Contents - LAC Biosafety

74 Ecology of insects in the forest environment
termite species were seen in procession, in the open, each carrying a cut piece of
grass, much like the leaf-cutting ants of tropical America (unpublished
observations). It is not known whether these termites use the grass to cultivate
fungi like the leaf-cutting ants.
The regeneration of some trees is facilitated by the activity of termites. It has
been shown (Chacko, 1998) that feeding of termites on the mesocarp of fallen
teak fruits (seeds) on the forest floor induces germination of the recalcitrant
seeds. On the other hand, insect seed predators may adversely affect
regeneration of some tree species. For example, Curran and Leighton (1991)
reported that in one year a dipterocarp seed crop of about 100 000 seeds ha 1 in
the lowland forest of West Kalimantan, Indonesia was entirely destroyed by
seed-feeding insects. The phenomenon of mass fruiting of dipterocarps in some
years is thought to be a strategy to escape complete seed destruction by
satiating the seed pests (Janzen, 1974).
Only some of the known relationships between plants, insects and their
environment have been discussed above and several other intricate relationships
remain little known. Because of the manifold interactions, forest fragmentation
and degradation which lead to loss of biodiversity will adversely affect the
proper functioning of tropical forest ecosystem processes.
3.2.7 Influence on forest primary production, succession and tree evolution
Mattson and Addy (1975) have argued that phytophagous insects
function as regulators (in the cybernetic sense) of primary production in forest
ecosystems. According to them, the activity and abundance of phytophagous
insects is dependent on the vigour and productivity of the forest ecosystem.
When the vigour and productivity of the ecosystem is lowered due to tree age,
stressful climatic conditions, low fertility of the site or bottlenecks in the flow of
certain vital nutrients, the insects respond by increase in their numbers, leading
to population outbreaks. This ultimately results in rejuvenation of the ecosystem
as insect grazing stimulates the host’s physiological system, increases the
penetration of sunlight, increases soil fertility through increased litter fall
(including insect excrement and cadavers) and kills weakened or old trees,
leading to the growth of more vigorous younger plants of the same species or
individuals of other species. Thus insect outbreaks help to maintain nutrient
cycling and primary production at optimal rates for a particular site. Each tree
species and forest ecosystem supports a variety of insects whose composition
varies with the seasonal and ontogenic development of the plants and at least a
few of these insects are thought to be capable of making dramatic population
changes in response to subtle changes in individual plant or ecosystem processes
(Mattson and Addy, 1975). In this manner, insects are thought to act as