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predation avoidance, as well as constraining metabolic rate and affecting the
rate at which interactions occurs between predators and prey ( Cohen et al.,
1993; Peters, 1983 ). By scaling the individual-level effects of body size up to
higher biological levels, such as population and community, our understand-
ing of patterns and processes at these organisation levels may be improved
(e.g. Jacob et al., 2011 ). Indeed, recent development in food-web research has
largely benefitted from the body-size-based approach. The assumption that
the body-size effect at an individual level may be scaled up to the species level
has provided new insights about how the food-web structure and dynamics
are constrained and associated to predator and prey body sizes ( Woodward
et al., 2005 ).
However, scaling up from individual to higher organisation levels may not
be valid when there is intraspecific variation in body size. Such variation adds
considerable complexity to the body-size-based view with respect to higher
levels of organisation. For example, intraspecific variability may arise
through individual growth. The majority of animal species undergo a sub-
stantial increase in body size during individual growth, and hence body size
varies considerably within species ( Ebenman and Persson, 1988 ). For exam-
ple, fish species grow by several orders of magnitude in size between hatching
and death, generally outweighing interspecific variations ( Hildrew et al.,
2007 ). Ontogenetic growth is often accompanied with dietary shifts in life
history parameters. This process is known as ontogenetic niche shift ( Werner
and Gilliam, 1984; Wilbur, 1980 ) and creates within-species variability in
resource use and the strength of trophic interactions. As a consequence, the
food web has a complex size structure, whereby different species have differ-
ent size structures, in which different individuals are characterised by differ-
ent body sizes. Understanding and ultimately predicting the dynamics of
such complex systems is a central goal of ecological research.
B. Predator-Prey Mass Ratio: Its Use and Problems
Despite variations in body size existing within a species, there is an expecta-
tion to identify a body-size-related pattern and its ecological consequences in
nature. This expectation has stimulated empirical research on body-size
differences of interacting predators and prey ( Barnes et al., 2008, 2010;
Brose et al., 2006a; Gilljam et al., 2011; Woodward and Warren, 2007 ), as
well as the development of food-web models that assume specific predator-
prey body-size relationships ( Brose et al., 2006b; Castle et al., 2011; Jennings,
2005; Maury et al., 2007; Petchey et al., 2008; Silvert and Platt, 1980; Thierry
et al., 2011 ). Currently, the empirical and theoretical study of the predator-
prey body-size relationship is being developed to utilise the useful concept of
predator-prey mass ratio (PPMR). PPMR is considered to be the most
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