(Poweret al., 1985); lemmings and voles feeding on dwarf shrubs (Oksanen,
1983 ; Hambacket al., 2004)). A large size disparity between predators and prey
tends to render prey uniformly susceptible to predation. This is because dis-
crimination between prey types becomes more difficult with increasing size
disparity and less worthwhile, unless there are considerable nutritional differ-
ences between prey species. If the size disparity is too large, visual selection
of prey becomes impractical at anything other than a patch scale. In many
cascades described from aquatic systems the herbivores are filter feeders or
scrapers feeding on a mixed community of algae (Paine,1980; Carpenteret al.,
1985 ; Poweret al., 1985; Jones & Sayer,2003; Humphries, this volume). The
strong effects in eutrophic lakes depend on the presence of large-bodied clado-
cerans (Jeppesenet al., 2003a; Jeppesenet al., 2004). Generalist herbivores are
also typical of those terrestrial communities where community trophic cascades
have been described (Hambacket al., 2004). Hence, predation has impacts across
many trophically similar species, particularly at the lowest trophic level, and
results in a community-wide response.
It has been suggested, however, that the predominance of cascades in aquatic
systems may be due to factors other than predation limiting the size of the
primary producers (Strong,1992 ). For phytoplankton, the constraints of remain-
ing suspended within the water column are size dependent and it is possible that
the restrictions imposed by sinking influence the pattern of body size among
trophic levels. Nevertheless, planktonic algae can aggregate as colonies of rela-
tively large ‘body size’, which makes them less vulnerable to grazing and several
algal taxa that produce large colonies are positively buoyant (e.g.Microcystis).
Even if sinking constraints on body size exist for phytoplankton, there does
not seem to be any such constraint, either environmental or phylogenetic, on
attached algae. Sinking does not affect attached algae, and several orders of algae
contain large-bodied multicellular representatives, yet clear examples exist of
cascades with attached algae at the base (Paine,1980 ;Poweret al., 1985 ; Jones &
Sayer,2003 ). Many algal species, both attached and planktonic, pass into a size
refugium where grazers are no longer capable of consuming them effectively
(Dodds,1991); chemical defences to deter grazing occur among algal taxa (Turner &
Tester,1997). It appears that there are no constraints on the body size of aquatic
organisms specific to their habitat that render them particularly susceptible to
predation, and aquatic communities more susceptible to trophic cascades.
As well as making prey more uniformly susceptible to predation, a large size
difference between predators and prey means that predators must consume large
numbers of prey to sustain their populations, as each individual prey item contri-
butes little to the predators’ production. This is born out by estimates of interaction
strength based on empirical data (Woodward, Speirs & Hildrew,2005), and models
estimating interaction strength from body size (Emmerson & Raffaelli,2004). In a
study of the flow of biomass and production through the community inhabiting120 J. I. JONES AND E. JEPPESEN