caused this difference in results between the two levels of comparison (between
communities versus within communities). It is possible that classification of the
species as bacterivores and omnivores is too simplistic and that accounting for
more fine-scale differences in their trophic strategies would change the results
of within-community level analyses. It is also possible that some of our findings
can be explained by processes that are not included in classic allometric theory,
such as the effect body size has on the diets of consumers, and therefore their
potential sources of energy (Marquet, Navarette & Castilla, 1995 ; Carbone &
Gittleman, 2002 ; Loeuille & Loreau, 2006 ).
Theory predicts that equilibrium abundances will be lower at higher temper-
atures (Allen, Brown & Gillooly, 2002 ; Brown, 2004 ). Our results are not consis-
tent with theory, because there are no significant effects of environmental
temperature in analysis two. Though the analysis of covariance should be
interpreted with caution, because of incomplete overlap of explanatory categ-
ories on the covariate axis, there was no indication that population density (given
body mass) was lowered by a warming event of greater than 10 8 C.
The explanation for this difference between theoretical expectation and
experimental result seems to be that the theory assumes no change in resource
supply rate accompanies a change in temperature (Brown & Gillooly, 2003 ). In
our experiments the resources are bacteria, which decompose the organic
matter present in the ecosystem. It seems likely that increasing temperatures
will increase rates of decomposition, bacterial production and resource supply
rates to the protist species. If the scaling of bacterial production with temper-
ature is the same as the scaling of consumer metabolism with temperature,
there should be no decrease in equilibrium abundance with increases in temper-
ature. A factorial experimental manipulation of temperature and resource sup-
ply rates could separate the effects of both on the population level consequences
of body size, but this experiment remains to be done.
Community consequences of body size – discussion
Links between biodiversity and ecosystem functioning have been alternatively
interpreted as consequences of traits of dominant species or diversity per se.
Some aspects of functioning (such as productivity or total community biomass)
may increase with diversity because more diverse communities may by chance
contain larger species, and not because of mechanisms associated with diver-
sity. However, allometric theory predicts that communities receiving the same
amount of energy will eventually support the same total amount of biomass,
regardless of organism size. Thus, the size of dominant organisms may deter-
mine functioning early in community development, but with enough time
thinning or increases in density may eliminate size-specific influences on func-
tioning. These hypotheses were supported in a recently published experimental
manipulation of body-size distributions in microbial microcosms (Long &
CONSEQUENCES OF BODY SIZE IN MODEL MICROBIAL ECOSYSTEMS 261