Environmental Engineering Reference
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detritus left from the meals of the anemones. The sea
anemones benefit because the clownfish protect them
from some of their predators.
A third example is the highly specialized fungi
that combine with plant roots to form mycorrhizae
(from the Greek words for fungus and roots). The
fungi get nutrition from the plant's roots. In turn, the
fungi benefit the plant by using their myriad networks
of hairlike extensions to improve the plant's ability to
extract nutrients and water from the soil (Figure 6-7c).
In gut inhabitant mutualism, vast armies of bacteria
in the digestive systems of animals break down (di-
gest) their food. The bacteria receive a sheltered habi-
tat and food from their host. In turn, they help break
down (digest) their host's food. Hundreds of millions
of bacteria in your gut help digest the food you eat.
Thank these little critters for helping keep you alive.
It is tempting to think of mutualism as an example
of cooperation between species. In reality, each species
benefits by exploiting the other.
Commensalism: Using without Harming
Some species interact in a way that helps one species
but has little, if any, effect on the other.
Commensalism is a species interaction that benefits
one species but has little, if any, effect on the other spe-
cies. One example is redwood sorrel, a small herb. It bene-
fits from growing in the shade of tall redwood trees,
with no known harmful effects on the redwood trees.
Another example involves epiphytes (such as some
types of orchids and bromeliads), plants that attach
themselves to the trunks or branches of large trees in
tropical and subtropical forests (Figure 6-8). These air
plants benefit by having a solid base on which to grow.
They also live in an elevated spot that gives them bet-
ter access to sunlight, water from the humid air and
rain, and nutrients falling from the tree's upper leaves
and limbs. Their presence apparently does not harm
the tree.
Figure 6-8 Natural capital: commensalism. This bromeliad—
an epiphyte or air plant in Brazil's Atlantic tropical rainforest—
roots on the trunk of a tree rather than the soil without penetrat-
ing or harming the tree. In this interaction, the epiphyte gains
access to water, other nutrient debris, and sunlight; the tree ap-
parently remains unharmed.
All communities change their structure and composi-
tion over time in response to changing environmental
conditions. The gradual change in species composition
of a given area is called ecological succession.
Ecologists recognize two types of ecological
succession, depending on the conditions present at
the beginning of the process. Primary succession in-
volves the gradual establishment of biotic communi-
ties on nearly lifeless ground, where there is no soil in
a terrestrial community (Figure 6-9) or no bottom
sediment in an aquatic community. Examples include
bare rock exposed by a retreating glacier or severe
soil erosion, newly cooled lava, an abandoned high-
way or parking lot, or a newly created shallow pond
or reservoir.
Primary succession usually takes a long time—
typically thousands or even tens of thousands of years.
Before a community can become established on land,
there must be soil. Depending mostly on the climate, it
takes natural processes several hundred to several
thousand years to produce fertile soil.
With the other, more common type of ecological
succession, called secondary succession, a series of
communities with different species can develop in
places containing soil or bottom sediment. This devel-
opment begins in an area where the natural commu-
Review the ways species can interact and see the results
of an experiment on species interaction at Environmental
ScienceNow.
6-4 ECOLOGICAL SUCCESSION:
COMMUNITIES IN TRANSITION
Ecological Succession: How Communities
Change over Time
New environmental conditions can cause changes in
community structure that lead to one group of species
being replaced by other groups.
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