Rainforest canopy trees in Peru. (Photo by R. Butler)
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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.
New research suggests rich Amazon soils
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?
- How do soils affect forest diversity?
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Continued: Short-term Variation, Ice Ages, Ecotones
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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WEEKLY NEWSLETTER
INTERACT
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Recent news
Majority of protected tropical forests "empty" due to hunting
(02/08/2012) Protected areas in the world's tropical rainforests are absolutely essential, but one cannot simply set up a new refuge and believe the work is done, according to a new paper in Bioscience. Unsustainable hunting and poaching is decimating tropical forest species in the Amazon, the Congo, Southeast Asia, and Oceana, leaving behind "empty forests," places largely devoid of any mammal, bird, or reptile over a few pounds. The loss of such species impacts the whole ecosystems, as plants lose seed dispersers and the food chain is unraveled.
New meteorological theory argues that the world's forests are rainmakers
(02/01/2012) New, radical theories in science often take time to be accepted, especially those that directly challenge longstanding ideas, contemporary policy or cultural norms. The fact that the Earth revolves around the sun, and not vice-versa, took centuries to gain widespread scientific and public acceptance. While Darwin's theory of evolution was quickly grasped by biologists, portions of the public today, especially in places like the U.S., still disbelieve. Currently, the near total consensus by climatologists that human activities are warming the Earth continues to be challenged by outsiders. Whether or not the biotic pump theory will one day fall into this grouping remains to be seen. First published in 2007 by two Russian physicists, Victor Gorshkov and Anastassia Makarieva, the still little-known biotic pump theory postulates that forests are the driving force behind precipitation over land masses.
Saving the world's biggest river otter
(01/30/2012) Charismatic, vocal, unpredictable, domestic, and playful are all adjectives that aptly describe the giant river otter (Pteronura brasiliensis), one of the Amazon's most spectacular big mammals. As its name suggest, this otter is the longest member of the weasel family: from tip of the nose to tail's end the otter can measure 6 feet (1.8 meters) long. Living in closely-knit family groups, sporting a complex range of behavior, and displaying almost human-like capricious moods, the giant river otter has captured a number of researchers and conservationists' hearts, including Dutch conservationist Jessica Groenendijk.
Big trees, like the old-growth forests they inhabit, are declining globally
(01/26/2012) Already on the decline worldwide, big trees face a dire future due to habitat fragmentation, selective harvesting by loggers, exotic invaders, and the effects of climate change, warns an article published this week in New Scientist magazine. Reviewing research from forests around the world, William F. Laurance, an ecologist at James Cook University in Cairns, Australia, provides evidence of decline among the world's 'biggest and most magnificent' trees and details the range of threats they face. He says their demise will have substantial impacts on biodiversity and forest ecology, while worsening climate change.
Protecting original wetlands far preferable to restoration
(01/26/2012) Even after 100 years have passed a restored wetland may not reach the state of its former glory. A new study in the open access journal PLoS Biology finds that restored wetlands may take centuries to recover the biodiversity and carbon sequestration of original wetlands, if they ever do. The study questions laws, such as in the U.S., which allow the destruction of an original wetland so long as a similar wetland is restored elsewhere.
More rainforest news
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