Geoscience Reference
In-Depth Information
( Lafferty et al., 2008 ). In the range of body sizes typically used for the
analysis of DMR, covering several orders of magnitude, large variations in
the strength of gape limitation are expected (see Brown et al., 2004; McNab,
2002 ). Recognition of systematic tendencies in gape limitation is essential to
account for an eventual increase in available resources with body size and a
reduction in predation pressure. The explicit consideration of these changes
could be important for the understanding of the DMR.
Population density is determined by the balance between (1) individual
demand of resources, (2) available resources for each individual, and (3)
reduction in individual numbers by predation or related processes. If any
of these determinants systematically changes with body size, it has the
potential to alter DMR. The energetic equivalence rule only considers the
first process, predicting that the increase in energetic demand is compensated
by a proportional reduction in density ( Damuth, 1981, 1987 ). Considering
that the same amount of resources R i is available to all the individuals, an
allometric increase in metabolic demand with body size aM a , and that the
temperature affects metabolic rate as exp(
E/kT), the expected DMR is the
ratio between available resources and the energetic demand of each individ-
ual ( Savage et al., 2004 )
R i a 1 M a exp E
D
ð
kT
Þ
ð
2
Þ
=
It should be noted that the scaling exponent of the DMR is determined by the
scaling in metabolic demand. In this formulation other determinants, such as
temperature and resource availability, affect the intercept but not the scaling
value. This assumes an equal effect in all size classes. However, the empirical
observation of larger scaling at higher temperatures suggests that changes in
scaling exponents could also be expected (e.g. Beisner et al., 1997; Yvon-
Durocher et al., 2011 ). Further, recent consideration of the eventual exis-
tence of different scalings in metabolic demands with body size at the
intraspecific or interspecific level ( Ginzburg and Damuth, 2008 ) should also
impact in the DMR at these two levels.
The general operation of gape limitation in resource acquisition implies
that R i could systematically change with body size, R i
f(M). In this context,
the structure of the function f(M) should be a determinant of the DMR. A
reasonable assumption is that gape limitation allows predators to consume
prey that are below a fixed fraction of their body size:
m
¼ b
M
where m is the maximum prey size that a consumer M can eat and
the
predator-prey size ratio. However, prey is not evenly distributed across
masses. Consider, for example, the frequency distribution of macroinverte-
brate sizes in a metacommunity of temporal ponds ( Figure 1 ), a system
further analysed below. As consumers become larger, they can feed on
b
Search WWH ::




Custom Search