Some detailed biogeographical/macroecological patterns - Nonequilibrium Ecology - page 143

Figure 9.3. Numbers of endemic reef fish (restricted to a single location) in the
Indo-Pacific ocean. The center has by far the greatest number of endemic species,
supporting the center-of-origin hypothesis explaining the great overall diversity
in the center. Reprinted from Mora, Chittaro, Sale, Kritzer, and Ludsin ( 2003 ),
with the permission of MacMillan Publishing Ltd., and the authors.
center of speciation from which species disperse); the center-of-overlap
hypothesis (high diversity in the center is the result of overlapping faunas
from more marginal regions); and the center-of-accumulation hypothesis
(speciation has occurred in several peripheral regions and ranges have
expanded due to prevailing currents into the center, some marginal popula-
tions may have been lost secondarily). Data permit a conclusion on which of
the hypotheses is correct. The last two hypotheses can be ruled out because
peaks of plots of species' longitudinal and latitudinal mid-ranges coincide
with the center, and because the center has the greatest number of endemics
(Figure 9.3 ) . There are some minor centers of endemism as well, mainly in
geographically isolated locations or in locations where the current direction
is mainly from tropical to temperate. The great diversity in the center can be
explained by the very great number of islands per unit of geographical area,
greater than in any other region, facilitating a high level of allopatric
speciation, further facilitated by recent geological sea level changes.
Species richness decreases, and dispersal ability increases, with distance
from the center (Figure 9.4 ) . In other words, community composition is
largely determined by the distance to the center and by the ability of species
to disperse; locally endemic species contribute little (about 2%) to commu-
nities away from the center (Figure 9.5 ).
In summary, not only latitudinal but also longitudinal gradients in
diversity exist, leading to a center of diversitywith particularly great richness
of endemic species from which outlying regions are colonized. Speciation
in outlying regions occurs but contributes little to community richness.
However, oddly, Hughes et al.( 2002 ) arrived at quite different con-
clusions. They found that the centers of diversity do not have the greatest