whether intrinsic local processes involving ecological mechanisms are more
important (Cornell & Lawton,1992 ; Witman, Etter & Smith,2004 ). Translated to
the body size/diversity problem, the question arises as to whether the local num-
bers of species of different sizes, in taxonomic/functional guilds or integral assemb-
lages, are simply a more or less random selection from the regional species pool, or
whether local environmental constraintsand species interactions are more impor-
tant in determining this pattern. Problems of definition then arise: what is a ‘guild’
(Adams & Shorrocks,1985 ) and how is the ‘regional species pool’ defined (Dupre,
2000)?
This chapter attempts to resolve some of the above uncertainties with respect
to marine faunas, with particular emphasis on the benthos, which has a much
higher diversity than the plankton, and a higher phyletic diversity than any
other realm on Earth (Warwick,1996).
Species guilds
Two generalized concepts with respect to species guilds are relevant here. First,
the controversial concept that there is a minimum size difference between
competing species that will permit their coexistence (Huxley, 1942 ), that ratio
being shown empirically to be 1:28 (Hutchinson, 1959 ). It has been argued that
this ‘Hutchinsonian ratio’ may be more to do with the generalities of assembling
sets of tools than with anything directly biological (Horn & May,1977), or indeed
may be an artefact (Eadie, Broekhoven & Colgan, 1987 ; Greene,1987). Also, tools
or musical instruments with which analogies have been drawn (Horn & May,
1977 ) are fixed in size, whereas animals increase in size during development
and are potentially in competition for resources with their congeners of differ-
ent sizes and also with other species in the guild (Woodward & Warren, this
volume). For taxa with distinct developmental stages (instars) the increase in
size between each moult (Dyar’s Constant) is the same as the Hutchinsonian
ratio of 1:3 (Dyar,1890), and this also applies to many marine taxa (see for
example Table11.1). Dyar’s Constant is much better substantiated than the
Hutchinsonian ratio, but Lawton and Strong (1981) argue that it is ‘nothing to
do with competition or partitioning of food resources’.
The second concept relates to the number of species in a guild that are
typically found to co-occur, usually seven to eight and never more than ten
(Adams & Shorrocks,1985). Again, the explanation for this also involves com-
petition for resources: more species would intensify competition for resources
beyond the limits of the number of species that could coexist. Reverting to the
analogy of inanimate objects there are instances where both these ‘rules’ may
apply. A new violin family introduced at a conference on musical acoustics in
Cambridge in 1997 had a ‘rich, even tone quality; particularly shines in contra-
puntal writing’ (Darius, 1977 ). There were eight viols in the consort, with size
ratios of 1:2, 1:2, 1:3, 1:3, 1:3, 1:3, 1:3 (Horne & May,1977).
BODY SIZE AND DIVERSITY IN MARINE SYSTEMS 211