whereNiis the number of individuals of speciesi,Miis the metabolism of an
individual of speciesi, andsis the number of species in the community. An
individual’s metabolism is related to its body size by allometry such that
Mi¼bB^0 i:^75 ; ( 13 : 2 )
wereBiis the body size of speciesiandbis a constant converting mass to energy
per second. Consequently,
CM¼b
Xs
i¼ 1NiB^0 i:^75 ( 13 : 3 )This prediction for community metabolism (CO 2 flux) was compared to the
explanatory power of total biomass using linear regression by ordinary least
squares.
Results
Population consequences of body size – results
In each of the three analyses of the relationship between cell mass and population
density (the mass–density relationship), cell mass was very strongly and relatively
linearly related to population size (Figs.13.1, 13.2and13.3). However, the slope
differed greatly between the first two analyses and the third analysis (0.93,
0.95 and0.52, respectively) (Tables13.1, 13.2and13.3). Communities in the
third analysis contained only one trophic level, while all of the communities in
the first and second contained species at several trophic levels.
However, effects of trophic group on the mass-density relationship within the
first two analyses were inconsistent with this between analysis difference. In
the first, slopes differed between trophic groups, with the slope becoming
shallower from autotrophs, to bacterivores, to omnivores (Table13.1). In the
second analysis, trophic group had a marginally significant effect on the inter-
cept (but not the slope), with bacterivores having the steepest slope and omni-
vores the shallowest (Table13.2).
The species richness of the community had a significant negative effect on the
intercept of the mass–density relationship in analysis one (Figure 13.1,
Table13.1). This is a signature of density compensation, in which populations
within less species-rich communities are on average denser than when in richer
communities (McGrady-Steed & Morin, 2000 ; Petcheyet al., 2002); density com-
pensation can occur because species compete for similar resources and are
therefore somewhat redundant. Density compensation was not evident in anal-
ysis two or three where there was no significant effect of species richness on
the slope or intercept of the mass–density relationship (Figs.13.2and13.3,
Tables13.2and13.3).
Perhaps surprisingly, there was no evidence that environment conditions
altered the slope or intercept of the mass–density relationship. In analysis
CONSEQUENCES OF BODY SIZE IN MODEL MICROBIAL ECOSYSTEMS 251