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Figure 8.5. Local versus regional species richness of stem-, bud-, and leaf-associated
cynipine wasps on Californian oaks. Each point between regional and local richness
represents a sampled local host population. Note that even at very high regional
richness the relationship does not become asymptotic, suggesting that local
communities are not saturated. From Cornell ( 1985a ). Reprinted by permission of
the author and the Ecological Society of America.
species tend to respond individualistically to environmental change. Types
of species, but not their abundances, are constrained by the need to avoid
ant predation. Competition for food is not important. Comparison with
other systems shows that each is different; there are useful generalisations,
but not at the level of intense local studies.
Studies of bracken insects were extended to other geographic areas,
i.e., New Mexico, South Africa, Papua New Guinea, and several British
sites. These studies also showed that bracken insects are probably type I
communities (see below), although data points are few. The size of the
regional pool is determined by how common and widespread bracken is
in each geographic area, indicated by a strong linear relationship between
area and species numbers. The presence of type I communities corres-
ponds to the finding that interspecific competition was shown to be
unimportant (see above). This is further supported by the finding that
there are many vacant niches (e.g., Srivastava et al. 1997 ). There are large
differences in the numbers of occupied niches between continents, and a
particular niche is sometimes occupied by one or many species.
Cornell ( 1985a , b ) censused 7 of the 15 Quercus oaks in California for
cynipine gall wasps and found a linear relationship between local and
 
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