An experimental test of the egg‐ratio method: estimated versus observed death rates

Abstract

1. The accuracy of death-rate estimates provided by the egg-ratio method was assessed by directly measuring the real death rates of six large laboratory Daphnia populations for 40 days.2. Of six models tested the Kawecki iterative model gave the best relationship to real death rates in terms of variance explained, bias, and fit at both 4-and 8-day sampling intervals. Although considered theoretically correct by persons familiar with the mathematics of egg-ratio models, the Edmondson-Paloheimo model gave slightly worse results in terms of bias and fit. 3. Using raw data, as opposed to smoothed data, gave the best fit between estimated and observed rates for both the Kawecki iterative and Edmondson-Paloheimo models. 4. Size-selective predation did not increase error in estimates of the death rate; however, non-steady-state dynamics were correlated with error in model estimates. 5. In general, the models gave a relatively good description of real dynamics, but they* regularly produced negative death rates, and were grossly wrong for some individual estimates. The present study also gives practical suggestions on application of this technique to field populations.

Mortality estimation for planktonic copepods: Pseudocalanus newmani in a temperate fjord

“…With the dynamics as simulated, the population surface method generated markedly more accurate parameter estimates than either cohort analysis or the population projection matrix method and somewhat better estimates than the systems identification technique. Tests of estimation procedures on simulated data remain a powerful means of assessing method accuracy and precision, although comparison with actual mortality measured in enclosures is also possible (Brett et al 1992). Unlike systems identification, the population surface method does not require prior specification of the functional form of birth and death rate variation over time nor does it suffer from the demographic instabilities to which systems identification is prone (Wood and Nisbet 199 1).…”

The significance of nonviable eggs for Daphnia population dynamics

“…Consequences -Overlooking egg mortality can have a serious impact on computing population birth and death rates, resulting in overestimates of both rates. For instance, Brett et al (1992) found that Paloheimo estimates of death rate overestimated observed death rates by 0.015-a bias that might have been caused by the occurrence of NDE. Moreover, the occurrence of NDE will lead to an apparent skewness of the egg-age distribution toward younger stages.…”

Estimates of the nonconsumptive mortality of mesozooplankton by measurement of sedimentation losses

“…While most studies have focused on the production of different groups of zooplankton and their interactions within the food web, comparatively little is known about the impact of nonconsumptive mortality on zooplankton population dynamics (Andersen 1997). Few articles address zooplankton mortality per se (Clarke and Carter 1974;Prepas and Rigler 1978;Ghilarov 1985;Gabriel et al 1987;Brett et al 1992). There is general agreement that consumptive mortality is the most important factor determining zooplankton population dynamics (Gliwicz and Pijanowska 1989).…”

“…In culture experiments, mortality rates can be measured relatively easily (Brett et al 1992). Under natural conditions, mortality is usually estimated as the difference between a calculated estimate of birth rate (b) and estimated population growth rate (r) (Clarke and Carter 1974;Prepas and Rigler 1978;Rigler and Downing 1984;Ghilarov 1985).…”

“…Estimates of death rate (d), calculated as d ϭ b Ϫ r, are confounded by errors in r and b and are therefore less precise than either (Taylor and Slatkin 1981). Consequently, it is difficult to obtain reliable estimates of mortality under natural conditions (Prepas and Rigler 1978;Brett et al 1992). …”