Tropical Forest Community Ecology

Disparity in Tree Species Richness among Biomes 37

Boreal

Temperate

Tropical

Eocene Oligocene

Miocene Pliocene

Last glacial maximum Mid-Holocene

Figure 3.1 Th emaps of th epast biom es, us ed to calculat eth ecomposit ear ea–tim em easur es in Figur e3.2.
Paleocoastlines from Smithet al.(1994) are shown for the Eocene, Oligocene, and Miocene maps. Equal area maps of
present-day coastlines are shown for the Pliocene, Last Glacial Maximum, and Mid-Holocene maps.

the relationship disappears when all biome areas
are considered together. Thus, the current size
of biomes does not explain tree species richness
totals, as predicted by the GAH.
The integral of biome area over time (log biome
area×age) exhibited a significant and positive
correlation with current tree species richness,
a r esult that holds for cumulativ etim ep eriods
sinc eth eMioc en e(R^2 = 0.35,P < 0.05;
Figur e3.4a), th eOligoc en e(R^2 = 0.51,P <
0.01; Figur e3.4b), and th eEoc en e(R^2 =0.67,
P<0.001; Figur e3.4c). Th eR^2 valu eof th e
correlation increases as time increases.
Extant tree diversity is also significantly and
positively correlated with minimum biome size

during the Pleistocene (R^2 =0.48,P<0.02;

Figur e3.5).This sugg ests that extinction via rang e

contraction during Pleistocene climate change

may also help explain the disparities in species

richness among the 11 biome areas we tested.

DISCUSSION

We found that current geographic area size

and species richness are not positively correlated

across biomes, a result that does not support the

GAH as a primary explanation for the latitudinal

diversity gradient. However, because the GAH is

based on factors influencingin situspeciation