Geoscience Reference
In-Depth Information
C. Application
PPMR is scale dependent and may vary depending on the method of estima-
tion. This fact has an important implication on how size-structured food-web
models should be constructed and parameterised (also see
Gilljam et al.,
2011
). Size-structured food-web models implicitly or explicitly assume par-
ticular definitions of PPMR, in accordance with the model structure. There-
fore, we suggest that these models should be parameterised based on the
particular PPMR that reflects the assumption. For example, species-based
allometric food-web models (
Brose et al., 2006b; Petchey et al., 2008; Thierry
et al., 2011
) exclude intraspecific variations by adopting the species averaging
procedure and thus should be based on species-based PPMR, such as species-
averaged and link-averaged PPMRs. On the other hand, size-based commu-
nity-spectrum models (
Jennings, 2005; Maury et al., 2007; Silvert and Platt,
1980
) assume that prey-predator interactions occur between individuals (not
species) and thus should rely on individual-based PPMR, such as individual-
predator and individual-link PPMRs.
Given this, conventional parameterisation of current modelling approaches
could be improved. In particular, the size-based approach has conventionally
employed PPMR
10
2
(e.g.
Andersen and Beyer, 2006; Blanchard et al., 2009;
Hartvig et al., 2011
) to quantitatively describe marine food webs. However, our
analysis of marine food-web data revealed that individual-link PPMRwould be
greater by about one order of magnitude, while individual-predator PPMR is
still close to 10
2
(
Figure 2
). The same results were obtained in a freshwater
invertebrate community, where individual-predator PPMR is estimated to be
close to 10
3
rather than 10
2
(
Woodward andWarren, 2007
).We therefore argue
that there should be a greater focus on the scale dependence of PPMR and that
size-structured food-web models should be more carefully parameterised using
an appropriate definition of PPMR.
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IV. DETERMINANTS OF PREDATOR-PREY MASS
RATIO
A. Statistical Analysis
The scale dependence of PPMR (
Figure 2
; also see
Woodward and Warren,
2007
) implies that PPMR may not be identical among individuals of the same
species and/or among species within the same food web. This casts a question
as to what determines each type of PPMR. Such studies remain limited,
despite previous analyses showing how PPMR is related to various factors,
such as predator body size, species identity or animal type, and habitat
