macrophytes, the plant-dominated state is more structurally complex than the
phytoplankton-dominated state and has greater species richness (Declercket al.,
2005 ), with both these factors feeding back to produce a complex food web
with weak links and frequent omnivory (Diehl,1992; Jones & Waldron, 2003 ).
Phytoplankton-dominated lakes tend to be characterized by simple, well-structured
food webs, with strong interactions between predators and prey and their
populations closely tracking one another (Townsend,1988). The shift between
these two states involves considerable change in community structure and
biomass of primary producers, a characteristic that is taken as the definition
of a community trophic cascade (Poliset al., 2000).
A similar pattern occurs in many of the communities where well-described
alternative equilibria are found (Table7.2). Primary producers can be either
small or large relative to consumers, with energy passing along a fast or a
slow cycle, respectively. Where primary producers have a large body size rela-
tive to consumers the community has a complex, stable food web, in contrast to
the simple, linear structure of the cascade food web where the primary pro-
ducers are small (relative to consumers). In each case there are two alternative
groups of primary producers that exhibit intense size-based competition,
with one group having access to a sized-based grazing refugium. Whilst these
patterns are of relevance in understanding the occurrence of alternative equi-
libria, they should also be used to guide future search for cascades. We suggest
that any such directed search will prove fruitful in any habitat, terrestrial or
aquatic.
Conclusions
It appears that the pattern of body sizes of the organisms in the community is
related to the occurrence of a trophic cascade. However, it is not body size per se,
Table 7.2Habitats where alternative equilibria have been suggested and the
dominant primary producers characteristic of the two states. The primary producers
typical of the two states are either rapid turnover, relatively small body size, low
standing stock (State 1) or slow turnover, relatively large body size, high standing
stock (State 2).
Habitat State 1 State 2
Lakes Phytoplankton Macrophytes
Coral reefs Microalgae Fleshy brown algae
Rocky shores Microalgae Fucoid algae
Woodlands Grass Woodland
Burnt heathland Heath Woodland
Deserts Ephemeral plants Perennial vegetation
BODY SIZE AND TROPHIC CASCADES IN LAKES 133