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(e.g.
Berlow et al., 2009; Brose et al., 2006b
). Another approach emphasises
body masses of individual organisms and does not consider taxonomic
identity at all (e.g.
Law et al., 2009
). In still another approach, both the
size and taxonomic identity of individual organisms are taken into account
(e.g.
De Roos et al., 2003; Hartvig et al., 2011
).
In the species-oriented approach, all individuals within a species are given
the same body size and ecological parameters like intrinsic growth rates and
per capita interaction strengths are assumed to be functions of this body size
(using allometric scaling relationships) (
Berlow et al., 2009; Brose et al.,
2006a,b; Emmerson and Raffaelli, 2004; Jonsson and Ebenman, 1998; Otto
et al., 2007; Yodzis and Innes, 1992
). Specifically, both empirical work
(
Emmerson and Raffaelli, 2004; Wootton and Emmerson, 2005
) and theory
based on metabolic considerations (
Lewis et al., 2008; O'Gorman et al., 2010
)
suggest that the per capita strength of interaction between predator and prey
should depend on the PPMR. Thus, it has been argued that the distribution
of species body masses and patterns of PPMRs in food webs should have
important consequences for their dynamics and stability (
Brose et al., 2006a,
b; De Ruiter et al., 1995; Emmerson and Raffaelli, 2004; Jonsson and
Ebenman, 1998; Otto et al., 2007
). Moreover, it has been suggested that
patterns in the distributions of body masses among species found in natural
food webs are ones that lead to stability and facilitate the long-term coexis-
tence of species (
Brose et al., 2006a,b; De Ruiter et al., 1995; Emmerson and
Raffaelli, 2004; Neutel et al., 2002; Otto et al., 2007
).
The purely species-oriented approach also assumes that all individuals
within a species have the same diet and the same enemies. It does not account
for the fact that individuals of many species pass through a broad range of
body sizes (many orders of magnitude) during their life cycle. Such large size
changes during the life cycle lead to ontogenetic and seasonal niche shifts
(
Ebenman, 1987, 1992; Ebenman and Persson, 1988; Hutchinson, 1959;
Rudolf and Lafferty, 2011; Werner and Gilliams, 1984; Woodward and
Warren, 2007; Woodward et al., 2010
). This means that an individual
might feed on different sized prey individuals and be preyed upon by differ-
ent sized predator individuals at different stages of its life cycle. Thus,
individuals with average body masses (species average) might never interact,
and hence, estimates of per capita interaction strengths based on average
body masses of species might be misleading: instead, only a subset of the
predator population interacts with a subset of the prey population, so size
refugia may exist that could promote coexistence within the food web.
Indeed, we find that predator-prey mass ratios calculated from individual
feeding events differ from ratios calculated from average body masses (see
also
Woodward and Warren, 2007
for the Broadstone stream food web and
Nakazawa et al., 2011
for a recent analysis of the
Barnes et al., 2008
marine
food web dataset). Specifically, species averaging tends to underestimate the
