Meta-analyses of the role of eutrophication in marine pelagic cascades
(Micheli, 1999 ) and in experimental systems (Boreret al., 2005) indicate that
productivity does not explain the strength of cascading interactions. Product-
ivity is intertwined with the turnover rate (rate of replacement of biomass) and
susceptibility to herbivory (a combination of palatability and size refugia; Polis,
1999 ) of primary producers. In fact these three can be summarized solely as the
turnover rate, as large or heavily defended primary producers do not replace
biomass rapidly, akin to a situation of low productivity. Where production is
sufficient, high losses to herbivory can select for either k (resistant, large or well-
defended, slow-turnover species) or r (resilient, small, rapid re-growth, fast-
turnover species) strategists. If the community is dominated by primary producers
of the latter type there is great potential for a change in herbivory to produce
a significant change in standing crop.
Increased herbivory often leads to increased specific primary production as
nutrients are recycled more rapidly and competitors suppressed. Hence, in
communities where herbivores are abundant, primary production may be
high even though standing stock is low (Liboriussen & Jeppesen, 2003 ). Thus,
the turnover rate of individuals, biomass and nutrients is higher where the rate
of predation is high relative to herbivory. Under such conditions it is likely that
small body size of primary producers is an advantage, as reproduction is achieved
early and less investment of production made in each individual. In bio-
manipulated lakes where zooplanktivorous fish have been removed, the result-
ant cascade leads to a shift away from slow-growing, large cyanophytes to
fast-growing small algae (Hanssonet al., 1998) as rapid growth is required to
compensate (partially) for losses to grazers. Large primary producers gain
competitive advantage only if they can achieve a size refugium (e.g.
Aphanizomenon flos-aquae), a condition more easily achievable where herbivores
are small and infrequent or when grazing is low.
We should, therefore, reconsider the well-described relationship between
productivity (though standing stock is often used as a proxy (Keddy,2000)) and
the size of primary producers that has been described for higher plants, both
terrestrial (Grime,1979) and aquatic (Willby, Pygott & Eaton, 2001 ). Although
small body size is an advantage in unproductive environments, increasing
productivity does not always lead to increased body size and a competitive
advantage. Where grazing is intense, small size and rapid turnover are an
advantage, with large size only conferring an advantage where production is
potentially high and losses to higher trophic levels are low (either because
herbivores are scarce or because a size refugium exists). Thus, we would expect
mean body size of primary producers to peak at intermediate productivity.
Again, rather than body size per se, the turnover rate of primary producers
may provide a more integrated measure of the potential for strong cascading
interactions, with cascades occurring where the community is structured such
BODY SIZE AND TROPHIC CASCADES IN LAKES 127