Biology Reference
In-Depth Information
dimensions, such as light levels; concentrations of nitrogen, iron, or potassium in the soil;
and relatively wet or dry spots on the forest floor. Niche theory holds that the ecological
separation of many species is a reflection of their differing abilities to use such limiting re-
sources; scientists merely lack the tools to detect all those dimensions just yet. However, a
visit I had made to a tropical forest research site in the Lambir Hills of Sarawak, Malaysia, a
few years earlier made me wonder if at least one part of niche theory deserved a recall. There
in the northern Borneo rain forests, as many as twenty-five species in the genera
Shorea
,
Parashorea
, and
Eugenia
all occurred on the same hectare, like the many species of
Inga
in Amazonian Peru! On what axis of limiting resources required by plants—nutrients, light,
moisture—could trees micromanage their needs in order to live so packed together?
The Lambir Hills site was part of a remarkable experiment to study and map rain forests
at an unprecedented scale—fifty hectares, about the size of a small vineyard. The first to do
this were ecologist Steve Hubbell and botanist savant Robin Foster. Their landmark study on
Barro Colorado Island, Panama, in the early 1980s illustrated the patterns seen everywhere
in the tropics: an incredible diversity of tree species—cresting at 125 to 150 species per hec-
tare—with virtually all appearing on their plot at very low densities. Among some species,
an adult tree was found only every few hectares. How different this is from a temperate zone
forest, where on a single hectare one might find the same number of individual stems but
fewer than two dozen tree species. Since then, twenty-three plots of similar size have been
intensively studied across equatorial forests from Peru to the Philippines. Each has yielded
census results similar to those from Hubbell and Foster's fifty-hectare samples.
Reanalysis of some of these large data plots yielded, in the first decade of this century,
yet another paradigm of tree diversity and local rarity. Steve Hubbell proposed a theory of
“functional equivalence” in which he suggested that many tropical tree species function as
ecological duplicates of one another and the species that grew in a given location in the forest
was determined purely by chance. Thus, many plant species exist in tropical forests because
they all have an equal chance of occupying spaces that open up when trees fall or die. Rarity
in tropical trees and vines is thus in part a function of limited habitat required for successful
propagation of a new generation, in particular a shortage of space for seedling survival, such
as the infrequent light gaps, or openings, in the forest for species whose seedlings and sap-
lings require intense sunlight. A tree that is the only one of its kind in a forest stand could
also be a recent immigrant whose seed somehow managed to disperse and survive far from its
clan. The take-home message of Hubbell's work was that the forest was not in a permanent
state of balance. The commoners might fluctuate in number, but species that were extremely
rare—say, one individual per fifty hectares—could also just as easily disappear.
Hubbell may be correct in saying that trees so rare that only a single individual is found
on average per fifty hectares could easily disappear. For example, tropical ecologists have
discovered that the best germination sites for each species, such as open areas created by
