Travel Reference
In-Depth Information
Just as this process of frequency-dependence can pack a population full
of dif erent alleles of a gene, the Janzen-Connell process can pack an ecosys-
tem full of dif erent species. Each species is at an advantage when it is rare
and at a disadvantage when it is common.
The Janzen-Connell hypothesis has been tested directly in several ways,
using both tropical and temperate tree species. Perhaps the most elegant
experiment was carried out by Alyssa Packer and Keith Clay of Indiana
University. They showed that temperate-zone black cherry trees from an
Indiana forest showed exactly the kind of growth pattern that had been pre-
dicted by Janzen and Connell.
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Packer and Clay found that cherry seedlings thrived when they were
planted in soil that had been dug from a part of the forest that had no adult
cherry trees. But when they raised the seedlings in soil that was taken from
areas close to adult trees, the seedlings tended to get fungal infections and die.
They then sterilized samples of soil taken from sites close to an adult tree
and from a part of the forest without cherry trees. They found that the seed-
lings thrived in these samples of heat-treated soil, regardless of where the soil
came from. This experiment showed that it was organisms living in the soil
near the adult trees that were inhibiting growth. To help close the circle of
proof, they used a classic approach that had been pioneered by Robert Koch,
the nineteenth-century microbiologist who discovered the anthrax bacillus.
They cultivated a root rot fungus from diseased seedlings and showed that
it was able to kill a new set of seedlings. They had not proved that this par-
ticular fungus was entirely responsible for the deaths of seedlings in their
greenhouse experiment, but they did show that it was likely to have been a
contributor to their deaths.
Some tree species in tropical forests have been tested for the Janzen-
Connell ef ect in similar experiments, though not as thoroughly or elegantly.
Some of these experiments gave positive results and others showed no Jan-
zen-Connell ef ect, a point that I will return to in a moment.
Figure 79
(
opposite
) Trees festooned with lianas dominate this rainforest in peninsular
Malaysia. The balance between life and death that maintains diversity in such a rainforest may
be invisible to the casual visitor, but it is essential for the forest's health.
