Plants vs. Insects
Ecologists debate the degree to which insect
communities in the tropics exhibit greater host
specificity and specialization compared with those at
higher latitudes. Some data suggest that lepidopteran
(butterfly and moth) species are proportionally
more host- specific in the tropics. Is increased insect
specialization a general characteristic of tropical
ecosystems, particularly rain forests? The answer is
equivocal. It depends on how specialization is defined.
Further complicating the question is that the exact
magnitude of insect species richness is still relatively
poorly documented for rain forests.
Defense compounds, protective ants, and tough
leaves all provide selection pressures affecting
the evolution of insect herbivores. Those insects
that evolve enzyme systems that detoxify defense
compounds or somehow sequester them are able to
specialize on specific plant species. Plant compounds
may be repellent but not actually be toxic. Insects may
overcome the repellency and adapt to recognize a host
plant by its repellent compounds.
Insects may evolve behaviors that minimize exposure
to defense compounds. For example, the caterpillars
of the butterflies of the genus Melinaea feed on plants
in the nightshade family (Solanaceae). Philip DeVries
and Irene Baker showed that the caterpillars cut the
leaf veins of their host plants, preventing the defense
compounds from reaching the leaf blade, where the
caterpillars feed.If you want to understand what goes on routinely
in nature think of the fight against computer hacking
as an analogy. Operating systems come with virus
protection and various firewalls, and hackers figure out
away around them. Then the programmers develop
more protective software, and the hackers in turn hack
that. Nature has been doing that sort of thing ever since
there has been nature.Plants vs. Howler MonkeysPrimates are largely herbivorous, and many eat mostly
fruit. Some, however, such as howler monkeys, are
primarily leaf eaters, or folivores. Thus they are exposed
to plants’ defense compounds. In a famous study by
Ken Glander in Costa Rica, Mantled Howler Monkeys
(Alouatta palliata) were observed to occasionally
appear disoriented and even to fall from trees. Further
observations showed that the howlers were apparently
sickened by consuming toxic leaves and that affected
their equilibrium in the trees. Howlers must learn
to be extremely selective in exactly which trees they
dine upon (plate 11- 40). Within one tree species,
Gliricidia sepium, represented by 149 individual trees
in the howlers’ range, the troop fed on only three of
the trees and always the same three. The leaves of these
trees were less difficult to digest than those of their
conspecifics. Mantled Howlers favor young leaves,
which are relatively high in nutritional value but not
yet concentrated with defense compounds. WhenPlate 11- 40. Mantled Howler Monkeys are folivores, but they
do not eat just any leaves in any tree. They appear to pick and
choose in order to avoid plants’ defense compounds. Photo by
John Kricher.Plate 11- 41. Spider monkeys are mostly frugivores, not
folivores, and their intestinal system is adapted accordingly.
Photo by Jill Lapato.chapter 11 evolutionary arms races: more coevolution, more complexity 195