Physiological mortality rates of planktonic ciliates

Contrasting physiological mortality with predator-induced mortality is of tremendous importance for the population dynamics of many organisms but is difficult to assess. I performed a meta-analysis using planktonic ciliates as model organisms to estimate the maximum physiological mortality rates (δmax) across pelagic ecosystems in relation to environmental and biotic factors. Data were compiled from published numerical response (NR) experiments and experimentally determined rates of decline (ROD). Variables reported are ciliate species and order, ciliate specific growth rates (rmax), prey species, temperature, habitat (marine vs freshwater), the coefficients of the numerical response experiments, and reported or calculated ciliate mortality rates. The median δmax of planktonic ciliates was 0.62 d−1 and did not differ between marine and freshwater species. Maximum ciliate mortality rates were species-specific and affected by their rmax, cell volume, and ability to encyst. Cyst-forming species had, on average, higher δmax than species unable to encyst. Maximum mortality rates of ciliates were positively related to rmax but appeared unaffected by temperature. I conclude that (i) in the ocean, physiological mortality is more critical for controlling ciliate population size than ciliate losses imposed by microcrustacean predation, but (ii) in many lakes, the opposite holds; (iii) cyst-formation is an effective ciliate trait to cope with the high mortality of motile cells upon starvation. The lack of a temperature effect on δmax deserves further study; if correct, planktonic ciliates may take advantage of rising ocean and lake temperatures, with important implications for the pelagic food web.

Methods
I used ISI Web of Science and Google Scholar to search for experiments that measured growth and mortality rates of ciliates as a function of prey concentration (i.e. numerical responses). The search terms were “growth (rate)” or “numerical response” in combination with “ciliate*” to search for numerical response experiments and “starvation” or “starved” in combination with “ciliate*” to search for mortality experiments. In addition, I searched the literature cited in these publications for further datasets. I considered only planktonic ciliates. When studies did not report all parameters of the NR curve, the data were extracted from figures with DataThief III or WebPlotDigitizer (Version 4.6) and fitted with a modified Michaelis-Menten equation that included the threshold prey concentration (P’) as an additional parameter. Mortality rates obtained by ROD experiments used the δmax reported in the respective study or calculated δmax from the maximum rate of decline after digitizing the data from the original curves, as described above. The literature search yielded δmax reported from 41 studies investigating 56 species or strains in 81 NR experiments and 19 ROD experiments. The final dataset (n = 77) included 37 studies and 48 species. I analyzed the dataset using the R Statistical Software using the packages lme4, lmerTest, AICcmodavg, and MuMIn.