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Hutchinson, 1959
). However, the change in trophic position of predators with
body size, and the consequent change in available resources, was only recently
considered as determinant of the DMR (
Brown and Gillooly, 2003; Cohen
et al.,2003
). If the organisms analysed are members of a single trophic level, the
DMR should be shallower than in those cases where larger individuals occupy
upper trophic positions with less resources (see
Brown and Gillooly, 2003;
Brown et al.,2004
). Further, this idea was formalized providing quantitative
predictions about the effect on the DMR of energy loss between trophic levels
(see
Brown and Gillooly, 2003; Brown et al., 2004; Cohen et al., 2003; Reuman
et al.,2009
). Considering efficiency
a
and a size ratio between predators and
prey of
b
, the expected association is (
Reuman et al.,2009
)
log M
log
ðÞ
3
4
log N
ðÞ¼
ðÞ
ðÞþ
a
ð
1
Þ
log
The consideration of this ecological mechanism could represent a main expla-
nation for the variation in slope of the DMR reported in the literature. This
analysis indicates that species with high efficiency in trophic transfer—such as
ectotherms in relation to endotherms—should have shallower slopes and food
webs with larger predator-prey size ratios should have steeper slopes. In
addition, it is important to note that this mechanism could also account for
the existence of non-linearity in the DMR. In this sense, a decrease in predator-
prey body size ratio as the trophic position of the predator increased has been
recently reported (
Riede et al.,2011
). FromEq.
(1)
, it is expected that the DMR
becomes steeper at larger masses. It has also been predicted that larger indivi-
duals within communities are prone to be energy-limited and, consequently,
restricted in the trophic position they can achieve (
Arim et al., 2007; Burness
et al.,2001
). When the consumption of resources at lower trophic positions
implies prey of small size—for example, filter feeders—the DMR could become
shallower at larger sizes. Further, as this kind of change in trophic behaviour
can involve particular adaptations (e.g. baleens inwhales, bigmouths in sharks,
and beaks in birds), the transition to a different DMR can be abrupt. These
considerations indicate the eventual existence of both gradual and sharp transi-
tions in DMR in gradients of body size. Therefore, it is essential that the
methods used to analyse the DMR have the potential to detect the alternative
patterns expected from theory.
B. Gape Limitation and DMR
The Eltonian abstraction of communities, with discrete trophic levels and low
efficiency in energy transfer between them, implies a reduction in available
resources along the food chain (
Brown and Gillooly, 2003; Cohen et al., 2003;
