On the nature of errors involved in estimating stage-specific survival rates by Southwood's method for a population with overlapping stages |
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Authors: | A J Sawyer and D L Haynes |
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Institution: | (1) Department of Entomology, Cornell University, 14853 Ithaca, NY, USA;(2) Department of Entomology, Michigan State University, 48824 East Lansing, MI, USA |
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Abstract: | Summary This paper has examined the effect of within-stage mortality on the estimation of stage-specific survival rates bySouthwood's (1978, p. 358) method. As pointed out bySouthwood, both the severity and timing of mortality affect the mean duration of a life stage, and consequently the estimate of the
number of individuals entering that stage. Knowledge of the form of the survivorship curve permits correction of the estimate
under certain circumstances. The use ofSouthwood's method with two overlapping stages having different rates and patterns of mortality leads to complex errors in the estimation
of survival for the first stage. The nature of these errors is examined analytically and via a simulation model.Southwood's method is fairly robust, with moderate differences in mortality rates leading to acceptable errors in estimating survival
for the first stage. When both the rate and pattern of mortality in both life stages are the same, then the survival estimate
is made without error. Precise estimates of stage-specific survival will not usually be possible withSouthwood's method because of the errors introduced by the very parameters being measured. Direct measurement of mortality rates and
survivorship patterns (seeSouthwood, 1978, p. 309) is strongly advised, at least in preliminary work. |
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