Biology Reference
In-Depth Information
inner circle. In only 1 out of 23 comparisons was mortality greater in the
inner circle than the outer ring. Only 5 out of 100 comparisons between
the two inner zones and the area ''beyond'' showed greater mortality, and
twice mortality was even lower. The authors conclude that the few cases of
compensation could perhaps be explained by chance. Field experiments
were conducted using seedlings of two tree species. In one, mortality was
closer to a conspecific adult, in the other there was no difference.
The authors also tested for intermingling of species. As indicated
above, seedlings of common species have greater mortality near conspe-
cific adults. This could result in creating opportunities for rarer species
and lead, in time, to a higher degree of intermingling. Use of Pielou's test
for ''pattern diversity of communities'' showed that individuals of all sizes
have conspecifics as neighbors more often than expected by chance for
most size classes of seedlings. However, pattern diversity increased sig-
nificantly with age, and it also increased significantly in the period since
initial mapping, although among the eight older classes, there was only
one significant increase. The conclusion has to be that most intermingling
occurs between the seedling and sapling stages.
Another possible compensatory mechanism (later called the
Janzen-Connell effect by others) is that herbivores could be attracted by
high-density clumps composed of single species or by seedlings close to
adults of the same species. Connell (
1979
) dismisses the effectiveness of this
mechanism, because it implies a great degree of specialization: herbivores
must be able to distinguish between tree species for it to become effective.
Herbivores in tropical forests were indeed shown to have low host speci-
ficity. Data from 900 herbivore species feeding on 51 plant species in
New Guinea were analyzed by Novotny et al.(
2002
), and most were
found on several plant species. According to the authors, monophagous
herbivores are probably rare in tropical forests, because species-rich genera
are dominant. Also, most herbivore communities shared a third of their
species between phylogenetically distant hosts. However, differential pre-
ferences of herbivores for different host species were not considered. (It can
ance of seeds or seedlings near conspecific adults in 15 out of 19 popula-
tions with an insect as the main herbivore; it was worse in 2 out of
27 populations with a vertebrate as the main herbivore (references in Wright
2002
). Nevertheless, Hubbell (
1980
) argued that the Janzen-Connell effect
could not be important, considering its short-distance effect and the great
species diversity, an argument not accepted by all (see below, Wright).
