naı ̈ve to each other. Consequently, the community and ecosystem properties that
emerge result from similar ecological interactions as might occur in natural
systems. Second, in each of the four experiments analyzed, the species present
in particular communities were selected at random from the laboratory pool. The
frequency distribution of body sizes in this pool is approximately log-normally
distributed (unpublished data). It therefore seems unlikely that we have biased
body-size distributions by choice of particular species. However, extinctions often
occur during an experiment and alter community composition (Weatherby,
Warren & Law, 1998 ; McGrady-Steed & Morin, 2000 ). Presumably this change in
composition results from ecological interactions and influences body-size distri-
butions and their consequences. Third, because the experiments are long relative
to the generation time of the organisms, great changes in population size occur.
For example, a bacterivores species may initially occur at ten individuals per ml,
but in a week have multiplied to thousands of individuals per ml. So although cell-
size distributions are somewhat constrained, population densities, population
biomasses and ecosystem process rates are emergent.Description of the four experiments and data sets
Experiment 1
This experiment was designed to address two questions (Long & Morin, 2005 ).
First, whether diversity or the size of dominant species affects ecosystem func-
tioning, and second, how effects changed in importance through time. Food
webs were assembled using two size fractions (small species or large species)
and two levels of diversity (four or nine species) using autotrophic and hetero-
trophic species of protist and small metazoan. The two size fractions of organ-
isms differed in average cell mass by approximately two orders of magnitude.
There were two different community compositions in the low diversity food
webs. The resulting six food webs were replicated five times. Population den-
sities were measured once a week for each of the six weeks of the experiment.Experiment 2
The experiment was designed to investigate the relationship between biodiver-
sity and ecosystem functioning and the variability of ecosystem functioning,
and is described in detail elsewhere (McGrady-Steed, Harris & Morin, 1997 ).
There were 10 different communities with from 3 to 31 autotrophic and
heterotrophic species of protists and small metazoans distributed among mul-
tiple trophic levels. There were five replicates of each community, and population
densities were recorded at least once a week for six weeks.Experiment 3
This experiment was designed to examine the effects of environmental warm-
ing on ecological communities (Petcheyet al., 1999). There were four different248 O.L. PETCHEYET AL.