268 A. Elizabeth Arnold
defined by only a few square centimeters of a
leaf lamina, and leaves may in turn serve as
provinces within the complex geography of indi-
vidual branches, trees, and forests. The abun-
dance and diversity of little-known endophytes
have led some authors to characterize tropical
plants as chimaera (Herreet al.2005b), wherein
plant tissue is interlaced with attendant fungal
hyphae – or as “inside-out lichens” (Atsatt 1988),
whereby plants are functionally inseparable from
the genetic, structural, and evolutionary contri-
butions of their fungal symbionts. The ecological
roles of these hidden symbionts are just beginning
to emerge, promising many decades of research
at the interface of endophyte biology and tropical
ecology.
It has been estimated that the vast majority
of microfungi in tropical forests represent unde-
scribed species (95%; see Arnold 2002), and some
authors have suggested that as many as 1 million
species of endophytes may exist. If workin gesti-
mates are correct, then diversity of endophytes
is likely many times higher than that of trop-
ical plants. Why are there so many species of
tropical endophytes? The answer may be rooted
in rates of speciation and extinction, and in
the case of symbionts, a subsequently complex
interplay of specificity and generalism that can-
not yet be reliably estimated. As indicated by
Leigh (1999), the more apt question may be:
What factors facilitate coexistence of such diverse
species? The multiplicity of species sharin gsim-
ilar substrates in tropical forests may push for
a continuous process of character displacement
in tropical endophyte communities (see Kitajima
and Poorter Chapter 10, this volume). Moreover,
the nearly infinite combinations of genotypes,
chemical exudates, and interactions are likely to
create endophyte communities with distinctive
emergent properties that are even more diverse
than the fungi themselves.
ACKNOWLEDGMENTS
I thank Lissy Coley, Tom Kursar, Gre gGilbert, and
especially Lucinda McDade and François Lutzoni
for guidance and helpful discussion, as well as two
anonymous reviewers and Stefan Schnitzer for
improvin gthe manuscript. I gratefully acknowl-
edge funding support from the National Science
Foundation (DEB-9902346 to L. McDade; DEB-
0200413; DEB-0343953 to J.W. Dallin gand AEA;
and DEB-0640956), the College of Agriculture
and Life Sciences at the University of Arizona,
and the Smithsonian Tropical Research Insti-
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