Zealand streams invaded by trout is a consequence of the behaviour of the
invertebrate prey, which seek refuge under stones and release benthic algae
from grazing pressure (McIntosh & Townsend, 1996 ).
Structural complexity also affects the extent to which predators feed omni-
vorously (Diehl, 1992 ), and omnivory has potentially a stabilizing influence on
species’ dynamics (Polis & Strong,1996). The wider array of potential prey in
complex habitats enables species to undergo ontogenetic shifts in their diet
(Diehl & Kornijo ́w, 1998; Svanba ̈ck & Eklo ̈v, 2002) and introduces life-history
omnivory into the web (Persson, 1999 ). Even within a single life stage, the
diversity of prey, coupled with an increased proximity before a prey item is
encountered and the reduced apparent density (encounter rate), will result in an
increased probability that non-preferred prey are taken, a wider diet width and
an increased likelihood of omnivory. Fish that feed in structurally complex
vegetated lakes are frequently generalist and omnivorous (Diehl,1992 ;Diehl&
Kornijo ́w, 1998; Jones & Waldron, 2003 ; Okun & Mehner, 2005 ).
However, the extent to which habitat complexity, food-web complexity and
biomass turnover interplay with each other and with other factors is as yet
unresolved. Habitats witha high proportion of primary producers with largebody
size and a slow rate of turnover are physically the most complex, the most species
rich, and have the most complex food webs (Box7.3 ). Whilst separating the
influence of these effects on cascade dynamics will require careful experimenta-
tion, this area is ripe for investigation and liable to lead to greater insight into the
mechanisms that cause trophic cascades.Alternative equilibria
Anybody working in the field of shallow-lake ecology cannot ignore two hypoth-
eses, namely trophic cascades and alternative equilibria (the presence of two
alternative community structures in lakes dominated by either phytoplankton
or macrophytes; Schefferet al., 1993). These are intrinsically linked. Alternative
equilibria are the manifestation of the presence or absence of a trophic cascade
(Paceet al., 1999) and are characterized by many of the features we have
discussed so far. In shallow lakes the primary producers can be of small or
large body size, with corresponding fast or slow turnover (phytoplankton or
macrophytes). As a consequence of their large body size, macrophytes achieve a
grazing refugium and, although consumed by numerous species, do not suffer
losses at the same rate as phytoplankton (Lodgeet al., 1998). Macrophyte domi-
nated lakes typically have a large standing crop of primary producers
(Liboriussen & Jeppesen,2003), and the majority of the production passes
through a detrital stage before passing to primary consumers (Carpenter &
Lodge,1986). A grazing refugium is also possible for phytoplankton, but is
usually only achieved when zooplankton grazing is low to moderate (Jeppesen
et al., 1997, Hansson et al., 1998). Due to the presence of large-bodied132 J. I. JONES AND E. JEPPESEN