sense that trophic cascades are characterized by clear trophic levels, there
seems a disparity between the view that cascades are prevalent and the rejection
by Jennings and Reynolds (this volume) of integer trophic levels for most
aquatic ecosystems, as well as Woodward and Warren’s (this volume) findings
for simple freshwater benthic communities. Jones and Jeppesen (this volume)
argue that cascades are common where there is a large size disparity between
predator and prey, such that ‘prey’ are so small that they are fed on unselectively
by the consumers, and where small productive producers turn over very rapidly
and can sustain a large biomass of long-lived predators. These features may be
more decisive than body size per se in determining the occurrence of cascades.
Several chapters took a biogeographical or macroecological approach. These
include Finlay and Esteban; Rundle, Bilton and Foggo; Warwick; and Schmid
and Schmid-Araya (all this volume). Finlay and Esteban (this volume) consider
the biogeography of organisms in relation to their notion of a ‘biogeographical
divide’, a body size below which species, by consequence of their enormous
population abundances (thus linking this concept to macroecological theory),
are cosmopolitan, but above which they are not. This body-size transition seems
to be in the range 1–10 mm. For macro-organisms, Rundle et al. provide evidence
of a positive relationship between range size and body size in actively dispersing
dragonflies. For passively dispersing freshwater organisms, they find that those
within the body-size transition of Finlay and Esteban, such as small crustaceans,
do indeed have a biogeography and are not cosmopolitan. For passively dispers-
ing marine and freshwater organisms, range size increases with body size,
perhaps related to the covariance between body size and propagule output,
and hence dispersal probability. It might be that microbial species below
about 1 mm tend to be cosmopolitan, but that above this body size, geographical
range again increases, as the ability to disperse actively goes up with body size in
active dispersers, and the likelihood of dispersal goes up with propagule output
(and therefore with body size) in passive dispersers. It seems clear that data on
range size for aquatic organisms lag well behind those for terrestrial organisms,
which may explain the general paucity of (biogeographical) macroecological
relationships for aquatic organisms.
Warwick(this volume) deals with a macroecological pattern for which there is
a long history of research in aquatic ecosystems, the species–body size spec-
trum. He reports a regularly bimodal spectrum for the marine benthos, appar-
ently contrasting with a unimodal one for freshwater. In the marine benthos the
small mode consists of meiofauna and the large mode the macrofauna, with few
intermediate species. Warwick attributes this pattern not to local ecological
interactions or habitat architecture, as originally advocated by Schwinghamer
(1981), but to the evolutionary history of the marine benthos. The trough of
species in the spectrum equates to the size of the planktonic, larval forms of the
larger macroinvertebrate benthos. These cannot be benthic because the smaller
BODY SIZE: IMPORTANT, BUT NOT THE WHOLE STORY 331