that there is a decreasing turnover rate with trophic level. To enable comparison
across habitats we suggest that differences in predators need to be taken into
account, and that any measure of turnover rate of primary producers is relative
to that of predators.Turnover rate
The turnover rate of primary producers has important consequences for the
rest of the community, as it influences the potential rate of supply of energy
(biomass) to the upper trophic levels. The biomass contained within upper
trophic levels is governed by the rates of supply and loss (from the system and
to further trophic levels) of production; if the supply is low (relative to the
biomass of primary producers) biomass cannot be sustained at more than the
trophic level below, a characteristic of the trophic cascade. In other words, if
primary producers turn over at a slow rate, then a large amount of biomass
will be retained within the lowest trophic level. If this is repeated at every
trophic level the trophic pyramid will tend towards a triangle. If the turnover
of primary producers is fast, there is the potential for herbivores to be main-
tained at a higher biomass than primary producers, and a square or inverted
triangle trophic pyramid is possible. However, it has been suggested that
such a structure is unstable (Raffaelli, 2002 ), with the upper trophic levels
prone to oscillation unless supported by external subsidies or via two routes,
one slow and one fast with top predators receiving 20–60% of their energy
from either pathway rather than 100% from a single pathway (Mooreet al.,
2004 ).
To investigate whether the turnover rate of primary producers influences the
shape of the trophic pyramid and the potential for a trophic cascade we con-
structed trophic pyramids for a number of shallow lakes spanning a gradient of
top-down effects of fish (Box7.3). There was a significant relationship between
the shape of the trophic pyramid and the ratio of slow- to fast-turnover primary
producers. The distribution of biomass among trophic levels became more
triangular with an increasing proportion of slow-turnover primary producers
(macrophytes and macroalgae). A similar shift in biomass distribution has been
seen in cave communities with differences in the turnover among the basal
species (Moore, Deruiter & Hunt,1993); bacteria (small, fast-turnover) are
associated with a square trophic pyramid, fungi (large, slow-turnover) with a
triangular pattern.
As a note of caution, however, it might be that long time scales are required to
detect change in communities with long-lived primary producers, far in excess
of those typical of experimental methods (but see Sinclairet al., 2000), and that
our opinion is constrained by the practicalities of experimental design. Few
experiments are undertaken at a temporal scale that incorporates multiple
generations of predators (though some biomanipulation experiments have128 J. I. JONES AND E. JEPPESEN