Geoscience Reference
In-Depth Information
1. Size Structure Dimension Set #1: Trophic Orderings
a. Predator Mass-Prey Mass. The first response variable examined
(regressed against predator body mass) was prey body mass. In the resolution
comparison, aggregations A and D were used (
Table 2
). That is, for A, the
log
10
body mass of each predator individual was plotted against the log
10
body masses of each prey item found in their gut. For D, species average
log
10
body mass of each predator species was plotted against the species
average log
10
body mass of each prey species fed on by that consumer.
In the groupings, comparison aggregations C and F were used. For the
taxonomic approach link average log
10
body mass of each predator species
was plotted against the link average log
10
body mass of each prey species fed
on by that consumer. Whereas for the size-class-based approach, the middle
of the size class on a log
10
scale consuming size class was plotted against the
middle (on a log
10
scale) of each prey size class fed upon by that consumer
size class.
b. Predator Mass - Predator-Prey Mass Ratio. The predator-prey body
mass ratio (PPMR) was calculated for all aggregations as log
10
(focal body
mass)-log
10
(non-focal body mass). For the resolution comparison, aggrega-
tions A and D were used. For A, the log
10
body mass of each predator
individual was plotted against the PPMR for each prey individual that a
predator consumed. For D, the species-averaged body masses were used; that
is, predator species average log
10
body mass versus PPMR for each prey
species that a predator species consumed.
In the grouping comparisons, the aggregations used were C and F. In the
taxonomic approach, the relationship was predator link-averaged log
10
body
mass versus the PPMR calculated for the link-averaged body masses for each
feeding interaction. While for the size-class approach, it was mid (predator
size class) versus the PPMR calculated using the midpoints of the consumer
and resource size classes.
c. Species Mass-Trophic height (TH). The third response variable exam-
ined was TH. This response could only be used for the grouping comparisons
as it required food web aggregations D* and F*. Prey-averaged TH was
calculated for each species (
Williams and Martinez, 2004
). This is equal to 1
þ
the mean of all of a consumer species' or size-class' trophic resources' TH
(species or size classes without prey are defined as having TH
1). It assumes
that consumers feed upon all their prey species or size classes equally, making
it suitable for binary food webs (
Williams and Martinez, 2004
).
¼
