(Fig.13.7d,r^2 ¼0.08, p¼0.38) while predicted community metabolism explained
35% of the variance in observed community metabolism and was marginally
significant (p¼0.04).
Discussion
Population consequences of body size – discussion
Brown and Gillooly ( 2003 ) hypothesize that the slope of the relationship
between body size and population abundance will be shallower when measured
across species that share a common resource pool (e.g. with an exponent
of0.75) and steeper when species acquire resources from different sources,
in particular, when species occupy multiple trophic levels and therefore forage
on each other (e.g. with an exponent of1). The inefficiency of energy transfer
from a lower to higher trophic level means that less energy is available to
species at the higher level, and consequently their abundance should be lower
relative to their biomass.
We found some support for this hypothesis, but there was also evidence that
was inconsistent with this idea. Analyses of communities that contained mul-
tiple trophic levels (analysis one and two) revealed much steeper mass–density
relationships than in the communities that contained only species that con-
sumed bacteria (analysis three). This is consistent with the hypothesis. In con-
trast, analyses of mass–density relationships within communities (i.e. within
analysis one and two), did not consistently show that species at higher trophic
levels (classed as omnivores here) had lower intercepts (and the same slope) as
species at lower trophic levels (the bacterivores). We are uncertain about what
Table 13.6Summary of analysis three of the community consequences of body
size on total community biomass. The response variable is total community density.
M¼body mass, S¼species richness.
ANOVA table
Df Sum sq Mean sq F value Pr(>F)M 1 3.2283 3.2283 22.469 1.109e05
S 1 2.8076 2.8076 19.541 3.575e05
Residuals 69 9.9136 0.1437
Coefficients
Estimate Std. error t value Pr(>|t|)(Intercept) 1.62628 0.39459 4.121 0.000103
M 0.37154 0.07407 5.016 3.93e06
S 0.12205 0.02761 4.421 3.58e05***
CONSEQUENCES OF BODY SIZE IN MODEL MICROBIAL ECOSYSTEMS 259