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3.2: Balance and Change in Ecosystems
pg. 82
Equilibrium: is the state of an ecosystem in which the biotic and abiotic
features remain relatively constant over a long period of time.
Succession: are the observed changes over time in an ecosystem, following
a disturbance.
A State of Equilibrium
Biomes are usually in a state of equilibrium. Energy flows through the
ecosystems, and nutrients are recycled through food webs in a constant
manner. The cellular processes, cellular respiration and photosynthesis, are
balanced. Populations are healthy and stable when ecosystems are in a state
of equilibrium.
Short Term Changes
Smaller ecosystems are usually in state of influx or change. The biotic and
abiotic factors can cause short-term changes. Examples are volcanic
eruptions, forest fires, and diseases, which can change the number and types
of species in small ecosystems. Population size can also change over time.
The Process of Change
Volcanic eruptions can drastic change the state of equilibrium of an area;
such as the ST. Helens eruption in the United States. The eruption was a
drastic change, but over time the changes were not permanent and the
ecosystem recovered.
When ecosystems form in areas that did not have life before, or in areas that
were wiped out by a catastrophe, and then reformed is called ecological
Biotic and Abiotic factors that are reestablished after a disturbance lead to
ecological succession. These disturbances can be natural (forest fire, earth
quake, and volcanic eruptions) or caused by human actions (deforestation,
agriculture, and development). Succession benefits an ecosystem because it
may increase the number and variety of species living there.
Ecological succession sees the reestablishment through a series of steps or
progressions from one type of ecosystem to another. (grasses, shrubs and
young trees, pine forests, and maple-beech forests)
Figure 3: Each square of land in the diagram shows a new time period of the progression
from grassland to forest ecosystems
Fire and Ecosystems
In some ecosystems; forests and grasslands, fire can play an important role.
Some plants require fire (heat) to release seeds or germination. Fire damaged
trees can also become habitats to other organisms, as well as nutrients for the
Lightening strikes can cause fires which burn larger trees but give
opportunity to smaller younger trees to survive by giving more space and
sunlight. More food is then available for animals to eat and survive.
Setting Fire Management Policies
Once it was thought that forest fires should be fought and extinguished as
quickly as possible, not understanding the natural cycle, but once understood
fire fighting polices have changed and now fires are allowed to burn in a
controlled manner. The Natural Burn Policy establishes the control burn of
areas by wildlife and forest managers, which started on purpose to mimic
natural fires.
Wildfires and Ecosystem Health
Aboriginal and early settlers would burn grassland areas to remove old dead
grasses and other plants. These burns allowed for early plant growth in
larger numbers in the spring and recycle nutrients back into the soil to
support the new growth. These new grasses and plants becomes a source of
food for animals and humans.
Humans and Succession
Succession allows ecosystems to recover from disturbances and keeps them
in balance over long periods of time. Some human disturbances can not be
repaired by natural ecological succession, sometimes humans need to take
action to help restore the area. Humans can plant trees in deforested and
mining areas and management processes.
Case Study: The Milton Quarry
The Milton Quarry was brought back through human support. Engineers
reshaped the quarry cliffs, and the area was restored to encourage the growth
of different types of habitats (wetlands, wooded areas, and lakes). With the
new habitats development, many species of organisms returned (insects,
birds, and mammals).
Check Your Learning:
Questions 1 – 6, pg. 87
Wrap Up:
- When the biotic and abiotic parts of an ecosystem remain relatively
constant over time, they are in equilibrium.
- Ecological succession is the natural, gradual changes in the biotic and
abiotic features of an ecosystem following a disturbance.
- Human activity can upset the state of equilibrium of an ecosystem, and it
can interrupt the natural process of succession.
- Fire management policy must consider the relationship between wildfires
and ecosystem health.
- Ecological succession allows an ecosystem to recover following a human
or natural disturbance.