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Rainforest canopy trees in Peru. (Photo by R. Butler)
CANOPY STRUCTURE
The canopy system characteristic of tropical rainforests
further increases diversity by creating new niches in the form of new
sources of food, new shelters, new hiding places, and new areas for
interaction with other species. In fact, it is estimated that 70-90
percent of life in the rainforest is found in the
trees. One of the best examples of a canopy niche which multiplies diversity
are the epiphytes, many of which form tiny ecosystems of their own.
The tank bromeliads of New World forests can hold over eight liters
(two gallons) of water in catchments formed in their stiff, upturned
leaves. These pools of water serve as nurseries for frog tadpoles and
insect larvae specifically adapted to life in this tiny obscure niche,
and provide water for millions of other canopy dwellers. Over 28,000
epiphyte species are known to science, although many more have never
been catalogued.
In addition to epiphytes, other plant species including lianas and creepers,
create new means for ground-dwelling animals to access the resources
of the canopy. Many of the ground-dwelling animals of the temperate
zone, like porcupines, kangaroos, anteaters, earthworms, and crabs,
have moved up into the canopy in tropical regions.
AREA
The size of a habitat is another factor in the great diversity of the
rainforest. Area increases diversity because a larger plot is likely
to have more habitats, hence niches, to support a greater variety of
species. In addition, many species require a large range for adequate
prey or seed forage. The basis for this idea was set forth by MacArthur
and Wilson in The Theory of Island Biogeography (1967) using
small islands in the Florida Keys. Soon after the work was published,
research focused on whether island biogeography could be applied to
fragments of habitat. Evidence for this concept was found in an experiment
devised by Thomas Lovejoy in the late 1970s. The experiment was known
as the Minimum Critical Size of Ecosystems Project and measured ecosystem
decay in forest patches ranging in size from 2.5 acres (1 hectare) to
2,500 acres (1,000 hectares). During the late 1970s the Brazilian government
was encouraging widespread clearing of rainforest by offering tax incentives
to landowners. However, in an area known as the Manaus Free Zone, just
north of the Amazonian city of Manaus, the government required that
50 percent of the forest on a developed area must be saved.
Lovejoy used this stipulation for his experiment, convincing landowners
to leave their required forest patches in neatly cut squares.
The experiment, today known as the Biological Dynamics of Forest Fragments
Project, found that the most seriously degraded forest with the least
diversity were the smallest, one-
hectare reserves, while the reserves that retained the most diversity
were the ones of the largest area. In the smaller reserves, drying winds
reached the interior, affecting tree species and resulting in more tree
falls. Gaps in the canopy allowed more sunlight to reach the forest
floor, further altering the understory microclimate and causing changes
in the makeup of resident species. Larger herbivores left the patches
since the limited number of trees could not provide sustenance, soon
followed by predators, which could not cope with the loss of prey. The
loss of predators caused an imbalance in the food chain,
and the populations of small herbivores and omnivores increased, adding
pressure on forest seed banks and impairing the reproducing ability
of forest trees. Troops of army ants could not be supported by meager
forest patches and they too left, along with the bird, butterfly, and
other insect species that depended on the troop. Shade-
loving plants and animal species died off as more sunlight penetrated
the diminished canopy, and "gap" species, like vines and certain
bird and insect species, proliferated. These losses continued to set
off a chain reaction that caused profound changes in the system, eventually
resulting in its collapse.
Similar experiments carried out around the world have yielded similar
results (although in some cases diversity among certain groups may actually
increase). The colonization of forest patches by forest-
edge species, light-gap specialists, and savanna species can
counter the loss of species less tolerant of the changed forest and
maintain the diversity of the patch. In some cases, forest fragment
diversity may hold steady, but overall (global) diversity declines as
some unique species lost from the forest patch are not replaced. Floor-
dwelling species appear more affected by forest fragmentation than canopy
species. Declining biodiversity in accordance with decreasing land area
is an important trend to consider for conservation (see section
10).
In global studies, larger forest patches lost fewer of their species.
Diversity declined but at a rate and to a degree inversely proportional
to the size of the patch. In other words, the larger the patch, the
more organisms survived and were successful in reproducing. Thus these
experiments demonstrated that the area of an ecosystem directly affects
biodiversity.
SOILS
The soils of a rainforest affect the diversity of the forest.
Although nearly 70 percent of tropical rainforest exists on
poor acidic soils, it retains its fertility in a large part thanks to
nutrient recycling and other processes. However, in some areas, soils
are so poor that only a limited number of tree species can grow (though
these forests are still highly diverse by temperate standards). One
example is the so-called "white-sand" or "blackwater"
forests that grow on rocky, sandy soils. Some of these forests grow
on nothing but rocks and the roots of other trees. Trees that grow under
these conditions tend to be species with tannins in their leaves, which
in turn, turn local rivers into "blackwater" rivers. The bitter
tannins in their leaves limit insect populations, thus reducing
the number of animals the forest can support (insects serve as a major
food source for larger animals in most rainforests). These "blackwater"
forests are self-perpetuating, since the "blackwater" rivers
that result from the decay of their leaves only make the soils more
acidic and prevent other tree species from growing
on the already nutrient-lacking soils.
Forest tree diversity, and hence total diversity, may also be reduced
in forests with soggy soils like those of the igapò or "swamp
forest." The limited number of tree species like Cecropia and palms
that can tolerate these wet soil conditions means that these few trees
species tend to dominate these areas. Subsequently only the animals
that feed on their fruits, leaves, and seeds are abundant in these areas.
High-diversity forests are often found on nutrient rich—sometimes volcanic—
soils that are well-drained. These forests are frequently found in areas
protected from major disturbances like strong wind and regular flooding.
Review questions:
How does the canopy amplify rainforest biodiversity?
How does area impact biodiversity?
Does forest fragmentation reduce forest diversity?
Biodiversity key to fighting climate change
(4/29/2008) Scientists from Brown University have discovered that an ecosystem's productivity is directly linked to its diversity of plant species. The discovery has granted biodiversity new importance in the fight against climate change: the more productive the ecosystem the more carbon it captures.
Saving the world's most recently discovered cat species in Borneo
(4/10/2008) Last year two teams of scientists announced the discovery of a new species of clouded leopard in Borneo. The news came as conservationists launched a major initiative to conserve a large area of forest on an island where logging and oil palm plantations have consumed vast expanses of highly biodiverse tropical rainforest over the past thirty years. Now a pair of researchers are racing against the clock to better understand the behavior of these rare cats to see how well they adapt to these changes in and around Danum Valley in Malaysia's Sabah state. Andrew Hearn and Joanna Ross run the Bornean Wild Cat and Clouded Leopard Project, an effort that aims to understand and protect Borneo's threatened wild cats, which include the flat-headed cat (Prionailurus planiceps), marbled cat (Pardofelis marmorata) leopard cat (Prionailurus bengalensis) the endemic bay cat (Catopuma badia) and the Bornean clouded leopard (Neofelis nebulosa).
Bats protect crops from insects
(4/4/2008) Bats eat as many insects at night as birds do during the day, according to research published in the journal Science.
Bats eat as many insects as birds
(4/3/2008) Bats eat as many insects at night as birds do during the day, according to research published in the journal Science.
'CAT scan' shows Hawaiian forests invaded by alien species
(3/3/2008) Invasive plant species are altering the ecology of Hawaiian rain forests, reports a study published in the Proceedings of the National Academy of Sciences (PNAS).
