,
loss of habitat may result in the permanent
loss of species or genetic variety in species. Decreased biodiversity in an area also
decreases the stability of the region and makes it less resilient to environmental changes.
This is not only problematic for the plants and animal in the region, it also decreases
resources available for humans.
As the human population grows and expands farther into natural areas, habitat loss is
increasing and biodiversity is decreasing. The careful balancing of human wants with
environmental needs is essential for maintaining healthy ecosystems for people as well as
other organisms.
© 2013-2014 Accelerate Learning - All Rights Reserved
5
ECOLOGICAL SUCCESSION
What Do You Know?
Put the following images in order for the process of secondary succession, starting with
number 1 for the image that represents a natural disaster. Below the images, describe the
typical organisms that grow in each step.
1:
2:
3:
4:
5:
© 2013-2014 Accelerate Learning - All Rights Reserved
6
ECOLOGICAL SUCCESSION
connecting with your child
Measuring Biodiversity
Biodiversity can be challenging to measure
and often requires complex calculations.
However, one simplified way to estimate
biodiversity is to count the number of
different types of organisms present in
an area. This estimation can be used to
compare different areas and identify which
has a greater biodiversity.
1. Obtain the following materials:
a. 10 stakes, posts, or twigs —
anything you can use to mark a
point in the soil.
b. Twine, yarn, rope—any material
that can measure a straight
distance is fine. You should cut
three pieces to the following
lengths: 0.5 m, 3 m, 6 m.
2. Identify two different locations in a
natural area, such as the bank of
a stream, a desert ecosystem, or a
forest floor community. If possible,
identify two locations at different
stages of succession. If not possible,
any two locations can be used.
3. Use the 0.5-m length of twine and
seven posts to make a circle with a
radius of 0.5 m.
a. Use one of the posts as the central
point of the circle. Attach the twine
to this post and mark six points on
the outside of the circle with the
remaining posts.
4. Identify and count all the herbaceous
plants and seedlings in this circular
area. Herbaceous plants and
seedlings are defined as those having
stems with a diameter less than
2.5 cm.
a. If the plant is easy to identify, then
do so; however, it is not essential
that you identify the plant correctly.
You should try to determine how
many different species and how
many of each species there are.
These plants can be identified
simply as “herb 1,” “herb 2,” “shrub
1,” etc.
5. Identify and count any animals in the
circle. To do this, you may have to dig
through the litter on the soil surface.
You will likely find many types of
invertebrates living on or just under
the soil, such as worms, ants, or
beetles.
6. Using the original center post, mark
an expanded circle with a radius of
3 m.
7. Identify and count the different types
of shrubs, vines, and saplings present
in the plot. These are defined as
having a stem diameter of 2.5–10 cm.
8. Using the original center point, mark
an expanded circle with a radius of
6 m.
© 2013-2014 Accelerate Learning - All Rights Reserved
7
ECOLOGICAL SUCCESSION
9. Identify and count the different types of trees present in the plot. The data can be
entered in a table similar to the one below.
Type
Number present
in Area 1
Number present
in Area 2
Herb 1
Herb 2
Herb 3
Animal 1
Animal 2
Shrub 1
Shrub 2
Tree 1
Here are some questions to discuss with your child:
• Which area has greater biodiversity?
• What components of the environment may account for this variation in biodiversity?
(For example, does one area have a more readily available water source?)
• If areas in different stages of succession were selected, what is the correlation between
stages of succession and biodiversity?
© 2013-2014 Accelerate Learning - All Rights Reserved
8