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sensitive to the biological level at which body size is defined. Thereafter,
Woodward and Warren (2007)
performed a detailed analysis about how
PPMR varies with respect to the four definitions in a freshwater invertebrate
community (also see
Woodward and Hildrew, 2002
) and found that PPMR
using averaged body sizes (in particular, species-averaged PPMR) was lower
than individual-link PPMR by about one order of magnitude. As far as we
know, these are the only two studies (
Cohen et al., 2005; Woodward and
Warren, 2007
) that have thus far revealed the scale dependence of predator-
prey (or parasitoid-host) body-size relations. Although still limited, the
available evidence clearly indicates that the use of averaged body sizes to
evaluate PPMR may generate a misleading interpretation of the real feeding
relationships within food webs.
2. Variability of Predator-Prey Mass Ratio
The other critical issue regarding the empirical evaluation of PPMR is that,
contrary to the assumptions made for size-structured food-web models,
PPMR may not be identical among all individuals of all species in real
food webs. Only a few exceptional studies have dealt with the issue of intra
and/or interspecific variability of PPMR.
Cohen et al. (2005)
showed that
primary, secondary, hyper-, and mammy parasitoids have different body-size
relations with their host individuals and concluded that PPMR may vary
among trophic niche positions.
Brose et al. (2006a)
analysed global datasets
covering a wide range of animals and habitats and showed that PPMR varies
across different habitats (e.g. higher in freshwater habitats than in marine or
terrestrial habitats), predator types (e.g. higher for vertebrate than for inver-
tebrate predators) and prey types (e.g. higher for invertebrate than for
ectotherm vertebrate prey). Although these results clearly show that PPMR
may vary among animal types or habitats, the study was only based on
species-averaged PPMR, and thus the implications for individual-level
predator-prey interactions remain uncertain. More recently,
Barnes et al.
(2008)
compiled published gut content data of marine food webs, for which
the body sizes of individual predators (mainly fish) and prey in their guts are
available. Using this dataset,
Barnes et al. (2010)
illustrated that individual-
predator PPMR varies among sampling sites and predator size classes. Their
important finding is that PPMR increases with individual-predator mass,
which implies that the relationships between the log body masses of preda-
tors and prey are non-linear, and clearly diverge from the conventional
assumption that PPMR is common within species. All of the available
examples indicate that PPMR may vary with various factors, such as species
identity, body mass, and food webs. Logically, if all individuals of all species
had a common PPMR, the different definitions of PPMR should produce an
