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Forest light gap in Brazil. (Photo by R. Butler)
SHORT TERM VARIATION
Rainforests
and their diversity do not exist in a constant state, but are
the product of a series of impacts including fires, tree falls, small-scale
human clearing, and even lava flows. These events can increase forest
diversity by giving new species a chance to grow in the absence of the
towering canopy trees. The growth of new tree species spells new opportunities
for their symbiotic species (for example new pollinators or seed dispersers).
Forests that are regularly stressed, like those affected seasonally
by strong winds and storms, tend to be dwarfed with a less developed
canopy and reduced diversity. "Typical" tall rainforests
are typically found where
they are protected from strong winds, as
in valleys and certain geographical areas.
Within a relatively small area there can be great variations in forest
dynamics. For example, in the terra firme rainforests of the Central
Amazon—where average canopy tree age can exceed 300 years
and some trees can be more than one thousand years old—
forest turnover rates can be extremely low. In contrast, nearby floodplain
forests may have turnover rates of less than 70 years due to migrating
river channels that periodically undercut river banks and trees.
Diversity is usually sharply reduced in forests degraded by activities
such as logging, burning, and agricultural development. Generally, when
forest is logged, the dense canopy structure is disturbed, allowing
more sunlight to penetrate to the forest floor. The forest is more likely
to dry out, and less water can be recycled through the system of evaporation
and transpiration. Many rainforest species are unable to cope with the
changes in the forest microclimate and either move on or gradually perish.
In addition, the loss of certain valuable hardwood trees to logging
has a major impact on species with which they have interdependent relationships.
Studies suggest that logging in any form reduces tropical forest diversity—studies around the world show
declines of certain species, especially
primates, birds, and insects in degraded forests. While there may be
a local increase in the abundance and diversity of certain species,
there is an overall regional or global decline in biodiversity due to
the loss of species specially adapted to the conditions of undisturbed
forest. Degraded forest is also more prone to be developed or burned
by humans, severely reducing diversity. Heavy logging in the forests
of Indonesia and Brazil was partly responsible for creating
the dry forest conditions that drove the widespread forest fires of 1997-1998.
ECOTONES: FOREST EDGES
Recent studies suggest that ecotones, transition zones between habitats,
play an important role in the biodiversity of rainforests. Ecotones
bordering rainforests and savannas, secondary forests, plantations,
and other forest types are evolutionary hotbeds where evolutionary
competition may lead to the rise of new species. Scientists say that
populations in ecotones may specialize to the niche and diverge significantly
from populations of the interior of the forest. This new theory initially
appears to challenge the popular view that the ice ages had a highly
significant role in rainforest diversity. However, some scientists speculate
that the receding forest and fragmentation of the ice ages would create
a larger area of ecotones, contributing further to biodiversity. Therefore
the combination of both conditions may have contributed to the well-
known diversity of tropical rainforests.
ICE AGES/GLACIATION
The relative age of a tropical rainforest plays a role in its diversity,
although the role is still largely debated. Tropical rainforests are
probably the planet's oldest continuous ecosystems. Tropical rainforests
began to take their form some 140 million years ago during the age of
the dinosaurs, the late Cretaceous. It was during this period, when
much of the world's climate was tropical or sub-tropical, that flowering
plants originated and later spread across the globe.
Over their long history, species have come and gone, communities
have been destroyed and reformed, and entire systems altered. Along
with the changes, new relationships within the system form as new species
emerge. Generally the changes are relatively slow, although there have
been times of upheaval where drastic change occurred over a short period
of time. These natural upheavals appear to foster an increase in biological
diversity as evidenced by the effect of the ice ages,
especially on the Malay Archipelago in Southeast Asia.
Today, many of the 20,000 or so islands of the Malay Archipelago
are covered with tropical forest. Some of these rainforests have existed
in some form or another for the past 100 millions years, although, as
discussed in section
one, the ancient forests had fewer large mammals and no flowering plants.
When the ice ages came and ocean waters condensed or became locked up
in polar ice, the floor of the shallow South China
Sea was exposed, allowing the crossover of species from mainland
Asia. Although this region was less affected by the temperature drop
than other areas because of its proximity to the ocean and the equator,
the climate cooled significantly enough to cause tropical rainforest
to recede to scattered patches. The areas formerly forested with tropical
rainforest gave way to savannas and montane forest ecosystems. Most
of the region had a distinct short rainy season.
The IPCC explains what causes the sea level to change. Image courtesy of the IPCC.
When the ice ages came
to an end, a warmer climate returned and the ocean rose again to reflood
the shallow areas of the South China Sea. Many of the plants and animals
that had crossed over from the mainland were trapped on the reformed
island habitats. In addition, some of the montane and more temperate
species adapted to the gradually warming climate and became tropical
species. The small pockets of tropical rainforest that survived the
ice ages served as biological reservoirs to repopulate the expanded
tropical forest zone. Some of the tropical species that had been separated
into different pockets had radiated enough during their isolation that
when they did again cross paths, their habits and physiological features
had changed enough (adapted to their
niche within the tropical pocket) that they could no longer
breed, and could be considered distinct species.
Borneo's Elephants
A 2003 study suggests that Borneo's elephants may be a distinct population, isolated from mainland Asian elephants when Borneo was cut off from the mainland around 18,000 years ago.
Diversity was again
multiplied by subsequent ice ages which caused isolation and
subsequent adaptive radiation into more distinct species. For example,
take a hypothetical elephant species that began
as a single species on mainland Asia. During the ice ages, it expanded
its range to some of the islands of the Malay Archipelago, which, with
the lower ocean levels, had become connected to the mainland. When the
ice ages came to an end, elephants became stranded on the islands. On
the smaller islands, those elephants with a smaller body size tended
to survive and reproduce more successfully because their lower dietary
requirements could be sustained by the smaller amount of food available
on the island. The larger individuals tended to be less successful reproductively.
Thus evolution favored the dwarfing of elephants on the islands over
the course of several thousand years and when the next ice-age crossover
came, the island elephants would not breed with
the mainland elephants. Since the island elephants filled a different
role on the Asian mainland during the crossover, some dwarfed island
elephants remained on the mainland during the next drop in water levels.
These elephants, now isolated from their island ancestors could diverge
enough to be unable to breed with the island elephants during the next
crossover. Thus over the course of two ice ages, one species of elephant
became three, not considering the other forms that would develop on
islands with different niches, like those with more mountainous terrain,
swampy bogs, or different plant species on which to feed. And so the
process of evolution through geographic isolation, continues, and more species are formed.
The Amazonian rainforest was affected in a different way than Southeast
Asia by the ice ages because the change in sea level did not play the
same role as in the existence of islands. Instead, the cooler temperatures
may have lead to a great contraction of the tropical rainforest and
resulted in its replacement with savanna. During the ice
ages, carbon dioxide levels drop by as much as 50 percent,
causing the majority of plants, which require high levels of carbon
dioxide (known as C3 plants) to decline. Some plants, known as C4 plants,
especially grasses, grow well under low carbon dioxide conditions. Thus
(according to a leading theory), when carbon dioxide levels dropped
during glacial periods, rainforests full of C3 plants
retreated and savanna grasses (C4 plants) expanded their range. Rainforest
was broken up into islands separated by savanna, while communities of
species were divided in isolated pockets. Some communities diverged
and when the forests expanded and the communities were rejoined, they
were altered enough so they could or would not breed.
This "refugia" theory, though plausible and supported by some
pollen evidence, is not universally accepted. Recent studies in a few
limited locations suggest that the Amazon may have remained densely
forested during the past ice age. Recently Hooghiemstra and van der
Hammen (1999) suggested that pollen evidence supports both theories
and both scenarios may have occurred in different parts of the Amazon
basin and at different periods of time.
A new theory proposed in December 2005 argues that Amazon rainforest biodiversity
has much less to do with climate change than it does
with the biology of native species and the forest itself. Looking at
the "DNA-clock" of butterfly species in the Amazon basin,
scientists from University College London concluded that rainforest
butterflies evolved at very different rates, a finding that suggests
their evolution is largely independent of external factors like the
ice ages. Lead author of the study, Jim Mallet, says that research "rules
out geographic isolation caused by past climate change as the main cause
of species evolution. Instead the evolution of species must largely
be caused by intrinsic biological features of each group of species."
Review questions:
How can climate change affect the distribution of species?
Unilever calls for ban on rainforest destruction for palm oil
(5/1/2008) Unilever, the world's largest consumer good company, will start using palm oil from certified sustainable sources this year and aims to have all its palm oil certified by 2015, according to a speech delivered today by CEO Patrick Cescau.
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.
Photos of newly discovered species in Brazil's Cerrado
(4/29/2008) An expedition to Brazil's Cerrado has turned up more than a dozen undiscovered species. Conservationists say the discoveries add urgency to protecting the grassland habitat which is rapidly being converted for agriculture.
