Diversity in the Tropics: Is It a
“Perfect Storm”?Scores of researchers have attempted to summarize the
various possibilities for why tropical ecosystems are
so rich with species. A thought- provoking 1966 paper
by Eric Pianka, now a classic, attempted to summarize
the forces that might account for hyperdiversity in
the tropics and in lowland rain forests in particular.
We have already addressed some possibilities in the
form of broad patterns of climate and evolutionary
history. But what about the effects of day to day
ecological forces within tropical ecosystems as such?
What additional forces might act to increase species
numbers and to maintain diversity? To borrow a
well- known phrase from the writer Sebastian Junger,
the tropics may represent a “perfect storm,” with
ecological and biogeographic conditions combining
in an ideal manner for bringing about speciation and
diversity maintenance, just as atmospheric variables on
occasion combine to create monumental storms. The
result of the tropics’ perfect storm has been to produce
prodigious numbers of species and then to keep
them. Below I discuss four hypotheses (stability- time,
productivity- resources, interspecific competition, and
predation intensity) that have been offered to account
for why the tropics are so diverse.The Stability- Time Hypothesis
The stability- time hypothesis suggests that the tropics
are ancient and that such antiquity coupled with the
relatively stable and equitable climate has resulted in
the generation and persistence of high species richness.
The idea is closely akin to what Dobzhansky believed to
be the case (and yes, it is the museum metaphor again).
How do we look back into time? How can we ascertain
whether climate has been stable in the tropics? Carlos
Jaramillo and colleagues conducted a study of the fossil
record of plant pollen deposited in lakes from sites in
central Colombia and western Venezuela, spanning a
time sequence from the Paleocene to the early Miocene
(65– 20 million years ago). Pollen and spores from plants
are resistant to decay, especially in anoxic deep lake and
bog sediments, and thus they can be analyzed in what
is termed a pollen profile of lake sediment. The deeper
sediment contains the oldest pollen. The data analysis
showed low plant diversity during the early Paleocene.
This was the time just after the end of the Cretaceousperiod, which ended with one of the five mass extinction
events in Earth’s history. Thus a low diversity of plants
at that time is to be expected. During the early Eocene
there was a rapid increase in plant diversity. However,
diversity declined beginning in the late middle Eocene
and continued to drop until the early Oligocene (34
million years ago). The extinction rate was elevated
during the transition from the Eocene to the Oligocene,
and the speciation rate increased during the early Eocene.
Otherwise, both extinction and speciation rates were
steady. Species richness correlated with changes in global
temperature. Global warming during the early and middle
Eocene permitted the spread of tropical plant species
into higher and lower latitudes. Jaramillo also noted that
tectonic activity in the Andes Mountains (the Andean
uplift) acted as a major stimulus to plant speciation.
Most of that mountain building has been recent (within
the past 5 million years), so the clear implication is that
most speciation has been recent. The study shows that the
tropics have never been really “stable” but instead subject
to some degree of climatic fluctuation.
Speciation is not dependent on vast time periods. Six
species of kingfishers (family Alcedinidae), birds that dive
headfirst into water and capture fish with their long beaks,
inhabit Neotropical rivers and streams. This assemblage
represents the total species richness of kingfishers in the
Neotropics. Five are permanent resident species, which
differ fundamentally in body and bill size (plates 9- 8– 12);
the sixth is the Belted Kingfisher (Megaceryle alcyon), a
North American migrant that winters in parts of the
Neotropics. The fossil record, as well as biogeographic
studies, shows that kingfishers evolved in the Old World,
and some eventually colonized the Neotropics, probably
during the Pleistocene. What is interesting is that of the
120 kingfisher species in the world, only six occur in the
Americas. Four of the six, the “green- backed” kingfishers
(genus Chloroceryle; plates 9- 8– 11) are one another’s
closest relatives and evolved in the Neotropics from a
recent common ancestor. Interestingly, wherever in the
world kingfishers occur there are never more than five
species together, coexisting in the same habitat (this
raises a question worth pondering about how resources
affect species richness and coexistence). The five resident
Neotropical kingfishers differ in body size, so they must
obviously feed on different- size prey, though there
is some overlap. In a monograph on the Neotropical
kingfishers, Van Remsen pointed out that the five species
of Amazonian kingfishers span the same size range and
within- habitat species richness as the kingfishers foundchapter 9 why are there so many species? 141