Biology Reference
In-Depth Information
abundances at least of some species, i.e., food is not in unlimited supply for
some species, and not all fronds are equally suitable as resources, although
the reasons for this are not understood and it is not known whether these
differences lead to density-dependent limitations in resources.
There are distinct differences in the insect communities between the open
and woodland patches, but it proved impossible to determine the reasons.
Concerning top down effects, observations showed strong effects of this kind
in a sawfly species at the open patch: the species colonized individuals of
herbivore-free bracken in an experimental setting, but unaided. The bracken
was separated into two groups. In one group, the colonization led to a
population explosion in the following years. Fronds were strongly defoliated,
as never seen in the field. The population collapsed and disappeared com-
pletely, as the result of infection with an unidentified microorganism. In the
other patch, the sawfly population persisted and there was no disease before
observations ceased a few years later. Lawton concludes that the population
explosion was due to release from an unidentified disease, indicative of strong
top down effects. Similar top down effects were demonstrated for a delpha-
cid bug. Densities of the bug were raised in experimental areas and controls
were kept in unmanipulated areas. There was a rapid (over a few days)
convergence of densities both of controls and experimental fronds. Several
replicasgavethesameresults,whichwere not the result of density-dependent
dispersal nor of density-dependent interference. The decline was not
due to greater numbers of a predator, a spider. Reasons remain unclear, but
parasitoids may be responsible. Lawton concludes that, although results are
not ''as clean as I would wish,'' it seems that both bottom up and top down
processes are involved in determining abundance, but for most of these
processes it is unclear whether they are density-dependent or density-
independent. Ants, which feed on bracken nectaries, have no effect on
species richness or abundances of insects. Ants feed on caterpillars of other
plants, but the bracken caterpillars appear to be protected against ants by
special adaptations, such as living in galls, mines, or silken webs, possessing
distasteful haemolymph, or using avoidance behavior. To test for inter-
specific competition, statistical correlations were used, because experi-
ments could not be conducted. Does greater presence of one species
depress numbers of others? No such correlations were found.
For the community as a whole, Lawton concluded that, in spite of some
ambiguous results mentioned above, most insect species on bracken are
regulated by density-dependent processes; bottom up and top down pro-
cesses affect abundances. There is much predictability in the rank order of
abundances, but predictability decreases over time. Nevertheless, insect
