Travel Reference
In-Depth Information
ecosystem, they must begin by simply surviving. They can do so in many ways.
One way is to hitch a ride on the Janzen-Connell or the N-C bandwagons.
The two species are still so similar that they are preyed on by the same
set of parasites and herbivores. When both species are common in a certain
part of the forest, their progeny are equally selected against by the enemies
that they share. When both species are rare, their progeny can survive more
readily because they are not preyed on as much.
So far as the pathogens and herbivores are concerned, the two species are
equivalent. But there will immediately be an advantage to any members of
both species that happen to be dif erent, because they will be able to escape
some of the pathogens and herbivores that prey on them.
Host species are always evolving new defenses, and pathogens and pred-
ators are always evolving new ways to overcome them. But in the case of our
two species there is a new dimension to this continuing confl ict. If one of the
species can escape at least some of the pathogens that prey on it, it will be
able to move into parts of the forest where the other species cannot thrive.
This will enable it to take advantage of niche-complementarity. It will now
be able to utilize environmental resources that would otherwise have to be
shared with the other species.
In short, it is a win-win situation for the two species. If they continue to
move apart on many fronts—defense mechanisms, biochemistry, physiol-
ogy, and so on—they will be preyed on by fewer predators and parasites.
And because each of the host species is able to survive in parts of the forest
infested with pathogens that no longer harm them, they will have access to
physical resources denied to the other species.
Of course, this advantage is only temporary. Soon some of the preda-
tors and parasites will evolve to overcome the tree species' fresh defenses.
Repeated rounds of ever-evolving defenses and attacks will continue to drive
the two species apart, long after they have lost any possibility of hybridizing
with each other. At the same time, if they accidentally come to share preda-
tors and pathogens with other species in the forest, they will be driven away
from those species as well. This process contributes to the astounding diver-
sity that dazzles us when we visit complex ecosystems such as rainforests
and coral reefs.
