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New puzzles about diversity
It seems clear that frequency-dependence is an important mechanism for main-
taining diversity in these tropical forests. But, as with every answer in science,
the number of questions that the answer raises goes up exponentially. These
questions bear directly on what will happen to life's diversity in the future.
One question is a purely scientifi c one. What phenomena are actually
responsible for the frequency-dependence that we see? Does it, as Janzen
and Connell suggested, result from interactions between the trees and an
“invisible world” made up of bacterial and fungal pathogens and a swarm-
ing collection of insect and animal herbivores? The other major model for
the maintenance of diversity, the niche-complementarity hypothesis, also
requires selective mortality, but in this model deaths result from competition
for limited physical resources. Which of these two models predominates in
the rainforests? We do not yet know, but I suspect that each will be found to
play a role, and that Janzen-Connell will turn out to be more important for
some species and N-C for others.
Janzen-Connell and N-C both predict that to maintain the health and
diversity of an ecosystem there must be selective mortality, in which some
members of a species are more likely to die than others. Just as such dif eren-
tial mortality is essential for the evolution of a species, dif erential mortality
is essential to maintain the health and diversity of an entire ecosystem.
The burial service in the topic of Common Prayer says, gloomily: “In the
midst of life we are in death.” I prefer to think that, in the biological world, in
the midst of death we are in life.
Second, we can ask whether all the frequency-dependent selection that
we see is actually maintaining diversity. The answer to this question seems
to be yes.
My colleagues and I measured the diversity of several forest plots from
around the world's tropics. We found that the commoner species in each
part of the forests died of at a faster rate than the rarer ones. This process
maintained diversity, not by adding species, but by ensuring that species that
happened to be locally rare did not disappear.
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