ascertained that the diversification of the genus Inga is
no older than 6 million years and may be as young as 3
million years. The authors suggest that Inga divergence
was likely caused by the recent Andean mountain-
building episodes. Pleistocene climate changes and
the bridging of the Isthmus of Panama (chapter 8) also
likely stimulated speciation in Inga.
Though speciation in Inga has been remarkably rapid,
it may not represent a typical case among tropical tree
genera. Eldredge Bermingham and Christopher Dick
conducted a census of 25 ha (62 ac) of rain forest at a
site in Ecuador and 25 ha in Panama, resulting in the
following tally:
- Ecuador: 1,104 woody plant species, 333 genera, 81
families, 43 Inga species.
- Panama: 277 woody plant species, 174 genera, 56
families, 14 Inga species.
What is noteworthy about those numbers is that
in Ecuador, 161 of the 333 genera are represented by
but a single species. In Panama, 121 genera have but a
single species. Therefore Inga is unusual in exhibiting
a pattern of multiple closely related species throughout
its range. But it is not unique. Other plant genera such
as Psychotria, Piper, and Ficus, all widespread in the
Neotropics, show a similar pattern, exhibiting high
species richness in relation to other genera.
Tropical regions are clearly rich not only in species
but in genera and families, and those take longer to
evolve, showing that while Inga is clearly a “cradle”
species cluster, many families of tropical plants (and by
implication, birds, mammals, and other groups as well)
may be part of a very old “museum.”
Climate, Energy Availability, and
Species Richness
The latitudinal belt from the Tropic of Cancer to the
Tropic of Capricorn, spanning approximately 47° of
latitude, is by and large warm and wet. Winters are far
less severe than in temperate and polar regions. Light
flows more evenly and constantly, as there is less (and on
the equator no) variation in day length. Ecologists have
long believed that equitable climate and an abundance of
photosynthetic energy, termed net primary productivity
(chapter 5), ultimately support greater species richness,
helping account for the latitudinal diversity gradient
(LDG). Let’s look at climate as it pertains to the LDG of
one well- known group, birds.
Ever since Dobzhansky’s 1950 paper, avian species
richness has occupied the attention of researchers
seeking to explain the LDG. Bradford Hawkins and
colleagues correlated seven climatic variables with broad
global patterns of bird species richness. The resulting
analysis indicated that “actual evapotranspiration”
was the variable that explained 72.4% of the variance
in global bird species richness. Why is this of interest?
Because evapotranspiration is an indirect measure of
forest photosynthetic productivity, since it is related to
the metabolism of the trees. The conclusion was that
productivity, as reflected in actual evapotranspiration,
was the best hypothesis to account for the LDG in
birds. Why? Productive forests have lots of food and
space and thus accommodate numerous bird species.
It’s that simple.
Hawkins and his colleagues also looked at patterns of
plant diversity. For plants, water availability, as it varies
from region to region, was the key constraint limiting
species richness. This effect was most pronounced
in areas of warm temperature where adequate solar
energy would otherwise be readily available. It is well
known that dry seasons have pronounced effects on
ecosystems, so this result should not be surprising.
The studies by Hawkins and colleagues showed an
apparent shift with latitude in the influence exerted by
climatic variables on species richness. In the far north
(and by implication in the far south), solar energy was
the variable that placed the strongest constraints on
richness. In these regions sunlight varies dramatically
during the course of the year. But in areas of high
solar energy input, as typify the tropics, water was
the variable most responsible for constraining species
richness. With regard to animal species richness, it
should be noted that it was not clear whether solar
energy and water availability act to limit species
richness or whether richness is limited primarily
through plant productivity, which, itself, is limited
by solar energy and water availability. But the study
does show that climatic variables correlate broadly
with species richness patterns across latitudes. Climate
appears to rule.
Questions remain, however. Hawkins et al. noted that
while climate variables may explain up to 90% of the
variance in latitudinal species richness in some cases,
in other cases it explains less than 50%. Such factors
as evolutionary history and ongoing biotic interactions
(discussed below) also exert strong influences.
140 chapter 9 why are there so many species?
