Tropical Forest Community Ecology

338 Diane W. Davidson and Steven C. Cook

for total sugars (a) and amino acids (b). Reflect-
ing mainly sucrose, glucose, and fructose content
(Engelet al.2001, Blüthgenet al.2003), the
median value for total sugars in EFNs exceeds that
of SUCmin across Amazonian and Bornean ants
(Davidson2005andunpublisheddata)byafactor
of about five (19.85% versus 4.00%). In contrast,
the median amino acid concentration in EFN is
just a tenth that of AAmin (0.1% versus 1.0%).
Opposing patterns for sugars and amino acids
suggest that sugar concentrations in EFN were
not simply biased to high values by evaporation
during nectar accumulation after ant exclusion.
Overall, it appears that both EFN plants and
trophobionts offer ants sugars at concentrations
much greater than the minimum requirements
of many arboreal ant taxa, but amino acids at
much lower concentrations. Predictably, the ratio
of % total sugars to % total amino acids in EFN far
exceeds that of SUCmin to AAmin (Figure 20.2).
The few available values for sugar and amino
acid concentrations in honeydews (arrows in
figures) hint at a similar conclusion for those
secretions.
“Oversupply” of sugars relative to amino acids
in EFN may have both multiple causes and
important consequences. It could be favored, in
part, because myrmecophilous plants themselves
are more N-limited than C-limited (Schupp and
Feener 1991). Additionally, it may alter the out-
come of ant–ant competition in ways favorable
to deterrence of the plant’s natural enemies.
The most N-starved, behaviorally dominant, ter-
ritorial, widely foraging, and populous ant taxa
(e.g., Rocha and Bergallo 1992, see also below)
should provide better anti-herbivore protection,
and these same taxa should accept sugars only
at high concentration. Recent work by Blüthgen
and Fiedler (2004a,b and online material) in the
Australian tropics has established that the aggres-
sive, ecologically dominantOecophylla smaragdina
(authors’ unpublished ER=8) regularly monop-
olizes the highest quality exudates: EFN and
hemipteran honeydews with high concentrations
of both total sugars and total amino acids, and/or
highaminoaciddiversity.Distributionsof justfour
other ant species were correlated or marginally
correlated with solute concentrations in exudates;
these taxa most often used nectars of low or

8 2M

W

WCO C

6

No. of plant taxa

4

2

0

No. of plant taxa

10

30

50

0 200 400 600 800

% Sugar / % AA

0 1020406080

SUCmin / AAmin

(a)

(b)

Figure 20.2 Distributions of SUCmin/AAmin ratios

for Amazonian and Bornean ants (bottom panel; this

study), and ratios of % total sugars/% total amino acids

for 16 Australian EFN plants (top panel; Blüthgen and

Fiedler 2004; lettered arrows as in Figure 20.1). Note

the different scales of the abscissas in the two panels.

Unequal variances again required non-parametric

comparison: median SUCmin/AAmin ratios were

identical (4.00) for Amazonia and Borneo, but

combined ratios were significantly lower than the

sugar/amino acid ratio in EFNs of Australian rainforest

plants (median=110,X 12 =34.92,P<0.0001 in a

Wilcoxon test; additional details in Figure 20.1 legend).

low relative total sugar concentration and were

unaffected by amino acid concentrations. Eleven

remainingtaxawerenon-selective.Althoughhigh

quality resources may have been included in the

fundamental niches of the latter 15 taxa, they

were under-represented in their realized niches.

Because plants have limited control over the

recipients of ant rewards, all optimality models of

EFN composition must take into account multiple

and potentially competing ant species. Neverthe-

less, rather than focusing on all potential attend-

ing ant species individually, future studies might

profitably address reward composition in rela-

tion to ant functional groups, as determined from

feeding ecology (see below) and/or N-deprivation

(Blüthgen and Fiedler 2004a,b, Blüthgenet al.