|Aim In simulation exercises, mid-domain peaks in species richness arise as a
result of the random placement of modelled species ranges within simulated
geometric constraints. This has been called the mid-domain effect (MDE). Where
close correspondence is found between such simulations and empirical data, it is
not possible to reject the hypothesis that empirical species richness patterns result
from the MDE rather than being the outcome (wholly or largely) of other factors.
To separate the influence of the MDE from other factors we therefore need to
evaluate variables other than species richness. The distribution of range sizes gives
different predictions between models including the MDE or not. Here, we
produce predictions for species richness and distribution of range sizes from one
model without the MDE and from two MDE models: a classical MDE model
encompassing only species with their entire range within the domain (rangerestricted
MDE), and a model encompassing all species with the theoretical
midpoint within the domain (midpoint-restricted MDE). These predictions are
compared with observations from the elevational pattern of range-size
distributions and species richness of vascular plants.
Location Mount Kinabalu, Borneo.
Methods The data set analysed comprises more than 28,000 plant specimens
with information on elevation. Species ranges are simulated with various
assumptions for the three models, and the species simulated are subsequently
subjected to a sampling that simulates the actual collection of species on Mount
Kinabalu. The resulting pattern of species richness and species range-size
distributions are compared with the observed pattern.
Results The comparison of simulated and observed patterns indicates that an
underlying monotonically decreasing trend in species richness with elevation is
essential to explain fully the observed pattern of richness and range size. When the
underlying trend is accounted for, the MDE model that restricts the distributions
of theoretical midpoints performs better than both the classical MDE model and
the model that does not incorporate geometric constraints.
Main conclusions Of the three models evaluated here, the midpoint-restricted
MDE model is found to be the best for explaining species richness and species
range-size distributions on Mount Kinabalu.|