Independently of this line of inquiry, ecologists have suggested that the total
biomass of particular subgroups of organisms, plants for instance, can increase
as the diversity of organisms within that group increases (Hectoret al., 1999;
Tilmanet al., 2001). For this pattern to square with the pattern of constant total
biomass observed in natural systems, one of several things must hold. (i) Natural
communities may be at the maximum diversity that a particular habitat will
support – and the communities where diversity-dependent biomass increases
are seen must be below that level of diversity. (ii) An increase in the diversity and
biomass within a particular size fraction of the community will produce a
corresponding increase in the biomass of size fractions in the remainder of
the community – an interesting but little explored phenomenon.
Some of the work that we describe here was initially conceived and conducted
to evaluate critiques of early biodiversity experiments, specifically to determine
whether what are now known as selection effects caused by the chance occur-
rence of larger organisms can explain patterns of increased biomass in more
diverse systems. While it seems likely that such transient effects could arise as a
consequence of initial conditions, the size invariance of productivity and biomass
noted above make it seem unlikely that such differences would persist for long in
communities after population densities of organisms respond and adjust to the
availability of energy and nutrients. To address these ideas we first ask if diversity
per se can change the size–population abundance relationship within commun-
ities and among experiments. Second, we ask whether communities initially
constructed of organisms of very different size would rapidly converge in bio-
mass. Finally, we investigate the consequences of body size for an ecosystem-level
process: whole community metabolism. Specifically, we test the hypothesis that
the metabolism of a given biomass will depend on whether the biomass is divided
up among lots of small individuals or just a few large ones.
Why investigate these consequences using data from model microbial eco-
systems? Microbial microcosms can contain organisms with a wide range (about
seven orders of magnitude) of organism size. The organisms used in our micro-
cosms also fill a wide range of trophic roles, including photoautotrophs, decom-
posers, primary consumers and secondary consumers. This allowed us to evaluate
effects of size across multiple trophic levels, thereby extending the analysis
beyond primary producers (Duffy, 2002 ; Petcheyet al., 2004). Additionally, these
organisms have generation times ranging from hours to days. This allows for
rapid population dynamics, such that an experiment encompassing many gen-
erations of population dynamics can take place in a few weeks. Properly testing
the predictions of allometric theory may require multiple generations because it
may take many generations for changes in population sizes due to thinning or
density compensation to occur, so that initial conditions, transient dynamics, and
potential selection effects may be less important. We could also limit the effect of
the environment on abundance and yield by holding environmental conditions246 O.L. PETCHEYET AL.