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Rainforest along the bank of the Tambopata river in Peru. (Photo by R. Butler)
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Structure and Character
CANOPY STRUCTURE
Rainforests are characterized by a unique
vegetative structure consisting of several vertical layers including
the overstory,
canopy, understory, shrub layer, and ground level. The canopy refers to the dense ceiling of leaves and tree branches formed
by closely spaced forest trees. The upper canopy is 100-130 feet above
the forest floor, penetrated by scattered emergent trees, 130 feet or
higher, that make up the level known as the overstory. Below the canopy
ceiling are multiple leaf and branch levels known collectively as the
understory. The lowest part of the understory, 5-20 feet (1.5-6 meters)
above the floor, is known as the shrub layer, made up of shrubby plants
and tree saplings.
The heavy vegetation of the canopy effectively screens light from the
forest floor, and in a true (primary) equatorial rainforest, there is
little jungle-like ground growth to impede movement. Ground vegetation
in primary forest is minimal and usually consists mainly of lianas (vines)
and tree seedlings.
An important characteristic of the canopy system is the presence of
plants known as epiphytes, that grow on canopy trees. Epiphytes are not parasitic because they
draw no nutrients away from the host, but use the host tree only for
support. High in the canopy, epiphytes are better able to access the
strong tropical sunlight, which they require for growth. Epiphytes have
adapted well to their aerial environment, developing various means to
collect nutrients from their surroundings, the mechanisms for which
are discussed in detail in the canopy section.
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The rainforest canopy. Image by R. Butler
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An additional plant type characteristic of the canopy system is the liana—a sort of woody vine that begins life as a shrub on the forest
floor and makes its way up to the canopy by latching on to canopy trees.
A related plant type, the hemiepiphyte, begins life in the canopy and
grows long roots that eventually reach the forest floor. Once rooted,
hemiepiphytes do not have to rely on capturing nutrients from their
canopy surroundings, but can access nutrients from the forest floor.
Unknown numbers of plants and animals reside in the canopy, the vast
majority of which are specifically adapted to life in this leafy world.
In tropical rainforests, it is estimated that 90 percent of
the species that exist in the ecosystem reside in the canopy. Since
the tropical rainforests are estimated to hold 50 percent
of the planet's species, the canopy of rainforests worldwide may hold
45 percent of life on Earth.
INTERDEPENDENCE AND COMPLEX SYMBIOTIC RELATIONSHIPS
Interdependence—whereby all species are to some extent
be dependent on one another—
is a key characteristic of the rainforest ecosystem. Biological interdependency
takes many forms in the forest, from species relying on other species
for pollination and seed dispersal to predator-prey relationships to
symbiotic relationships.
Agouti in forest clearing
Brazil nut pods
Brazil nut tree
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These interdependent
relationships have been developing for millions of years and form the
basis for the ecosystem. Each species that disappears from the ecosystem
may weaken the survival chances of another, while the loss of a keystone species—an organism that links many other species together, much like the keystone
of an arch—could cause a significant disruption in the
functioning of the entire system.
For example, Brazil nut trees (Bertholletia excelsa) are dependent
on several animal species for their survival. These large canopy trees
found in the Amazon rainforest rely on
the agouti, a ground-dwelling rodent, for a key part of their life cycle.
The agouti is the only animal with teeth strong enough to open their
grapefruit-sized seed pods. While the agouti eats some of the Brazil
nut's seeds, it also scatters the seeds across the forest by burying
caches far away from the parent tree. These seeds then germinate and
form the next generation of trees. For pollination, Brazil nut trees
are dependent on Euglossine orchid bees. Without these large-bodied
bees, Brazil nut reproduction is not possible. For this reason, there
has been little success growing Brazil nut trees in plantations—they only appear to grow in primary rainforest.
Life in the rainforests is competitive and countless species have developed
complex symbiotic relationships with other species in order to survive.
A symbiotic relationship is a relationship where both participant species
benefit mutually. Symbiotic relationships appear to be the rule and
not the exception in the rainforest. For example, ants have symbiotic
relationships with countless rainforest species including plants, fungi,
and other insects. One symbiotic relationship exists between ants and
caterpillars. Certain caterpillar species produce sweet chemicals from
"dew patches" on their backs, upon which a certain ant species will feed. In return, the ants vigorously protect the
caterpillar and have even been observed carrying the caterpillar to
the nest at night for safety. This relationship appears to be species
specific in that only one caterpillar species will cater to a particular
ant species.
DIVERSITY
All tropical rainforests are characterized by tremendous biological
diversity. Section 3 concentrates on the diversity
of the tropical rainforest.
Review questions:
- Most of the plant and animal species live in what level of the rainforest?
- What are epiphytes?
- What is an example of an epiphyte? (Hint: think of a popular kind of flower)
- What are lianas?
- What is a symbiotic relationship?
- What is a keystone species?
- Why are agoutis important in the rainforest ecosystem?
[print version | spanish | french | portuguese
| chinese | japanese]
Continued: Rainforest biodiversity
This article was written by Rhett A. Butler [bibliographic citation for this page] and was last updated on the most recent date listed in the column on the right side.
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Recent news
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