Biology Reference
In-Depth Information
Amigos area alone (to put this in perspective, only 15 tree species are native to the United
Kingdom).
The distribution of tropical trees presents the central puzzle of rain forest ecology and, by
extension, of one major form of rarity—life at low density over a large range. Explain this
ecological brainteaser and many things fall into place. To answer how such rarity is gener-
ated requires an understanding of why rain forest trees are so diverse in the first place. From
New Guinea to the Amazon to the Congo or Indonesia—wherever you go, you typically find
an extraordinary number of tree species in a small plot of land, say one hectare, often with
each species represented by a single individual. Why is this pattern so prevalent in the tropics
but less so in the temperate zone and boreal forests?
All tropical trees use the same photosynthesis system, utilizing light to convert carbon di-
oxide and water into sugar and oxygen. Different tree species may “specialize,” growing bet-
ter in different light levels, but the tree species that occupy the canopy (as compared with
the understory) have roughly equal access to light and to the available nutrients in the soil.
If nature is said to abhor a vacuum, it also seems here to abhor a monoculture. Many trop-
ical scientists believe that specialized insects and pathogens that primarily attack a single
tree species check that species from dominating the forest. Foresters in both the temperate
and boreal forests know that on plantations of trees or in vast stands of few species, such as
the white or black spruce trees that blanket the upper latitudes, pest outbreaks are common.
Such eruptions are even more common in tropical tree farms where trees of the same species
are planted in large numbers—essentially a monoculture. Besides, in tropical regions, plant
herbivory can be more intense than in forests closer to the poles; in the latter, the long win-
ters suppress the activity of pest species. In contrast, tropical plant eaters are typically active
every day of the year, especially in the wettest rain forests. If a given tree became common
as a seedling, sapling, or small tree, the plant predators to which that species was suscept-
ible would proliferate in the presence of more abundant food and would soon devour the in-
creased supply. The combined effect of these leaf chewers, suckers, and defoliators is that
individual trees that are spaced far from other members of their species—often a distance of a
hectare or more—have higher survival rates. So the very nature of the hot, humid, and stable
climate, especially warm winters, that leads to considerable diversity of trees also fosters a
rich coterie of the creatures that inhibit their spread.
An allied theory accepted by many tropical biologists has held that patterns of seed preda-
tion and dispersal best explain the low density of canopy tree species. According to this hy-
pothesis, if most of the seeds of an adult tree fell beneath its crown, waves of seed predators,
from ants to agoutis, would finish them all off. Only those seeds carried a safe distance from
the parent by some animal, be it a fruit-eating bird, mammal, or even fish, would find a better
and safer germination site. Biologists note that as much as 90 percent of all plants in trop-
ical forests produce fruits that are dispersed by animals, whereas in the temperate zone the
