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| Although the mid-domain effect (MDE) model for species richness in
bounded geographical domains has proved controversial, several studies
have revealed its explanatory potential for patterns of species
richness. This paper investigates unexplored aspects of one-dimensional
MDE, based on a data set of latitudinal distributions of New World
birds (3706 species) on a V scale. Two previously published data sets
for other taxa are also considered. We adjusted band sums (number of
species per latitudinal band) for longitudinal area by constructing
species-area curves for each band. Area-corrected richness patterns
differed substantially from raw band sums, although both confirmed a
strong, mid-tropical peak in richness. An MDE model accounted for 47%
of the adjusted pattern, whereas area alone explained 13% of variation.
Area-adjusted band sum data proved preferable to coastal transect data
from the same data set. MDE was relatively more important in smaller
latitudinal domains and/or for taxa with relatively large ranges. On
fundamental grounds, we concluded that MDE randomizations based on
empirical ranges are more appropriate than those based on theoretical
range size distributions. Models that, like MDE, produce quantitative
richness predictions should be evaluated statistically against the null hypothesis of equality, not simply correlation, of empirical vs.
predicted richness values. | |
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