approximately 40% of all tropical terrestrial arthropod
species (including spiders, crustaceans, centipedes, and
millipedes, as well as insects), Erwin suggested that the
total arthropod species richness of the tropical canopy
might be as high as 20 million, and the figure climbs to 30
million when the ground and understory arthropods are
added. Note that Erwin’s estimates are indeed that, based
on assumptions about host specificity, a characteristic
that may vary considerably among species of trees and
other plants. Many tropical entomologists take a much
more conservative view of insect species richness and
have been critical of Erwin’s estimates. Insects, as a group,
remain rather poorly documented as to their actual
species richness, and future research is needed. Without
question, numerous species await discovery. But one thing
is certain whether one chooses to challenge Erwin or not:
there are multitudes of arthropod species throughout the
world’s tropics (plate 9- 6).
Species Richness and Diversity
Gradients
Most major taxa, including flowering plants, ferns,
mammals, birds, reptiles, amphibians, fish, insects,
spiders, millipedes, snails, and bivalve mollusks (such
as clams and mussels), all have their greatest number of
species in the tropics. Some tropical rain forests contain
so many species that they exhibit what ecologists have
termed hyperdiversity. Why are tropical ecosystems,
and rain forests in particular, home to so many species?
Charles Darwin realized that species numbers per
unit area tend to decline strongly with latitude as one
travels away from the equator, a point he noted in
chapter 3 of On the Origin of Species. Alexander von
Humboldt also observed this trend for plants and
believed it to be related to reduced tolerance for cold
in higher latitudes. The reduction in diversity with
increasing latitude is termed a latitudinal diversity
gradient or LDG. The evolutionary biologist Theodosius
Dobzhansky, in a seminal paper titled “Evolution
in the Tropics” (1950), noted that only 56 species of
breeding birds occurred in Greenland, while New
York had 195 breeding species (Dobzhansky’s figures
would need updating today but are still very much in
the ballpark.) Dobzhansky noted that Guatemala had
469, Panama 1,100, and Colombia 1,395 breeding bird
species. Breeding bird diversity increased by almost 25
times from Greenland in the Arctic to Colombia on
the equator. Regarding snakes, Dobzhansky noted that
22 species occurred in all of Canada, whereas 210 were
found in Brazilian forests and savannas.
Dobzhansky suggested that since plants and animals
are all products of evolution, any differences between
tropical and temperate species result from differences
in evolutionary patterns. Selection pressures vary with
latitude and, for some reason, result in more species
being in the tropics. But, as is often the case in ecology,
the devil is in the details. Just what evolutionary
selection pressures and other forces might account for
the greater richness and variety of the tropical fauna
and flora, compared with those of temperate and
polar lands? How does life in tropical environments
influence evolutionary potentialities?
One possibility is that speciation rates exceed
extinction rates in tropical regions, thus allowing an
incremental buildup of species richness with time.
There are other possibilities as well. For example,
tropical rain forests, because they remain warm and
wet throughout the year, fix more photosynthetic
energy per unit area than other ecosystems. Does more
energy somehow translate to greater species richness?
If so, how? Is high plant productivity essential to species
richness? There is high structural complexity in rain
forests. Might this greater three- dimensional space not
account for greater species richness in groups such as
birds, mammals, and arthropods?
Dobzhansky suggested that part of the answer to
high tropical species richness rested with the equable
nature of the tropical climate. Echoing Humboldt,
he argued that polar and temperate climates impose
significant physical selection pressures such that fewer
organisms have been able to adapt over evolutionary
time. Speciation is therefore less frequent in the higher
latitudes. The tropics, in contrast, offer a climate of
abundant rainfall for most if not all of the year, no
season of frost or cessation of plant growth, warm and
relatively invariable temperature, and less overall severe
meteorological fluctuation, all of which may promote
speciation to retard extinction to a degree greater than
is the case in the higher latitudes. As species are added,
many specialize to utilize unique resources. More and
more species are packed into the rain forest ecosystem.
The high level of plant productivity in the tropics serves
as a large and steady food base. Dobzhansky’s hypothesis
was obviously speculative and very challenging to
test, but it stimulated much thinking about latitudinal
diversity gradients.138 chapter 9 why are there so many species?