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Relationship between local and regional species diversity
Many authors have discussed how species comprising communities are
acquired from the regional species pool through filters. At least three such
filters can be distinguished, each of which leads to the loss of some species,
as follows:
(1) large-scale biogeographic processes (distance, isolation);
(2) landscape filters (patch size, density, configuration);
(3) habitat availability (Lawton 1999 , references therein).
But Lawton also points out that the importance of these filters may be
secondary to that of local ones. Concerning the relationship between
regional and local species richness, Cornell and Lawton ( 1992 ), also
Lawton ( 1999 , further references therein), following an approach intro-
duced by Terborgh and Faaborg ( 1980 ), distinguish type I and type II
systems, although in the real world systems may be intermediate. In the
former there is proportional sampling, that is, local richness rises in
proportion to regional richness. In the latter, local richness levels off
with increasing richness, that is, it never reaches the richness of the
regional species pool, because interactions between species prevent this.
Both false type I and type II communities may seem to exist under certain
conditions. For example, overestimation of regional species diversity may
falsely lead to the conclusion that communities are of type II, and a pattern
similar to type II may also result from ''stochastic equilibrium'' (coloniza-
tion and extinction are in balance). Lawton ( 1999 ) briefly discusses a few
examples and concludes that type I communities (or communities close to
them) are more common than type II communities. Cornell and Karlson
( 1997 ), Srivastava ( 1999 ), and Lawton ( 2000 , further references therein)
reviewed the literature and also concluded that most systems appear to be
type I communities. But even some of the examples for type II commu-
nities are probably not correct, as mentioned by Lawton ( 1999 ) himself.
Thus, fish parasite communities almost certainly are not saturated, and
interspecific competition is not important for them. Lawton, importantly,
stresses that a pattern does not give a mechanism, i.e., other mechanisms
than interspecific competition may lead to type II communities. For
detailed examples on the relationship between local and regional diversity
see pp. 130-131.
Rohde ( 1998b ), using computer simulations, has shown that an asymp-
totic relationship between local and regional species richness may simply
arise from different likelihoods of species occurring in a community
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