| Summary: | The differential effects of body size on species' demographic parameters
has long been hypothesized to be a powerful structuring force in zooplankton
communities. The size-efficiency hypothesis predicts that large species, due to
metabolic efficiency and-greater effectiveness of food collection, should displace
small species when food is limiting, in the absence of predation. According to
the threshold-food concentration hypothesis, small-bodied rotifers achieve r=0 at
a lower food concentration than large rotifers; however, large rotifers have
higher maximal reproductive rates. I attempted (1) to assess the importance of
food concentration in structuring the species and size composition of a natural
rotifer community in Deer Lake, Burnaby B.C., and (2) describe seasonal changes
in rotifer community structure with reference to temperature, competition and
predation.
The threshold food hypothesis relates specifically to rotifers, and its
significance has been tested in published laboratory studies. Therefore, I
predicted that the y-intercepts of regression equations relating food
concentration (measured as size-fractioned chlorophyll a) and reproductive
output would be higher for small species than for large ones, and that the slopes
of these lines would be higher for large species than for small ones. I found no
patterns with respect to body size in either of these two parameters; however, I
found some evidence for size-efficiency within a single species, Keratella
cochlearis. The large form of K. cochlearis reproduced at a significantly lower food
concentration than either of the two smaller forms. Average rotifer body size of
the whole community showed no change with chlorophyll concentration. Recent
research which suggests that threshold food levels change along several
environmental gradients may explain the lack of support my data provided for
the threshold-food hypothesis. Additionally, selective grazing may change the
food requirements needed for reproduction of various species.
Temperature was important in determining seasonal species abundance,
likely because of physiological responses of development rate to temperature. I
did not find that species with high loadings on those principal components axes
that were significantly correlated with Daphnia or cyclopoid copepod abundance
had attributes which conferred resistance to interference competition or
predation. However, spined, small Keratella cochlearis co-occured seasonally with
predatory cyclopoid copepods. Although competition and predation may not
have been measured adequately, or at a scale relevant to rotifer survival and
reproduction, it appears that temperature is the most important factor I
measured in organizing rotifer species into communities in Deer Lake.
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