Geoscience Reference
In-Depth Information
identical value. Thus, the intraspecific or interspecific variability in PPMR is
related to the issue that different PPMR values are obtained depending on
the definition. However, at present we do not yet know exactly how PPMR
varies with factors and which factors should be considered when using each
definition of PPMR.
C. Goal of the Present Study
In brief, there is a gap between the empirical evaluation of PPMR and its
application to the food-web modelling, which arises from the issues of scale
dependence and variability of PPMR values. Hence, our primary goal was to
address the two fundamental issues in more detail than previously, using the
recently compiled gut content data of marine food webs (
Barnes et al., 2008
).
First, we show how the PPMR value varies among different definitions and
suggest the potential mechanism that creates scale dependence. Second, we
evaluate how the PPMRs defined at different scales are affected by predator
species identity and body mass. Our analysis aims to provide insights
towards improving our understanding of PPMR, facilitating future research
with respect to PPMR.
II. DATA
We use the recently compiled gut content dataset of marine food webs
(
Barnes et al., 2008
). The original dataset comprised 34,931 records of
prey-predator individual interactions from 27 locations, covering a wide
range of environmental conditions from the tropics to the poles. The dataset
includes 93 predator species (mainly fish) with size ranges of 0.1 mg to over
415 kg, and 174 prey types with size ranges of 75 pg to over 4.5 kg. Prey
organisms are not always identified to the species level and are sometimes
placed in an 'unidentified' category.
Barnes et al. (2010)
analysed the data
from 21 sampling sites and examined the effects of habitat properties (e.g.
productivity and water temperature) and body mass on PPMR. However, as
neither the scale dependence of PPMR nor the possible effects of species
identity were measured, the relative role of species identity and body mass,
and its dependence on the scale at which PPMR was measured, remains
unclear. We first conduct an analysis on the scale dependence of PPMR by
using the existing data (
Barnes et al., 2010
;
Table 1
). In addition, we examine
the effects of species identity and body mass with respect to each definition of
PPMR, for which we select 11 sampling sites that included more than two
predator species.
