2005 ), and these authors also provide a good discussion of the difference
between a failure to reject a null hypothesis and verifying that null hypothesis.
In addition, for claimed invariant relationships, where a slope of zero is pre-
dicted by theory, it is incredibly difficult to distinguish such invariance in a data
set from a non- (and therefore meaningless) relationship.
Finally, different representations of the data could well generate quite differ-
ent perspectives on the significance of body size in ecology. Within taxonomic
collections (e.g. a family of water beetles or harpacticoid copepods), bivariate
plots of body size against abundance provide reasonable representations of
relationships. As the taxonomic grouping becomes coarser (e.g. freshwater
benthos or marine benthic invertebrates), regression techniques begin to fail
to provide an adequate model of the data. At the whole system level, a constraint
space (e.g. Brown, 1995 ) or biomass spectrum (e.g. Stead et al., 2005 ) might be
more appropriate, although formally quantifying such pattern and exploring
deviations from theoretical prediction is fraught with difficulty (Leaper &
Raffaelli, 1999 ).
Other empirical, theoretical and conceptual backgrounds
Most chapters in this book do refer to metabolic theory, but perhaps wisely do
not attempt explicit tests, instead setting their findings in the context of a
variety of other areas of theory and concept. For instance, Humphries (this
volume) considers suspension feeding, a mode of feeding virtually restricted
to aquatic systems, in relation to body size and the physics of water and flow.
Body size is crucial for aquatic organisms because it determines flow regime and
thus, for suspension feeders, food supply. We are particularly struck by
Humphries’ remark that body size is particularly ‘nebulous and difficult to
specify’ for suspension feeding organisms, because they stretch to the very
limits the relationship between the various measures of size that ecologists
can use. Consider for instance gelatinous pelagic filter feeders, of large physical
dimensions but very low body mass; benthic filter feeders with large and heavy,
but metabolically inactive, shells; and organisms that build extensive nets and
mucus sheets to capture food at a rate beyond that expected from their body size
alone. Perhaps for this reason there is still no clear relationship between body
size and food-particle size for suspension feeders, in contrast to the situation for
conventional predators.
Townsend and Thompson,Atkinson and Hirst,and Huryn and Benke(all this
volume) all deal with body size as a species trait in relation, respectively, to the
habitat templet hypothesis, life-history theory and metabolic theory.Townsend
and Thompson(this volume) consider body size against the habitat templet
hypothesis of Southwood (1977), and allied predictions specifically developed
for streams by Townsend and Hildrew (1994 ), and referring to traits in relation to
disturbance, productivity, land use, predation and others. They conclude that
BODY SIZE: IMPORTANT, BUT NOT THE WHOLE STORY 329