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individual predator and prey sizes. Hence, due to the technical ease of this
method, species-averaged PPMR is the most commonly used form, as shown
in the compilation of global datasets by Brose et al.(2006a) . However, species-
averaged PPMR may differ from the PPMRs defined at the individual level,
when feeding habits vary within a species, for example, due to ontogenetic niche
shifts ( Werner and Gilliam, 1984; Wilbur, 1980 ).
Link-averaged PPMR also utilises the mean body mass of multiple indi-
viduals within a species, but differs from predator-averaged PPMR by being
based on the individual body masses of predators and prey that actually
consume or are consumed by the interacting species ( Figure 1 B):
Mean mass of predator individuals consuming a prey species
Mean mass of prey individuals consumed by a predator species
ð
1b
Þ
Link-averaged PPMR
¼
Species-averaged and link-averaged PPMRs are species based, in the sense
that these PPMRs use the mean body mass of multiple individuals.
The other two definitions of PPMR are individual based and are measured
from the predator- and prey-centred viewpoints, respectively. Individual-
predator PPMR is evaluated by viewing a predator individual as a scale at
which predator body mass is defined and by averaging prey body masses
found in the gut of predator individuals ( Figure 1 C):
Mass of an individual predator
Mean mass of prey individuals consumed by a predator individual
ð
1c
Þ
Individual-predator PPMR
¼
Individual-link PPMR focuses on each predation event of a prey individual
( Figure 1 D) and is defined as
Mass of an individual predator
Mass of a prey individual consumed by a predator individual
ð
Þ
1d
¼
Individual-link PPMR
PPMR may have several other definitions, depending on the selection of scale
at which body mass and prey-predator pairs are defined. For example,
species-averaged PPMRmay be modified to fuse the prey species into a single
category. Alternatively, as a modification of individual-predator PPMR, the
prey individuals found in each predator gut may be divided by species.
The number of data samples that are available from the same gut content
data varies among the four types of PPMR. The number of species-based
PPMRs (species-averaged and link-averaged PPMRs) available from a given
dataset is equal to that of predator-prey species-pair combinations. Mean-
while, the number of available individual-predator and individual-link
PPMRs is equal to that of predator and prey individuals in the dataset,
respectively. More importantly, it should be also noted that the estimated
PPMR may vary depending on the definition (i.e. scaling from individuals to
species). In a pioneering study, Cohen et al. (2005) examined the body lengths
of 37 species of parasitoids and 12 species of their aphid hosts. This study
showed that the relationship between host and parasitoid body size is
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