Contents - LAC Biosafety

3.2 Role of insects in ecosystem processes of tropical forests
Insects play key roles in ecosystem processes at two trophic levels – as
primary consumers and as decomposers. They also play minor roles as secondary
and tertiary consumers. In addition, they interact with many other life forms in
innumerable ways. These direct and indirect effects of insects on trees, other
organisms and the physical environment can influence primary production,
succession and evolution of plant communities.
3.2.1 Insects as primary consumers
3.2 Role of insects in ecosystem processes 61
The phytophagous insect fauna of tropical forests is rich in species
(i.e. diversity) as we saw in Chapter 2, although under normal conditions
the number of individuals per species (i.e. abundance) remains low. In general,
each plantation tree species has 10 – 200 species of associated insects (Chapter 5,
Section 5.4). For trees in natural forests very little information is available.
Published records for 20 species in the moist – deciduous forest of Kerala, India
(Nair et al., 1986a) show an average of 38 species (range, 2–188) of insects per tree
species, but this is not based on a comprehensive search, most records being
incidental. The richness of the canopy insect fauna of tropical forests was clearly
brought out in several recent studies (see Chapter 2). In lowland seasonal forest
in Panama, 1200 species of beetles and 332 species of bugs were recorded from
the canopy of a single tree species, Luehea seemannii. The greater part of canopy
insects are herbivorous, feeding on the leaves or sap. Some studies indicate that
chewing insects consume 7–10% of the leaf area in tropical forest canopies (Wint,
1983) although higher levels of leaf consumption may occur in some seasons. For
example, in the lowland rain forests of Panama and Papua New Guinea, Wint
(1983) recorded 13–14% defoliation during the summer months. However, in a
study in moist–deciduous and evergreen forests in Kerala, India, Nair et al.
(1986a) found only about 2% annual foliage loss caused by insects. This estimate
was based on monthly visual scoring of leaf loss, on five trees each of 38
representative species in the natural forests over a two-year period. The effect of
sap-sucking, gall-forming and stem-boring insects was not assessed. Based on
several studies made in temperate countries, Schowalter et al. (1986) estimated
that insects normally consume less than 10% of annual foliage standing crop.
The few studies in the tropics mentioned above suggest that this may be
applicable to tropical forests as well, although quick post-defoliation regrowth of
leaves in tropical trees, following insect feeding, complicates these estimates.
Leaf consumption may reach extremely high levels when insect outbreaks
occur. Several examples of such outbreaks are described in Chapters 4 and 10.
During these outbreaks, huge quantities of foliage of particular tree species