Grazing
by brachionus plicatilis: prey dependent or ratio dependent?
O. Vadstein, L.M. Olsen, T. Andersen, H.R. Reinertsen-2003
Abstract:
The
functional response of an organism describes food consumption and
competitive ability, and determines the dynamic stability of predator-prey
relationships. Traditionally, functional responses have been considered prey
dependent, but recently it has been proposed that the ratio between prey and
predator is a better predictor for the functional response. We conducted
microcosm experiments with phosphorus (P) deficient growth medium, using the
rotifer Brachionus plicatilis as predator and the algae Tetraselmis sp. as
prey. To establish different carrying capacities, the total P concentration
was varied. The cultures were run for 2-3 months with a dilution rate of
0.01/d.
After
an initial algal bloom with a subsequent increase and collapse of the
rotifer population, a quasi-steady state was established. During the algal
bloom, the rotifers grew exponentially until the algal bloom was grazed
down. However, the growth rates of the rotifers were significantly reduced
with increasing total P concentration (-0.116 +/- 0.04/d/(µM P), p=0.024).
during the post-bloom period, rotifer and algal densities more or less
established a steady state, and no predator-prey oscillations were detected.
For both algae and rotifers, the average density during the post-bloom
period increased linearly with total P concentration. Slopes were
0.34+/-0.05 and 1.08+/-0.05 (+/- SE, dimension mg C/(µmol P)) for algae and
rotifers, respectively. Egg-ratio data suggested that the physiology of the
rotifers was comparable in all cultures during this period. The fact that
both prey (algae) and predator (rotifer) densities increased with increasing
total P concentration suggests that the functional response of B. plicatilis
is ratio dependent. Ongoing work will try to verify this by inverse
modeling, and possible mechanisms will be discussed.
(Norwegian
University of Science and Technology, Department of Biology, Trondhjem
Biological Station, 7491-Trondheim, Norway, e-mail: olav.vadstein@vm.ntnu.no)