Biology Reference
In-Depth Information
EIGHT
Hot Wet Places
RAINFORESTS ARE DIMLY LIT
and exceptionally diverse—claustrophobically dark and
fecund—so no wonder tropical biologists end up puzzling over existential questions. At
La Selva Biological Station in Costa Rica, giant trees with buttressed trunks tower over-
head, obscuring the sky, and every glimpse holds the vibrant greens and somber browns
of plants and their decaying remnants. After a torrential shower the air reverberates with
the buzzes, whines, and clicks of insects. Mantled howler monkeys sound of in the dis-
tance. All around us leaf litter reeks from the chemical adventures of microbes, and over
the course of hours my puny primate nose wrinkles toward some collared peccaries, then
heaps of rotting fruit and a pile of cat droppings. Rounding a trail curve I'm baffled by
a shimmering lavender stripe, dozens of yards long and a half-inch tall; then I drop to
my knees and contemplate thousands of leaf-cutter ants, each carrying a single delicate
flower petal. And from time to time, slogging along the muddy paths, I imagine being
overgrown by mosses and fungi, or devoured by spike-headed katydids the size of small
mice.
Setting aside matters of life and death for the moment, what do ecologists mean by
“exceptionally diverse,” and why might anyone care? A comparison among some famil-
iar places illustrates how numbers of species increase toward the Equator, culminating
in unparalleled tropical richness. California reaches from Death Valley's floor to Mount
Whitney's summit, spans parched salt flats to drenched redwood groves, and yet across
ten degrees of latitude boasts only thirty-five species of snakes. Almost twice that num-
ber occur in La Selva's five square miles, as if a house full of serpents were packed into
a thimble, and there are nearly four hundred species of birds, more than half as many as
in the continental United States. Tropical faunas encompass more lifestyles too, thanks
to rampant adaptive diversification; most temperate bats feed on insects, for example,
whereas some of their hothouse relatives specialize on fruit, nectar, fish, frogs, or birds.
This dramatic global variation has long intrigued naturalists, and its causes are partly
understood. Rainforests usually occupy middle latitudes, so Earth's most biologically op-
ulent regions are hot and wet. Some of them have been that way for millions of years,
during which rising seas and tectonic events fragmented landscapes, catalyzing the ori-
gin of new species. More land, more sun, and more rain, coupled with geographical isol-
ation and geological time, have fostered plant evolution—and thereby more plant-eating
insects, insect-eating frogs, frog-eating snakes, and birds and mammals that eat them
