High Fiber Content Is an Adaptation
Leaf toughness, nutrition value, and fiber content
strongly contribute to herbivore resistance. Phyllis
Coley (1983) examined rates of herbivory and defense
characteristics of 46 canopy tree species on Barro
Colorado Island. She compared young leaves with
mature leaves and gap- colonizing species with shade-
tolerant species. In general, young leaves were grazed
much more than mature leaves, even though many
contained phenols (indicating that phenols do not
prevent herbivory, though some forms may discourage
it). In another study Coley and her colleagues (2005)
learned that although leaves of shade- tolerant species
may live for several years, 75% of the lifetime damage
occurs during the early weeks when the leaves have
opened and are expanding, before they have added
essential fiber.
Soil Quality Affects Defense Compound
Abundance
Defense compounds are particularly abundant in plants
of lowland forest occurring on nutrient- poor white,
sandy soils, such as are found in the northern Amazon
region (chapter 6). Because of scarce soil nutrients, leaves
are metabolically costly to replace. These leaves are long-
lived (several years) and have such high concentrations
of defense compounds that when a leaf finally drops,
it must be leached of these compounds by rainfall
before it can be broken down and its minerals recycled.
Recall from chapter 6 that water from blackwater rivers
characteristic of white, sandy soil regions appears dark
and tea- like because of the concentration of leached
phenolics (e.g., tannins). Plants have greater fitness if
they manufacture enduring leaves with concentrated
defense compounds than replace leaves ravaged by
microbial pathogens, fungi, or herbivores. Given the
shortage of minerals in the soil, the replacement of leaves
is more costly than synthesis of defense compounds.
Most plant species in sunlit disturbed areas and
roadsides are adapted to maximize growth rates. They
synthesize alkaloids, phenolic glycosides, and cyanogenic
glycosides, all present in low concentration, collectively
representing a relatively low metabolic cost. In contrast,
plants on nutrient- poor soils invest in metabolically
more costly defenses such as polyphenols and fiber (e.g.,
lignin), retaining them in leaves and bark. The trees grow
more slowly but are better protected. These contrasting
patterns in plant defenses appear to be a function of
resource availability. On sites where resources are poor,
“expensive,” long- lasting defense compounds are favored.
On resource- rich sites, “cheaper,” shorter- lasting defense
compounds are favored, because the tree is able to both
devote sufficient energy to rapid growth and replace
defense compounds as needed.
Many species of disturbed areas are subject to
significant herbivore damage. Cecropias, for example,
are fast growing but routinely show extensively
damaged leaves (plate 11- 36).Ant- Plants and Plant Ants: Let the
Ants Do the Work (but Pay Them)Nectar is a rich, sugary substance strongly associated
with flowers. Recall from chapter 10 that it is the
“bribe” that is paid by the plant to pollinating animals:
“Come to the flower, have some nectar, but take along
some pollen and deliver it to another flower.” However,
nectar is not confined to flowers. Many tropical
plant species possess nectar- secreting glands, called
extrafloral nectaries (EFN), as well as other structures
that attract ants. Extrafloral nectaries are found in 90
plant families and 330 genera. Some of these plants,
which include ferns, epiphytes, vines, and trees, are
called myrmecophytes, or ant- plants, because of their
ant- attractant properties. Ant- plants occur widely in
the Old World tropics, especially Southeast Asia, and
in the Neotropics. They are also present in areas such
as the Sonoran Desert in the southwestern United
States. Ant- plants normally have some form of shelter
for ants (domatia) in addition to providing nutrition.Plate 11- 36. Cecropia showing leaf damage by herbivorous
insects. Photo by John Kricher.192 chapter 11 evolutionary arms races: more coevolution, more complexity