Rainforests, Diseases, and Speciation - The Darwinian Tourist - page 160

to coexist with the dinosaurs. If the mammals became too numerous or too
large, carnivorous dinosaurs would notice them and eat them up. It therefore
behooved the mammals that shared the world with the dinosaurs to keep a
low profi le. And this meant that there was no selective pressure acting on the
fl owering plants to produce bigger seeds and fruit.
Once the dinosaurs vanished, however, mammals could grow larger.
Large carnivorous and herbivorous mammals were able to take the place of
the large carnivorous and herbivorous dinosaurs. The herbivorous mam-
mals were used to eating fruits and seeds, and now they needed large and rich
sources of food to fuel their large bodies and brisk metabolisms. The fl ower-
ing plants rapidly co-evolved with them to provide these sources of energy.
The diversities of mammals, of birds, and of the fl owering plants them-
selves exploded at the beginning of the Cenozoic. The dinosaurs had kept a lid
on all this evolutionary activity. Their demise is regretted by some (especially
by small dinosaur-smitten children), but personally I would rather share the
planet with intelligent, behaviorally diverse mammals, brilliantly colored
fl owers and birds, delicious fruit, and forests that are far more diverse than
anything that existed during the Mesozoic.
Theories about rainforest diversity
In the green teeming world of the rainforest millions of organisms are living
and dying, all of them subject to the unrelenting pressures of natural selection.
How can we make sense of all these individual stories of life and death? How
can each of these stories help to explain the diversity of the entire rainforest?
Many theories that try to make sense of the biological cacophony of
complex ecosystems have been proposed. At the time I visited Manù I knew
something about one of them.
In the 1970s, Daniel Janzen and Joseph Connell had suggested that high
levels of tree diversity in tropical forests could be explained by the threats
that surround and besiege each tree. 4 Trees, they pointed out, live in a kind
of precarious equilibrium with entire collections of herbivorous animals,
insects, fungi, and bacteria that prey on them.