Disparity in Tree Species Richness among Biomes 41(TCH) recently proposed by Wiens and Donoghue
(2004). The TCH is based on three basic ideas.
First, if a clad eoriginat ed in th etropics, it is
expected to include more tropical species because
of the longer temporal duration of the tropics,
and hence greater opportunity for diversification
in tropical regions. Second, if tropical areas have
covered larger areas for longer durations than
extra-tropical areas, a correspondingly higher pro-
portion of extant lineages should have originated
in th etropics. Third, if adaptations to surviv e
freezing temperatures are necessary to invade
extra-tropical regions and these adaptations are
difficult to acquir eand maintain, th en nich econ-
servatism within tropical lineages will maintain
the disparity in species richness over time (Latham
and Ricklefs 1993, Ricklefs 1999, Wiens and
Donoghu e2004).
The GAAH and the TCH both predict peaks
in species richness in tropical areas, but they
approach the disparity of species richness between
biomes from different angles. The GAAH focuses
on the intrinsic properties of biomes that influ-
encein situdiversification of resident species. The
TCH focuses on lineages, particularly the phylo-
genetic distribution of tropical and non-tropical
taxa, as this bears directly on general inferences
about the geographic history of diversification
across many clades. The two hypotheses intersect
in their emphasis on the tropics being larger and
older than extra-tropical regions.
However, the GAAH provides a general rule
for why most lineages can be traced to the trop-
ics and why tropical lineages have undergone
greater diversification overall than temperate and
boreal lineages. For example, the TCH argues that
because frost tolerance is a difficult physiologi-
cal barrier for angiosperms to overcome, relatively
few lineages were able to colonize the temper-
ate zone. But phylogenetic niche conservatism
explains only why so few lineages cross into the
temperate zone – it does not address why the
lineages that do acquire frost tolerance have not
diversified to the same degree as their tropical rel-
atives. It is not likely that those lineages that did
cross th efrost-lin ear einh er ently constrain ed in
their potential for diversification. We suggest that
if temperate areas were large and stable through
time, an interval on the order of 55 million years
might be sufficient for the development of a com-
parably diverse temperate flora. However, climate
changes and glaciation at higher latitudes dur-
ing the last 55 million years have resulted in
much smaller effective areas for temperate and
boreal biomes than tropical ones. During this
time, tropical biomes overall have been larger than
temperate biomes (Figure 3.2), and are therefore
expected to be characterized by lower extinction
rates (and perhaps higher speciation rates as well),
leading to higher species richness in the tropics
(Fin eand R e e2006).
If the assembly of forest communities has been
characterized by evolutionary responses to phys-
iological thresholds that exist between biomes
(e.g., frost tolerance), then boreal lineages should
tend to be phylogenetically nested within tem-
perate lineages, and temperate lineages within
tropical lineages (Wiens and Donoghue 2004).
Th edata curr ently availabl esupport this pr edic-
tion (e.g., see Juddet al. 1994, Hoffmann 1999,
Scheenet al. 2004). If diversification within a
lineage could be dated and mapped onto recon-
structions of past biomes (similar to Figure 3.1), it
would allow for a powerful test of how the area of
a biome over time affects speciation rates (Ricklefs
2004). We caution, however, that a large number
of independent lineages would need to be studied
to avoid sampling bias in detecting any general
relationship. In addition, it is important to note
that some tropical lineages have crossed the frost-
line, but disappear from our analysis of temperate
areas because they include trees in the trop-
ics but only herbaceous plants in the temperate
zone (i.e., Clusiaceae).Thus, our non-phylogenetic
focus on “trees” rather than monophyletic groups
underscores the different approaches needed by
clade-based (like the TCH) versus biome-based
(like the GAAH) analyses of variation in species
richness.
Another exciting avenue to follow would be to
estimate the rates at which lineages cross biome
boundaries. For this, likelihood-based inference
methods for historical biogeography would be use-
ful (e.g., Reeet al. 2005). Using data on the
location, frequency, and timing of lineage expan-
sions across biom eboundari es, on ecould ask:
Do tropical and extra-tropical lineages diversify
at similar rates? Are some boundaries between