zooplankton biomass and increased phytoplankton (Carpenter, Kitchell &
Hodgson, 1985 ). The phrase trophic cascade has been variously applied, but it
is generally accepted that it describes a community-wide response of species,
including those near the base of the web, typically primary producers (Polis
et al., 2000). There has been much discussion as to how widespread within
trophic levels the response must be, but it has been argued that for a true
community trophic cascade the impact of changes in predator biomass must
result in a change in total primary producer biomass (Polis, 1999 ; Poliset al.,
2000 ). Cascading interactions within compartments of the community are pos-
sible (e.g. Stronget al., 1996), but here there is limited change in primary
producer biomass, typically resulting in a change in species composition rather
than biomass change per se. It may be that the ‘runaway consumption’ (sensu
Strong,1992) required for a cascade is characterized by a substantial shift in
community composition in circumstances where alternative primary producers
may exploit the vacated niche, but there is no consensus on the extent of the
compositional change required (Poliset al., 2000).
In an attempt to define the conditions that give rise to trophic cascades, Polis
et al.(2000) argued that trophic cascades are restricted to simple, trophically
stratified systems in homogeneous habitats where strong interactions link uni-
formly susceptible prey to predators. In these relatively simple systems, energy
(organic matter) passes through the community via a single direct route and
there are few, if any, alternative routes to buffer changes in predator popula-
tions. As the variety of prey tends to be low, either as functional type or as
species per se, there is little choice for predators and, as the system is homo-
geneous, there are few refugia for prey. It has been suggested further that these
characteristics are typical of aquatic environments and that as a consequence,
cascades are ‘all wet’ (Strong, 1992 ). Nevertheless, there is evidence to suggest
that cascades can occur in the speciose communities of structured, complex
environments, both aquatic and terrestrial (Paceet al., 1999; Moran & Scheidler,
2002 ; Jones & Sayer,2003; Preisser, 2003 , but see Gruner, 2004 ).
Here we discuss whether a structured pattern of body size among predators
and prey is involved in the occurrence of trophic cascades, and how such a
pattern can arise in communities. We will examine other parameters and their
relationship with body size, to determine if any patterns are the result of body
size per se or some other correlated factor.
Body size
Trophic cascades are dependent upon the presence of generalist herbivores
(Jiang & Morin,2005), which are often associated with large relative size at
least at some stage in the life cycle where the primary producers are vulnerable
(e.g. urchins feeding on macroalgae (Esteset al., 2004); large Cladocera feeding
on phytoplankton (Carpenter & Kitchell,1993); minnows feeding on algae
BODY SIZE AND TROPHIC CASCADES IN LAKES 119