Succession by prw4sQv

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									        SELF-STUDY GUIDE TO ECOLOGICAL SUCCESSION
                                (for Cunningham & Saigo, 10th edition)
Introduction
        The purpose of this guide is to provide a tutorial on ecological succession. The only resource
you will need is your textbook. You may work through the guide alone, or with a group of other
students. This material will not be covered in lecture, so study it carefully and ask me if you have any
questions about it.

Ecological Succession
         Ecosystems are damaged by many impacts, some natural, some caused by humans. Examples of
natural disturbances are volcanic eruptions (such as Mt. St. Helens) or forest fires started by lightening.
Humans disturb pre-existing ecosystems when we clear land for farms or when we dam rivers and create
artificial lakes. Such disturbances kill or remove many of the ecosystem’s plants and animals, and alter
environmental conditions such as soil and light. The result can be a very different ecosystem or, in
severe cases, no surviving system at all.

        Our skin has a built-in ability to repair damage. Can ecosystems repair damage done to them, or
are they permanently altered by disturbances? The answer lies in the process of ecological succession.

How does your textbook define ecological succession? (try the Glossary)



This definition is a good general one, but it leaves out the idea that ecological succession often enables a
disturbed ecosystem to return to its original state. It is a process that repairs damage. Let’s see how this
occurs.

        There are two types of ecological succession, primary and secondary. Read about them and then
define them in your own words:

   Primary succession:




   Secondary succession:



What do they have in common?



What is the key difference between them?



Think of an example of each that is not discussed in your textbook. Is the disturbing factor natural or
human?
Bio. 112                                                                 Ecological Succession, p. 2

Successional plant communities
        The first plant species to establish themselves in a region undergoing succession are termed
pioneer species. The term is taken from human society, where pioneers are people who establish
themselves in an area that was previously unsettled. Imagine that you are a plant trying to grow in a
recently disturbed site - what is the environment like for you if the site is:
         the top of a boulder:



         a sand dune:



         an abandoned farm field:



        In cases of primary succession, the initial environment is harsh, mostly because of the absence of
soil. Pioneer plants there suffer from lack of moisture, nutrients, and a way of anchoring their roots.


According to your textbook, what types of plant might begin primary succession on bare rock?


How do you think they cope with the harsh conditions present there?



         Now, here is the key idea of primary succession: pioneer species alter the environment so that a
different set of species can invade. How, according to the textbook, do lichens and mosses change a bare
rock site so that other plants are capable of living there?




        In cases of secondary succession, the soil is usually intact, so that conditions are not so difficult.
The pioneer species are normally those that get there first. Many plants that we call “weeds” are species
adapted to invading open sites of secondary succession rapidly. They use two different techniques:
         Seed dispersal: think of dandelion seeds - how are they adapted for reaching open patches of
        ground?


         Resistant seeds: many weeds have seeds that can remain viable while buried in the soil for
        long periods of time.


        In either primary or secondary succession, there’s plenty of sunlight, so the pioneer species are
(1) sun-loving and (2) fast-growing. Later species take longer to get there and longer to grow to full size.
Bio. 112                                                             Ecological Succession, p. 3

Species replacement during succession
        You should understand now that pioneer species change the environment so as to favor the
growth of new plant species (call them mid-successional species). But why do these species replace the
pioneers? Why can’t the pioneers continue living there? The main answer to this question is simple:
shade. Each succeeding group of plants is taller than the previous species and out-competes them for
light. In fact, one author has described ecological succession, particularly secondary succession, as a
“race for the sun.”

        Here’s a sequence of plants for a typical example of primary succession, a beach dune in
northern Indiana:

        beach grasses -> shrubs -> cottonwoods -> pines -> oaks -> beech/maples
          (pioneers)                                                  (climax)

Complete the table below during the class field trip: list a typical sequence of species for secondary
succession in an abandoned field in western New York State. What is the approximate timing of each
stage? How does the site and its environment change during the successional series?

           Dominant Plant Species          Time (years)             Community Characteristics




Other notes:
Bio. 112                                                               Ecological Succession, p. 4

Characteristics of pioneer and climax species

The characteristics of the plants change predictably during both primary and secondary sequences:

               Pioneer species                                  Climax species
   Adapted to dry soils with few nutrients               Require moist, fertile soils

   Sun-loving; can’t grow in shade                       Shade-loving; seedlings can grow under shade
                                                              of adults
   Full-grown plants short, cast little shade            Full-grown plants tall, cast dense shade

   Rapid growth; little permanent (woody) tissue         Slow growth; trees with large woody trunks

   Most energy put into growth and reproduction          Most energy put into maintaining adult plants

Describe, in a few sentences, how these characteristics adapt each type of species to the conditions
present during their position in the succession process:




Why do mid to late-successional species put so much of their energy and nutrient resources into woody
tissue?




The “endpoint” of succession
        Once the climax vegetation for a given region takes over, the pace of succession slows way
down. The ecosystem enters a period of stability or steady state. There’s a simple explanation for this
fact: once the area is taken over by a plant species which is adapted to the environment and has seedlings
capable of reproducing under the shade of adults of the same species, that species can block further
replacement. Climax species in the eastern U. S., such as oak, maple and beech trees, can live to ages of
several hundred years, so any remaining changes are slow. At the same time, a climax is not permanent.
New disturbances can remove the climax vegetation and begin the process of succession over again. For
example, a maple forest growing on an old sand dune can be damaged by severe storms, toppling trees
and “blowing out” the soil. A patch of the forest is thereby returned to bare sand and primary succession
begins again. Or, a forest fire may kill mature hardwoods and trigger new secondary succession.


Mosaic landscapes
       Imagine for a moment two extremes:
(1) Humans take an extreme position of environmental preservation and treat all disturbances as evil.
       Ecosystems are protected from all changes. Now wait 100+ years. What has happened to
       pioneer and mid-successional species, and why?



(2) Human disturbance of ecosystems is frequent and widespread. Forests are cut and land cleared for
       farming or development every 20-30 years. What sort of species are left in the region, and why?
Bio. 112                                                                Ecological Succession, p. 5

The pace of ecological succession
       It might seem as if we can damage ecosystems without concern, because they have this
marvelous ability to renew themselves through succession. There are two reasons to be cautious,
however. First, ecosystems take a long time to recover from disturbances. How long would it take a
hardwood forest in the eastern U. S. to recover from being completely cleared?

        In cold or dry climates the pace of succession is even slower; in some parts of Alaska which are
recovering from glaciers, fully fertile soils and climax communities take over 400 years to appear.

        Second, some disturbances are so severe or widespread that the ecosystem cannot recover. To go
back to the analogy of human skin, not all wounds heal. What is the message of Figure 14-23 (p. 321)?




Human management of ecosystems
       Ecological succession is more than a fact about ecosystems: it affects our lives in several ways.
For example, what would happen if you stopped mowing your lawn for a few years?




The time and energy required to maintain a lawn or golf course in the eastern U. S. is partly due to an
attempt to fight nature, to keep ecological succession from occurring.

        Our fight against weeds is also an example of succession at work in our lives. Weeds are pioneer
plant species. Weeds can be thought of as plants that (1) are good at dispersing their seeds rapidly into
freshly disturbed soil (gardens or farm fields); (2) have strong root systems; and (3) grow rapidly and
choke out the plants we want to grow. How are these the adaptations of pioneer species?




        We tend to maintain our gardens and farms in early successional stages for a good reason: early
successional species grow rapidly and put most of their energy into root, leaf and seed production, rather
than woody growth. Roots, leaves and seeds, of course, are what we mainly use for food. Even when we
grow trees, why do we prefer fast-growing, mid-successional species such as pines to climax species such
as oaks and maples?




        There is even some evidence that the development of cities may follow a path analogous to
ecological succession. Pioneer societies exhibit rapid population and economic growth. As cities “age”,
more of their resources (tax dollars??) go into maintaining the “infrastructure” (roads, utilities, police…)
and less is available for economic growth. Study the following graph of the costs of services
(=maintenance) per capita (person) for different size cities in Ohio. Is it possible that large cities are
reaching a “climax stage” in which they cost so much to run that there is no potential for future growth?
Bio. 112                                                            Ecological Succession, p. 6




Use the space below to summarize the major points you’ve learned about ecological succession in a brief
outline. Are there any unclear points?

								
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