Disparity in Tree Species Richness among Biomes 43Because different explanations for the latitudi-
nal diversity gradient may be valid for different
kinds of organisms, some have recommended that
w ec eas etrying to find a univ ersal caus e(Gas-
ton 2000). W ear esympath etic to this vi ew. For
example, the mechanisms causing the famous
reverse latitudinal gradients, such as in salaman-
d ers, ar eunlik ely to b eth esam eas th em echanism
causing forward latitudinal gradients (Williget al.
2003). But it also seems appropriate to consider
that organisms like trees take on a greater impor-
tanc eb ecaus eof th eir rol ein providing habitat,
food, and shelter for so many other organisms
(e.g., Huston’s 1994 structural versus interstitial
organisms). If w eund erstand th emost impor-
tant causes of global diversity gradients in trees,
we will also understand one element controlling
gradients of organisms that are dependent on
them for survival, such as specialist herbivorous
insects, which may represent most of the world’s
biodiversity.
Human activity is currently causing both mas-
siv eloss es of habitat and rapid climat echang e
(Corlett and Primack Chapter 26, this volume,
Laurance Chapter 27, this volume). Understand-
ing the consequences of fluctuations of biome
area through time on global tree diversity is crit-
ical as we strive to develop effective conservation
strategies. If we consider conservation of natural
habitats at the largest scales, the results reported
here suggest that tropical areas should be more
s ensitiv eto habitat loss than high-latitud ear eas
b ecaus e extinctions during th elast glacial p eriod
have likely set temperate and boreal biomes well
below their equilibrium diversity values. For this
reason, we might predict a lower extinction rate in
temperate or boreal biomes with moderate losses
in eff ectiv ear ea, whil eth esam eamount of habi-
tat d estruction in th etropics should caus emuch
higher extinction rates. Thus, because of their
greater potential for higher extinction rates, areas
in tropical biomes should be given the highest
priority for conservation.
ACKNOWLEDGMENTS
We are grateful to R. Condit, E. Dinerstein, R.
Foster, P. Grogan, C. Hawkins, H. Helmisaari,
H. Hytteborne, J. LaFrankie, R. Leemans, S. Linder,
W. Pruitt, A. Solomon, A. Svidenko, and
W. Wettengel for responding to our queries relat-
ing to information on the world’s biomes and tree
diversity estimates. We thank S. Schnitzer, C. Dick,
Z. Miller and one anonymous reviewer for help-
ful comments regarding the manuscript, and the
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