Environmental Engineering Reference
In-Depth Information
Figure 28.5
Vertical stratification of a
tropical rain forest.
Source: After Collinson (1977).
Coupled with diversity, studies of biological interactions 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 certain types of 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. Destruction of the forest can break these
interactions and consequently the whole structure of the ecosystem.
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
