The Sixth Extinction: An Unnatural History

Although the seventy-third meridian misses Central America altogether,
it’s worth noting that tiny Belize, which is about the size of New Jersey,
has some seven hundred native tree species.
The seventy-third meridian crosses the equator in Colombia, then
slices through bits of Venezuela, Peru, and Brazil before entering Peru
again. At around thirteen degrees south latitude, it passes to the west of
Silman’s tree plots. In his plots, which collectively have an area roughly
the size of Manhattan’s Fort Tryon Park, the diversity is staggering. One
thousand and thirty-five tree species have been counted there, roughly
fifty times as many as in all of Canada’s boreal forest.
And what holds for the trees also holds for birds and butterflies and
frogs and fungi and just about any other group you can think of (though
not, interestingly enough, for aphids). As a general rule, the variety of life
is most impoverished at the poles and richest at low latitudes. This
pattern is referred to in the scientific literature as the “latitudinal
diversity gradient,” or LDG, and it was noted already by the German
naturalist Alexander von Humboldt, who was amazed by the biological
splendors of the tropics, which offer “a spectacle as varied as the azure
vault of the heavens.”
“The verdant carpet which a luxuriant Flora spreads over the surface
of the earth is not woven equally in all parts,” Humboldt wrote after
returning from South America in 1804. “Organic development and
abundance of vitality gradually increase from the poles towards the
equator.” More than two centuries later, why this should be the case is
still not known, though more than thirty theories have been advanced to
explain the phenomenon.
One theory holds that more species live in the tropics because the
evolutionary clock there ticks faster. Just as farmers can produce more
harvests per year at lower latitudes, organisms can produce more
generations. The greater the number of generations, the higher the
chances of genetic mutations. The higher the chances of mutations, the
greater the likelihood that new species will emerge. (A slightly different