The Tropics: Cradle or Museum?
Ecologists have offered two metaphoric views of the
tropics to account for their high species richness. One,
called the cradle, with a nod to Dobzhansky, views the
tropics as uniquely suited to high rates of speciation
and thus species accumulate in tropical ecosystems,
far more than outside of the tropics. The other view,
called the museum, presumes that speciation rates
are not higher in the tropics but extinction rates are
low, so the tropics, like a good museum, “keep” their
more ancient species along with whatever new ones
evolve, thus maintaining a high species richness. In the
museum view, extinction rates outside of the tropics
are presumed higher. Dobzhansky’s reasoning also
alluded to this view but less directly.
Perhaps the tropics can act as both cradle and
museum. Most folks do not visit the tropics to see
bivalve mollusks such as clams and mussels, but these
creatures fossilize well, and so a look at the fossil record
of bivalves can offer insights into past patterns of
speciation. A study by David Jablonski and colleagues
suggests this possibility. The researchers examined the
fossil record of 163 genera and subgenera beginning
11 million years ago, in the Miocene epoch. What
they learned was that most bivalve families, including
those at high temperate latitudes, originated in the
tropics, a finding that supports the cradle view of the
tropics. Their data showed that first occurrences of
bivalve mollusk genera were greatest in the tropics and
that many genera subsequently radiated to temperate
latitudes. The data further showed that substantially
higher extinction rates occurred at higher latitudes
over the 11- million- year period covered by the study,
supporting the museum view of the tropics (i.e., lower
extinction rates at lower latitudes). They concluded
that genera originating in tropical areas extended their
ranges with time while still occupying the tropics.
The study suggests that endemism (the restriction
of a species to a specific narrow geographic range)
should be proportionally more common in the tropics
and decrease with increasing latitude. As we discussed
in chapter 8, there are numerous areas of endemism
throughout the Neotropics. Conservation priorities
(chapter 18) should consider both high species richness
and levels of endemism, both of which characterize
the tropics and explain why the tropical regions of
the world have gained such a focus in conservation
biology.Does speciation occur more rapidly in the tropics,
thus packing the ecosystems with species? There are
relatively few studies that examine this question, but
some studies have utilized molecular techniques to
compare similarities in DNA among species. The results
of these studies have thus far generated contrasting
conclusions. Some suggest that speciation is actually
more rapid (for some groups) in the temperate zone.
Some temperate species of birds and mammals are more
recently evolved than most equivalent tropical species.
This is really not surprising. Higher latitudes have been
strongly affected by Pliocene and Pleistocene climatic
changes, particularly glaciation, with its multiple
effects on ecosystems. Such climatic shifts would act to
promote speciation in isolated populations (chapter 8).
Nonetheless, some speciation patterns have been high
throughout the tropics.
Consider the Inga, a common tree in shade coffee
plantations (plate 9- 7). Inga is a widespread Neotropical
genus, with some 300 species ranging from Mexico
into Argentina. Did these 300 species accumulate over
many millions of years or are they more recent in origin?
Using molecular techniques that allow comparison of
DNA segments, a research team led by J. E. RichardsonPlate 9- 7. The elongated pods of Inga, a member of the
large legume family, Fabaceae, are easy to recognize. There
are many Inga species throughout the Neotropics; they are
evolutionarily young. Photo by John Kricher.chapter 9 why are there so many species? 139