Geoscience Reference
In-Depth Information
ABSTRACT
The density mass-relationship (DMR) between abundance and body size is a
key attribute of biodiversity organisation. The identification of the determi-
nants of the DMR has consolidated as a major research area, focused on
both statistical and ecological issues. Here, we advance the connection be-
tween food webs and DMR, by showing how gape limitation could deter-
mine the amount of resources available and consumption by enemies, the
number of modes, scaling exponents, and intercepts of the DMR. The widely
used statistical approach of applying ordinary least squares (OLS) regres-
sions to log-transformed data of recorded densities—or histogram frequen-
cies—and mass has been shown to be biased and to present statistical
problems. Improvements have been suggested for all these methods, with
the maximum likelihood (ML) approach emerging as the best one for both
frequency distributions and fits to untransformed data in bivariate relation-
ships. The combination of these methods with tools to detect more than one
scaling in a dataset, such as segmented regressions, could detect more com-
plex patterns, to test and validate theoretical expectations. At least five
different DMRs have been reported in the literature to date, but it is not
evident whether variations in the reported patterns originate from attributes
of the studied systems or if they are determined by properties of particular
DMR used. We analysed these five DMRs and related statistical tools in a
metacommunity composed of 18 local communities of temporary ponds.
DMRs presented steeper slopes than those usually reported, with evidence
for changes in the scaling regime across size classes. Evaluation of the
performance of alternative statistics confirmed ML estimates as the best
method available, even with small sample sizes. To understand DMR, it is
clear that explicit attention should be paid to the ecological mechanisms
involved in each one of the alternative approaches, and to the statistical
tools that can be used for its detection.
I. INTRODUCTION
It is generally accepted that larger animals are less abundant than smaller
ones, particularly if their trophic position increases with body size (
Cohen
et al., 2003; Elton, 1927
). The observation of reductions in density compen-
sating for the increase in energetic demands with individual size has been
suggested to derive from an energetic equivalence where, on average, popu-
lations use the same amount of energy independently of their individuals'
body size (
Damuth, 1981, 1987, 1991; Nee et al., 1991
). The density-mass
relationship (hereafter DMR; also commonly referred to as mass-abundance
scaling) has been connected to sound ecological and evolutionary
