School of Engineering and Science - Jacobs University

CHAPTER IV
Favella ehrenbergii (Figure 1b) generally grew faster (t = 6.27, df = 22, p < 0.001, twotailed
t-test) in the single predator treatment when compared to G. dominans (Figure
2b). Mean growth rates of the tintinnid increased constantly from 0.54 to 0.98 d -1 and
were on average 0.77 d -1 . Specific ingestion rates dropped from 165 to 86 cells
predator -1 d -1 and averaged 64 cells predator -1 d -1 during the course of the experiment.
As found for G. dominans the grazing rates of F. ehrenbergii hardly exceeded the
growth rates of S. trochoidea for the first two days (0.43 and 0.51 d -1 ). However, during
the last day of the experiment as a result of the increasing predator population the
grazing rate amounted 2.55 d -1 leading to a sharp decline in the prey population.
Predator specific filtration rates recorded for F. ehrenbergii were not different (t = 0.05 ,
df = 22, p = 0.96, two-tailed t-test) from those recorded for G. dominans and ranged
between 0.31 and 0.64 mL µg predator -1 d -1 (mean value 0.49 mL µg predator -1 d -1 ).
In treatments with G. dominans as sole prey for F. ehrenbergii (Figure 1c), the larger
predator displayed a mean grazing rate of 0.20 d -1 . Although prey was always available
at high concentrations, grazing rates decreased from higher values (0.21 and 0.34 d -1 ) to
a value of 0.04 d -1 on the last day of the experiment. This was also reflected by the
specific ingestion: The ciliate initially ingested 115 and 111 cells predator -1 d -1 on the
second day but then ingestion dropped to a value of 6 cells predator -1 d -1 (mean 70 cells
predator -1 d -1 ). Even if F. ehrenbergii ingested high numbers of prey cells it was not
able to increase growth rates as with the prey S. trochoidea. Growth rates amounted to
0.32 d -1 on average and dropped from 0.50 to 0.08 d -1 during the experiment.
While both predators in the single predator treatments roughly only consumed the daily
production of S. trochoidea until day three of the experiment, a completely different
picture emerged in the two predator treatments (Figure 1d, 3). Although the initial
predator and prey biomass were the same as in the single predator treatments, S.
trochoidea biomass already decreased after 24 hours and was completely grazed down
at the end of the experiment. Consequently, grazing rates in the two predator treatments
were consistently higher (0.60 and 1.89 d -1 ) than those measured in the single predator
treatments on the first two days of the experiment (after 24 hours: t = 5.91, df = 10, p <
0.001, after 48 hours: t = 16.50, df = 10, p < 0.001, two-tailed t-tests: pooled g of single
predator treatments against g in the two predator treatment). After 72 hours this effect
was only measurable in one replicate due to the extinction of the prey (Figure 3).
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