Biology Reference
In-Depth Information
the results of multivariate analyses incorporating both range area and
Quaternary age-abundance, he argued that range area accounted for
essentially all of the variability. He therefore concluded that length of
postglacial presence had little to do with insect species richness and that
faunas in equilibrium with each tree species developed within a few hun-
dred years and were related primarily to distributional area.
Birks (1980) noted that the Quaternary age-abundance data used by
Southwood (1961) and Strong (1974b) were crude, and presented esti-
mates of the time of appearance of tree species in Britain based on radio-
carbon dates for postglacial pollen profiles and other dateable fossils. He
found that significant correlations existed for numbers of insects and time
that various tree species had been present, with the correlation being
strongest for angiosperms. He also emphasized that correlation does not
prove causation and suggested that evolutionary time might have been
more important than Strong (1974b, 1974c) inferred. Nevertheless, these
analyses do suggest that area is probably an important factor and that
British trees probably accumulate many herbivorous insects soon after
they invade. The analyses of insect diversity unfortunately do not distin-
guish generalist versus specialist species for the tree taxa.
Kennedy and Southwood (1984) reexamined a refined data set on
insects associated with British trees. Using stepwise multiple regression
techniques, these authors confirmed the importance of an index of area
and abundance as the strongest indicator of number of insect species asso-
ciated with each tree. However, they also found that the time data used
by Birks (1980) remained a significant variable in multivariate analyses
including area and abundance. In addition, they found that the number of
close relatives a tree species had in Britain had a positive influence on the
number of associated insects.Thus, this series of studies over more than 20
yr showed that both abundance and evolutionary time seem to influence
the diversity of insect herbivores that an introduced tree acquires.
Strong and Levin (1975) examined the richness of parasitic fungal
species for British trees, including natives and those introduced in recent
historical time.They also found that species richness was closely related to
distributional area. The slope of the relationship of number of species to
area was much lower for fungi than that for herbivorous insects, presum-
ably reflecting the fact that fungi exhibit a much greater dispersal ability
than insects. Fungal species thus appear to have attained the equilibrium
diversity expected by species-area relations within a few hundred years of
introduction of their hosts.
Many other studies have focused on the arthropods associated with
