Tropical Arboreal Ants 335moreforhighantabundancethanforhighspecies
richness (Stork 1988); exceptional abundance is
typical of one or a few species and is principally
a phenomenon of the arboreal zone (Stork 1988,
Tobin 1994, Yanoviak and Kaspari 2000). Many
of theabundantarborealtaxaarespatiallyterrito-
rial and/or behaviorally dominant in interspecific
encounter competition (Yanoviak and Kaspari
2000). Together, superabundance and behavioral
dominancedefinevariousantspeciesasecological
dominants (Davidson 1998).
Based on high abundance, behavioral domi-
nance,andperhapsthelackof evolutionaryoppo-
sition to risky behaviors in reproductively sterile
workers, ants are rivaled only by bees as func-
tionally important canopy arthropods. They can
deterinsectandvertebrateherbivores(e.g.,Janzen
1972, Bentley 1976, Dejean and Corbara 2003),
magnify damage from sap-feeding herbivores
(Queiroz and Oliveira 2001, Dejean and Corbara
2003), interfere with plant reproductive services
(Davidson and Epstein 1989, Willmer and Stone
1997, Raineet al.2002), and provide enemy free
space for nesting birds (e.g., Koepke 1972, Young
et al.1990, andCyphorhinus araduson myrme-
cophyticTriplaris). By one estimate, fully a third
of tropical woody dicots and herbaceous vines
produce extrafloral nectar (EFN) and/or pearl
bodies to attract ants for anti-herbivore protection
(Schupp and Feener 1991, see also Blüthgen and
Reifenrath 2003). Still other plants, principally
herbs, epiphytes, and primary hemiepiphytes, use
ants to disperse their seeds (e.g., Davidson and
Epstein 1989, Kaufmannet al.1991, Horvitz
and Schemske 1994, Kaufman 2003); some such
plant taxa may be keystone resource species for
rainforest frugivores (Terborgh 1986). Ants also
interact strongly with other arthropods, espe-
cially sap-feeding Hemiptera (Auchenorhyncha
and Sternorhyncha [e.g., Buckley 1987]) and
other ant species (Jeanne and Davidson 1984).
Wilson (1959) first noted an important asym-
metry between arboreal and terrestrial ants.
Although few terrestrial taxa forage in the arbo-
real zone, many arboreal species forage terres-
trially. Coupled with larger mean colony sizes
in many arboreal species (Tobin 1994, Yanoviak
and Kaspari 2000), this observation suggests that
arboreal ants may often be competitively superior
to their terrestrial counterparts. Numerous recent
studies identify specific arboreal taxa as eco-
logical dominants, defending mutually exclusive
territories ranging from treetops to the ground
(e.g., Majer 1993, Dejean and Corbara 2003,
Blüthgenet al.2004b). Nevertheless, territorial
defense is selective, occurring against certain
other ant species and not others, though the basis
for selectivity remains poorly understood.
Why are arboreal ants so abundant? Why may
they outcompete terrestrial species, and what
accounts for the competitive superiority of eco-
logical dominants? These questions may have
common answers, related in part to how resource
imbalances have molded ant biology (Tobin 1991,
Davidson 1997, 1998, 2005, Yanoviak and
Kaspari 2000). Extraordinary dependence on
carbohydrate (CHO)-rich, nitrogen (N)-poor exu-
dates of associated plants and sap-feeding insects
appears to have strongly influenced the ecology
and evolution of arboreal ant taxa, together with
their functional roles in tropical ecosystems. In
turn, effects of dietary resource ratios on the
elemental and ecological stoichiometry of ants
have likely driven evolution of these associates.
Here, we briefly highlight some theoretical aspects
of the nascent fields of elemental and ecological
stoichiometry, and then illustrate how such prin-
ciples may play out in the biology and ecology of
tropicalarborealantsandothercloselyinteracting
species. We then consider how digestive anatomy
and function help to define the nature of ant
interactions with one another, plants, and tropho-
bionts. Our focus throughout is on ant nutrition,
but we do not dismiss the importance of other
aspects of ant ecology (e.g., nest site limitation
and top-down forces), already better understood
and summarized elsewhere (Herbers 1985, 1989,
Fonseca 1999, Orret al.2003, Foitziket al.2004,
Lebrun 2005).ELEMENTAL AND ECOLOGICAL
STOICHIOMETR YOF TROPICAL
ARBOREAL ANTS
The parallel theories of elemental/ecological
stoichiometry and the geometric framework
explicitly recognize mass balance of energy and