Chapter 40 (Communities and Ecosystems), Pages 809-811
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Ecosystems in Time
The content of today's lecture will help you answer
question #3 on
Note that exam questions and written assignments will be
the learner objectives included in this lecture outline. Not all the
questions provided at the chapter ends in the text or on the text web
pages may be appropriate study aids. Use those that reflect the lecture
After studying this material you should be able to:
Describe the effects of disturbance, or lack there of, in
and managed) ecosystems and explain its relationship to the process of
Define the term 'invasive species', give an example of an
plant or animal and describe its impact on an ecosystem.
Distinguish between the terms "primary succession"and
succession" and describe some examples of each.
Distinguish between the terms "soil" and "mineral
Describe how pioneer species in primary and secondary
change nonliving components of an ecosystem (temperature, light,
moisture, humidity, mineral substrate, etc.) during the early stages of
Give an example of a biological community that is
maintained in an early stage of succession by repeated disturbance and
describe the disturbance regime and its effect on the community.
Disturbances and Community Change
Community change is initiated by disturbances of all
Ecological Succession - Overview
From the Latin, succedere, to follow after
"Change in the species composition of a community over time."
Primary Succession follows the formation of new
consisting of rock, lava, volcanic ash, sand, clay, or some other
exclusively mineral substrate.
This means that there is NO SOIL present.
Soil is a mixture of mineral material, decaying organic
and living organisms.
Secondary Succession follows the destruction or
destruction of the vegetation of an area by some sort of disturbance,
fire, windstorm, or flood that leaves the soil intact.
Pioneer species initiate recovery following
both primary AND secondary successions
Pioneers "pave the way" for later colonists by altering the
and abiotic environment:
Species composition tends towards a
through succession resulting in the development of the vegetation
common for the biome.
The climax community describes an end product of succession
persists until disturbed by environmental change.
Succession occurs at large scales involving higher plants and
animals, but may involve microbial communities on a smaller scale.
Illustration of Primary Succession from Eastern
The processes involved in changing an area from one
community (no plants, no animals, no insects, no seeds, AND NO SOIL) to
one consisting of individuals, populations, communities, and
ecosystems. CAUTION!! The text definition of
succession is misleading.
Starts WITHOUT SOIL.
This is the confusing part - there may have been a previous
community, but if a disturbance removes or in some way covers the soil
so only mineral substrate is left to support pioneer plants we would
classify it as Primary succession.
No organic matter, only mineral material (e.g. sand, bare
gravel from glacial outwash, volcanic ash and lava.
PIONEER PLANTS of primary succession (e.g. lichens
Illustration of Secondary Succession from Eastern
Follows disturbance of an existing community that removes
the vegetation, but does not remove, destroy, or cover the soil.
Starts WITH SOIL.
PIONEER PLANTS of secondary succession (the first
become established after the disturbance) start from roots or seeds
remaining in the soil or from seeds carried in by wind or animals from
Faster than primary succession.
Many communities are often held at earlier successional
stage by repeated but unpredictable disturbances that prevent
succession from reaching the climax community that might be expected
for the climate of the area.
The species are adapted to the dominant modes of
disturbance in their communities. A change in the distubance
regime -- whether it be an increase or a decrease -- can result in the
loss of biodiversity and species extinction.
The original prairies of Illinois are an example of such a
community. The early successional grass and perennial plants are fire
tolerant because of their underground roots and stems. Repeated fires
destroy shrubs, young trees, and other plants that would change the
environment and result in further successional changes that would
eventually result in the establishment of a deciduous forest.
Agricultural practices are essentially an artificial form
of disturbance. Crops like corn and soybeans as well as the common
weeds found in agricultural fields have the characteristics of pioneer
species and require repeated soil disturbance.
The three factors that characterize a disturbance regime
size, frequency, and intensity of disturbance.
Fire supression in western forests has caused a build up
that promotes large infrequent, high-intensity crown fires rather than
small, frequent, low-intensity understory fires. In these situations
restoration of the natural fire regime can be difficult and costly,
but failing to do so is a risk both to humans and nature.
A summary of changes that occur during
Pioneer species colonize first.
Pioneer species alter the environmental conditions
Eventually new species of plants become established in
conditions altered by the pioneer species and displace the pioneer
Animals come in with or after the plants they need to
Further environmental change by the new plants and
the establishment of different species.
With infrequent disturbance, a stable climax community
plants and animals that can reproduce themselves in the existing
conditions will become established.
Disturbance of the ecosystem will start the process of
In a given area there are usually small patches of
stages of succession, depending on the time and severity of the last
disturbance. This adds diversity in the types of vegetation and animals
living in the greater region.
Various stages of succession in one area