including people, if they should brush against the tree
(a personal experience not easily forgotten). Ants
appear agitated, swarming out of the thorns and over
the foliage, at any disturbance.
The ants obtain shelter within the thorns as well as
nutrition from two kinds of extrafloral nectaries. One
type is termed Beltian bodies, small orange globules
growing from the tips of the leaflets of the compound
leaves; the other type is the foliar nectaries, located on
the petioles.
Janzen performed a field experiment that discriminated
between the protectionist and exploitationist hypotheses.
He treated some acacias with the insecticide parathion,
and he clipped thorns to remove all ants from the
treated trees. The antless trees did not survive nearly as
well as the control trees, which were permitted to keep
their ants. Janzen estimated that antless acacias were
not likely to survive beyond one year, either falling prey
to herbivores or being overtopped by other, competing
species of plants. He concluded that the ants and acacias
are obligate symbionts, depending entirely upon each
other. The relationship between plants and ants is thus
one of obligate mutualism.
More recent research has shown that the acacias
attract protectionist ants in a biochemical manner.
They synthesize the enzyme invertase, which cleaves
the disaccharide sucrose in the EFN, rendering it to
glucose and fructose. Protectionist ants avoid sucrose
but are attracted to glucose and fructose. Researchers
manipulated the sucrose concentrations in EFNs of
acacias and were able to attract or repel Pseudomyrmex
ants on the basis of presence (repel) or absence (attract)
of sucrose.
Extrafloral nectaries are known from some
temperate- zone plants but are far more abundantly
represented among tropical plant species. Though
many plants with EFNs house ants, the degree to which
the ants act to protect their hosts may vary considerably
among species. But in the case of swollen- thorn acacias
and some cecropia species, ants have clearly assumed
the function of defense compounds.
Extrafloral Nectaries Promote Multispecies
Interactions
Because they represent an energy resource, it should not
be surprising to learn that extrafloral nectaries attract
numerous arthropods, not merely ants. There is constant
selection pressure operating in any ecosystem to screen
arthropods as antagonists or otherwise. Ant defenders
are subject to selection pressures by organisms other
than plants. Philip DeVries documented a remarkable
example in Panama, observing that caterpillars of the
butterfly Thisbe irenea entice ants to protect them rather
than their host plant (Croton), and the caterpillars then
eat the leaves from the very plant the ants were once
protecting. These caterpillars, termed myrmecophilous,
for their “ant- loving” habits, have evolved at least three
separate organs that act to attract and satisfy ants: nectary
organs that produce protein- rich ant food; tentacles that
release chemicals mimicking those of the ants themselves
and signaling them to defend; and vibratory papilla
that, when the caterpillar moves its head vigorously,
make sounds that travel only through solid objects, but
which immediately attract ants. The ants appear to have
a stronger preference for the protein- rich caterpillar
nectar droplets than for the carbohydrate- rich food
supplied by the Croton nectaries. The ants are essential
in protecting the otherwise vulnerable caterpillars from
predatory wasps. By providing nectar for the ants, the
caterpillars have succeeded both in averting the main
protective adaptation of the plant and in ensuring their
own relative safety from their major predators, wasps.
Amazing.
The ecological significance of extrafloral nectaries
remains an area of active study. Because EFNs are so
widespread among tropical tree species, they may be
a widely utilized resource responsible for significant
energy movement through tropical food webs.The Evolutionary Arms Race
Given that so many species of tropical plants possess
numerous defense compounds as well as mechanical
defenses, it may seem surprising that any kind of
herbivore is able to consume them. But evolution
is never static. Once a defense compound evolves, it
in turn acts to exert a counter- selection pressure on
herbivores and pathogens to evolve some form of
adaptation that circumvents the plant defense. Once
again vulnerable, the plant is then under enhanced
selection pressures to evolve yet another defensive
adaptation, which, should that occur, merely acts as
yet another selection pressure on the herbivore, an
evolutionary arms race of sorts. There are numerous
examples of this chain of events. Here are a few of the
most noteworthy.194 chapter 11 evolutionary arms races: more coevolution, more complexity