Geoscience Reference
In-Depth Information
diversity or richness is high, but species' distributions are
often restricted to small areas (
Plate 27.2
).
Coupled with diversity, studies of biological interac-
tions between species have demonstrated the complexity
and fragility of the system. The classic example of this
situation is the Brazil nut tree (
Bertholletia excelsa
). The
flowers of this tree need to be cross-pollinated with other
individuals of the same species. They have a complex
physical structure which means that they are largely
pollinated by the female long-tongued orchid bee during
the one-month flowering period in November. The bees
also pollinate other tree and orchid species which provide
nectar at other times. If only Brazil nuts were grown, as
on a plantation, there would be no suitable insects for
pollination, as there would be no food supply at other
times of the year. It appears that the Brazil nut will not
set fruit without these other tree and plant species to
provide bee food when the Brazil nut is not in flower. A
further complication for dispersal of the Brazil nut is the
shell case. After mature nut cases have fallen to the forest
floor, they are opened only by agoutis (a small guinea-pig-
like rodent), which break through the shells and bury the
nuts as a food store. Inevitably some of these are forgotten
and eventually germinate. Survival of the Brazil nut
therefore needs the bees, food for the bees at other times
of the year and the agouti to break into the shell cases and
disperse the nuts. At present about 45,000 t of nuts are
collected from the wild in Brazilian Amazonia each year.
Unfortunately collection is often so intense that few nuts
survive to become seedlings and most of the nuts are from
old trees; the system is not sustainable.
Many different types of animals are present, taking
advantage of the diverse niches provided by the vegetation.
A large majority of the animals are
arboreal
(living in
trees). Those at the canopy level rarely descend to the
forest floor. In general, the high habitat and niche diversity
result in high species richness. Often, animals and plants
are ultimately dependent on a few plant species for their
existence. Because these diverse and complex ecosystems
can change frequently over short distances, relatively
minor disturbances, like logging, can cause species
extinction. This is one of the reasons why ecologists and
conservationists are so concerned about uncontrolled
exploitation of the rain forest.
The history of the rain forests appears to be more complex
than was once thought. The diversity of species in tropical
rain forest was for long believed to be the result of
environmental stability. It was thought that the impact of
the Pleistocene glacial periods, which had been dominant
in temperate and polar regions, had not extended into the
equatorial areas. Indeed, there is still controversy over the
amount of change which has occurred. Some scientists
believe that the wet tropical regions of Earth have been
stable for at least the last 40 Ma. In that case the rain forest
development should have taken place over a long period
of time. The idea is that this period of uninterrupted
development has allowed rain forest plants and animals
to evolve and adapt. Hence there is richness and diversity
of forest species to exploit the resulting ecological niches.
Other experts believe that the forests should no longer
be considered ancient biomes that have survived since the
Tertiary. Instead it is argued that they
did
experience
dramatic changes during the Quaternary period, and
probably owe much of their present diversity to the
the wide height range of species and their variety.
Photo: Peter Smithson
