Chapter 20

TropicalArborealAnts:

LinkingNutritiontoRoles

inRainforestEcosystems

DianeW. Davidson and Steven C. Cook

OVERVIEW

Arboreal ants are extraordinarily abundant and functionally important in tropical rainforests worldwide. With
carbohydrate-rich, nitrogen (N)-poor diets, many such ant taxa invest “excess carbon” in chemical weaponry, wide-
ranging and high tempo foraging, spatial territoriality, support of populous colonies, and nutrition of N-contributing
microsymbionts. Despite such investments, and their potential contributions to nitrogen gain and/or conservation,
feeding assays document greater average N-limitation in arboreal exudate-feeders than in predatory/scavenging
species.
Better anti-herbivore protection may accrue to extrafloral nectar (EFN) plants attracting the most N-starved, eco-
logically dominant, and territorial arboreal ant species. Such taxa should take sugars only at high concentration but
amino acids even at low concentrations. Not surprisingly, EFNs average higher total sugar concentrations and lower
amino acid concentrations, compared with minimum sugar and amino acid concentrations acceptable to foraging
workers of arboreal taxa as a whole.
Rankingsof exudate-feedersbyincreasingdietaryNcontributionsfrompredationandscavenging(increasingδ^15 N)
generally correspond to expectations of myrmecologists and may be reasonable predictors of the relative capacities of
different ant taxa to deter plant herbivores. Exceptionally low values for certain “herbivorous” taxa may be explained
in part by differential recycling of light N by microsymbionts.
In the most prominent exudate-feeding taxa (Formicinae and Dolichoderinae), foraging functional groups may be
defined by a combination of digestive anatomy and body size. Among formicines, derived proventriculi and large,
size-adjusted crop capacities and liquid uptake rates correlate with solitary leaf-foraging, visual navigation, diurnal
activity, diverse diets, and high species richness. In contrast, large-bodied dolichoderines (Dolichoderusspecies) have
plesiomorphicproventriculi,poorerforagingperformances,uniformdietsof trophobionthoneydew,andcomparatively
lowspeciesrichness.Wide-rangingleaf-foragersshouldbegoodexploitativecompetitors,whereastrophobiont-tenders
may be better interference competitors. Foraging in aggregate, the latter taxa may also encounter fewer herbivorous
prey, and often damage hosts via resource parasitism and disease transmission. Small-bodied taxa (including some
myrmicines) are generally highly N-limited and accepting of lower quality resources. Finer distinctions are inevitable
within all functional groups. Although foraging functional groups correlate strongly with taxonomy, it remains
uncertain how they “map” to guilds defined by structure and intensity of interspecific interactions.

INTRODUCTION

Measured in terms of both numbers and biomass
in canopy fogging samples, ants (Formicidae:
Hymenoptera) are the dominant arthropods
of tropical rainforest canopies. Insecticidal
fogging may oversample cursorial insects and

undersample some sap-feeders (Dejean et al.

2000), but the abundance of ants is so striking

(regularly half of all sampled arthropods; Tobin

1995, Davidson and Patrell-Kim 1996) that their

pre-eminence should survive correction for these

biases. Relative to ant communities in temperate

forests, tropical communities are remarkable