et al., 2005). If this occurs, distinct food webs that experience similar abiotic
conditions should fall on the single constant yield line described by logNtotal¼
logblogMaverage, whereNtotalis the total number of individuals in a commun-
ity,Maverageis the average mass of an individual in a community, andbreflects
environmental influences like resource supply rates and temperature. The
slope of1 means that a community containing cells twice the average mass
of those in another community will contain half the number of individuals.
Consequently, differences in the mass of individuals are accompanied by oppo-
site and perfectly compensatory changes in the number of individuals. To test
this hypothesis we used the same three analyses as described earlier except:
log 10Ntotalwas the response variable, log 10Maveragewas the continuous explan-
atory variable, datapoints were communities (rather than species), and there
was no ‘trophic group’ categorical variable.Ecosystem consequences of body size
Metabolic rate scales with individual mass0.75(Peters, 1983 ; Damuth, 1987 ;
West, Brown & Enquist, 1997 ) and consequently lots of small individuals should
have higher total metabolism than just a few large ones, even if they represent
the same amount of total biomass. Of course, the microbial communities that
we analyze are composed of a mixture of small or large individuals, so we
predict community metabolism from the size and abundance observed for
each of the species in the community. This prediction is compared with
observed flux of CO 2 , a measure of whole community metabolism. Observed
ecosystem level CO 2 flux was estimated using a closed-circuit microrespi-
rometer (Micro-oxymax, Columbus Instruments, OH, USA). This measures the
CO 2 concentration in the gas above the liquid in the sealed culture vessels at
multiple discrete times, and flux can be calculated from these measures.
Concentrations were measured during at least an eight-hour period and flux
averaged across that. Can the total biomass or body sizes of the organisms in
the community in a particular vessel predict ecosystem-level CO 2 flux?
To test this we assumed that CO 2 flux is proportional to community metab-
olism (CM) and this is the sum of the metabolism of all the individuals in the
community. We limit the analyses to the data from the constant temperature
replicates of the only experiment that included only heterotrophic species
(Experiment 4). The other three experiments all contained autotrophs. If the
individuals are grouped into species, community metabolism is the sum of the
species’ metabolisms, where a species’ metabolism is the sum of the metabo-
lisms of the individuals of that species:CM¼Xs
i¼ 1NiMi; ( 13 : 1 )250 O.L. PETCHEYET AL.