constant (Gilloolyet al., 2001;Savageet al., 2004). Conversely, controlled manip-
ulation of environmental conditions, particularly temperature, allows investiga-
tion of how it alters the consequences of body size.
Methods
General methods
The analyses presented here draw on data from four experiments that share much
methodology in common. In all, experimental communities were 240–250 ml
glass or pyrex jars each containing 100 ml of nutrient medium (1 litre of well
water, 0.55 g Carolina Biological Supply Protozoan Pellet, and 2–6 wheat seeds)
housed in incubators at 18 8 C unless otherwise stated below. This medium was first
autoclaved and then inoculated with three or four species of bacteria (Serratia
marcescens,Bacillus subtilus,B. cereusand sometimesProteus vulgaris). To this basal
food-web level were added various combinations of the protist (algae, ciliates and
sarcodines) and small metazoans (rotifers) that were kept in laboratory stock
cultures. Bacterivores and autotrophs were typically added before predators and
herbivores, to allow resources to grow to levels that could sustain consumers.
The experiments lasted six or seven weeks and were sampled between two and
six times. All analyses are of only the last week of collected data. Sampling involved
swirling the microcosms to homogenize the distribution of organisms, removal of
first0.3 ml and then potentially 5 ml of media. The number of individuals of each
species was recorded in the smaller volume and if some species were not observed
the larger volume was searched. Autotrophs were sampled by a third method:
enumeration using a haemocytometer, with a minimum detection threshold of
approximately 10^4 ml^1. All counts were transformed into number of individuals
per ml to give the population density of each species in each community.
Cell size (mass here) was estimated from linear dimensions of up to ten
cells of each species and the rough shape of the cells (Wetzel & Likens, 1991 ).
Transformation of the linear dimensions to volume, and assumption of
1mm^3 ¼1 milligram resulted in the mass of each cell and is reported in milligrams
here. Some variation in cell size occurs during the growth of protist populations
and can also result from changes in environmental conditions that occur during an
experiment (e.g. depletion of available resources) or from experimental treatments
(e.g. a cool versus warm environment). Though here we ignore this intraspecific
variation in cell mass, it seems unlikely that it could invalidate the analyses of
many orders of magnitude of interspecific variation in cell mass presented.
Choice of species
One might argue that studying the consequences of body size in communities
with artificial body-size distributions is meaningless. We do not believe this is the
case. First, all of the species in our laboratory stocks potentially co-occur in natural
aquatic communities and so are not necessarily evolutionarily and ecologically
CONSEQUENCES OF BODY SIZE IN MODEL MICROBIAL ECOSYSTEMS 247