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promising parameter for studying size-structured food webs and has been
used to model food-web structure and dynamics (
Brose et al., 2006b;
Jennings, 2005; Maury et al., 2007; Petchey et al., 2008; Silvert and Platt,
1980; Thierry et al., 2011
). PPMR represents the number of magnitude by
which predator individuals are larger than their prey individuals and is
ideally measured by direct gut content observations. A number of studies
and reports have provided such data for a wide range of animal species. The
compilation of these studies has recently revealed that, in general, the body
mass of predators is about 100 times larger than that of their prey, although
marked variations have also been found (
Barnes et al., 2010; Brose et al.,
2006a; Woodward and Warren, 2007
).
In theoretical studies, two main classes of size-structured food-web models
have been developed, specifically species-based and size-based models. While
both modelling approaches utilise PPMR as a key parameter, there are
differences in the basic assumptions. The species-based approach assumes
that body size is a characteristic of species (not individuals) and that feeding
relationships between species are systematically determined based on a
PPMR value (
Brose et al., 2006b; Petchey et al., 2008; Thierry et al., 2011
).
This modelling approach inevitably omits intraspecific variations in body
size and resource use. Meanwhile, the size-based approach describes the size
spectrum of a community, in which it is assumed that a single PPMR value
regulates the frequency that prey-predator interactions occur between indi-
viduals (not species) to govern the dynamics of the community size spectrum
(
Jennings, 2005; Maury et al., 2007; Silvert and Platt, 1980
). This modelling
approach incorporates intraspecific variation in body size, but often excludes
species identity. Although species-based and size-based models are distinct in
basic model structure, they share a common assumption that all individuals
have an identical value of PPMR, irrespective of species identity and body
mass. In other words, PPMR is regarded as a community-specific parameter
representing some trophic characteristics of food webs.
While PPMR has been widely used in size-structured food-web modelling,
the empirical estimation of PPMR is not straightforward and still remains
controversial. There are two critical issues that may influence PPMR estima-
tions, specifically scale dependence and variability.
1. Scale Dependence of Predator-Prey Mass Ratio
PPMR may be defined at various biological scales, depending on the way in
which predator and prey body mass is defined. As a result, several analytical
procedures may be implemented for the empirical evaluation of PPMR.
Woodward and Warren (2007)
presented four definitions of PPMR ranging
from low to high resolution, which we term (i) species-averaged PPMR,
