Geoscience Reference
In-Depth Information
Box 1
Interdependence among scaling relationships
Some of the response variables (scaling relationships) in our analysis are
strongly correlated. Indeed, if we know the relationship between preda-
tor body mass and prey body mass, the relationship between predator
body mass and PPMR can be predicted (see also Riede et al., 2011).
Using data from individual feeding events from one marine and
six freshwater (stream) food webs, we find the following relationship
between predator body mass, M
P
, and the body mass of its prey, M
R
:
log
10
M
R
¼
a
þ
blog
10
M
P
ð
1
Þ
where the values of the intercept a and the slope b aresystemspecific.The
slope is smaller than 1 (b
1) in all systems (except for Trancura River).
Moreover, we find the following relationship between predator mass
and PPMR:
<
¼
M
P
M
R
log
10
c
þ
dlog
10
M
P
ð
2
Þ
where the values of the intercept c and slope d are system specific.
Using Eq.
(1)
, the PPMR can also be expressed as:
¼
M
P
M
R
log
10
log
10
M
P
log
10
M
R
¼
log
10
M
P
a
b log
10
M
P
ð
3
Þ
¼
a
þ
ð
1
b
Þ
log
10
M
P
Thus, c
¼
a and d
¼
(1
b). Now, as b
<
1 for all webs (except for
Trancura River), it follows that d
0 for all webs (except for Trancura
River). This means that the size difference between a predator and its
prey will increase with the size of the predator in the webs analysed
here. In a study of 21 marine food webs,
Barnes and colleagues (2010)
found that d
>
0 in 14 of the webs.
>
themselves.
Woodward and Warren (2007)
also illustrated that the predator-
prey mass ratio (PPMR) in a system can be severely underestimated when
using species averages, when compared with data derived from individual
feeding events. The difference in scaling of PPMR with predator body mass
depending on resolution is also in line with a recent study by
Nakazawa et al.
(2011
).
Why do PPMR patterns based on species averaging differ from those
based on individual-level data? The main reason is that intraspecific size
