Morin, 2005 ). Early in the experiment, total community biomass was lower in
communities containing species with small individuals than in communities
containing larger individuals. However, after 25–50 generations, there were no
detectable differences between the biomass of the two types of community.
The results of the analyses presented suggest that certain ecological differ-
ences among communities and experiments might affect the relationship
between individual size and density, so that the total biomass of all commun-
ities is not equal, even if energy supply to the communities is. The first two
analyses showed much greater levels of biomass equivalence than the third, in
which communities with larger individuals tended to have greater total bio-
mass. One finding that is recovered in our analyses is that communities with
greater numbers of species tend to have greater total biomass than those with
fewer species (McGrady-Steed & Morin, 2000 ; Petcheyet al., 2002; Hooperet al.,
2005 ). Data in Fig.13.7also clearly show that factors other than energy supply
must influence total community biomass, because there are three communities
with nearly an order of magnitude greater biomass than others.Ecosystem consequences of body size – discussion
For a subset of the communities in one of the experiments (Experiment 3) there
was evidence that the size distribution of individuals influenced community
metabolism, as measured by the amount of CO 2 produced by a community
during a fixed time. Communities that contained many small individuals tended
to have higher metabolism than communities with fewer individuals. It is
perhaps surprising that any significant relationships between biomass and
community metabolism, and predicted and observed metabolism, were
detected considering that a major taxonomic component of the microcosms
was completely ignored (the bacteria). Their small size might suggest a very
large contribution to community metabolism and future analyses would benefit
from taking their size and abundance into account.
The evidence was equivocal, however, and further studies are needed to test
the robustness and generality of the importance of size distributions for com-
munity metabolism. In general, the analyses presented above should be con-
sidered preliminary in nature and can perhaps be best used to make
recommendations for future experiments. These could include predictions
based on the size distribution of individuals within species, as opposed to the
assumption made here that all individuals of the same species are the same
mass. Furthermore, experiments could manipulate the size distribution of
species (and thereby individuals) present in different communities (e.g. Long &
Morin, 2005 ). Here, communities were assembled mostly from species with
small sized individuals, or mostly from species with large sized individuals.
We did not analyze Long and Morin’s ( 2005 ) data in this context because their
communities contained photoautotrophs and including their contribution to262 O.L. PETCHEYET AL.