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in
Figure 1
B, a mean mass of all predator individuals was calculated for a
given predator species (blue (open) shapes).
Level C: The next level of resolution has been termed link averaging
(
Woodward and Warren, 2007
) and considers only individuals involved in a
particular trophic link between focal and non-focal entities. That is, prey body
mass is calculated from those individuals measured in a particular predator
species' guts, not simply all individuals of that prey species found in the
environment. Similarly, predator body mass is derived from those individuals
with that particular prey species in their guts, rather than all individuals of that
predator species in the environment. In the example in
Figure 1
C, the mean
body mass of all individuals of a given predator species that feed on a particular
prey species is calculated and used as the value for the x-axis grouping and
plotted against the mean body mass of all individuals of that particular prey
species fed upon by that predator on the y-axis. At this level of resolution, it is
possible that both the focal and the non-focal entity can be represented as
having a different average mass depending on the particular interaction. In the
case of cannibalism, for example, it is most likely that a species would have
different focal and non-focal masses.
Level D: Lastly, to achieve the lowest level of taxonomic resolution, the
data were aggregated by species (or the next most resolved taxonomic level
for those taxa that could not be described to species). Here, whether an
individual was recorded as a prey or a predator was disregarded when
calculating the average body mass of a species (
Figure 1
D): this is equivalent
to many of previous studies which have used masses taken from the literature
(e.g.
Brose et al., 2006a; Cohen et al., 1993
; Riede et al., 2011). Both focal and
non-focal entities will now always have the same body mass regardless of
which interaction they are engaged in. Cannibalistic species will in this case
be represented as an entity that either is feeding on, or is eaten by, at least one
non-focal entity with the same average body mass as itself.
2. Different Levels of Aggregation Based on Size-Class Groupings
In addition to the taxonomic aggregations described above, two levels of
aggregation were performed on the size-class grouped data, depending on the
response variables we wished to calculate.
Level E: First, only the predator or prey individuals (depending on which
was the focal entity) were grouped into body size classes. The non-focal
entities are then the individuals associated with those individuals grouped
into body size classes (
Figure 1
E), in a manner that is analogous to the
taxonomic grouping in
Figure 1
B.
Level F: The second aggregation then involved also assigning the non-
focal individuals into body size classes (
Figure 1
F). In both levels E and F,
