Body size has been widely recognised as a key
factor determining community structure in ecosystems. We
analysed size diversity patterns of phytoplankton, zooplankton
and fish assemblages in 13 data sets from
freshwater and marine sites with the aim to assess whether
there is a general trend in the effect of predation and
resource ...»»»»
Body size has been widely recognised as a key
factor determining community structure in ecosystems. We
analysed size diversity patterns of phytoplankton, zooplankton
and fish assemblages in 13 data sets from
freshwater and marine sites with the aim to assess whether
there is a general trend in the effect of predation and
resource competition on body size distribution across a
wide range of aquatic ecosystems. We used size diversity
as a measure of the shape of size distribution. Size diversity
was computed based on the Shannon-Wiener diversity
expression, adapted to a continuous variable, i.e. as body
size. Our results show that greater predation pressure was
associated with reduced size diversity of prey at all trophic
levels. In contrast, competition effects depended on the
trophic level considered. At upper trophic levels (zooplankton
and fish), size distributions were more diverse
when potential resource availability was low, suggesting
that competitive interactions for resources promote diversification
of aquatic communities by size. This pattern was
not found for phytoplankton size distributions where size diversity mostly increased with low zooplankton grazing
and increasing nutrient availability. Relationships we found
were weak, indicating that predation and competition are
not the only determinants of size distribution. Our results
suggest that predation pressure leads to accumulation of
organisms in the less predated sizes, while resource competition
tends to favour a wider size distribution.^^^^