Evaluation of Mortality Factors in Insect Life Table Analysis

An individual animal may suffer simultaneously from several different maladies, which lead to its eventual death. Such "overlaps" between contemporaneous mortality factors create some problems in partitioning, in a consistent manner, the animals killed among several specific causes. The aim of this paper is to develop the notion of "killing power" of a mortality factor, so that we can properly evaluate and incorporate in a model the effect of any given factor on the dynamics of the animal population concerned. I first introduce the basic notion of killing power under the assumption that the mortality factors involved are operating independently of each other. The killing power of a given factor is then shown to be equivalent to the marginal probability of an animal being killed by that factor. The assumption of independence of the mortality factors is relaxed, and the more general notion of conditional killing power is introduced. This is equivalent to the conditional probability of an animal being killed by a given mortality factor, when that individual has not been simultaneously affected by other contemporaneous factors. The concept of mutually exclusive mortality factors is discussed to clarify an apparent confusion in some published literature. Typical examples of this type of mortality factor are an extended drought and excessively wet weather, both of which may kill some animals, but which will not occur simultaneously. Therefore, unless the stage division in the life tables so broad that these types of factors can occur in the same stage, we do not need to consider mutually exclusive mortality factors. How to evaluate the killing power of a mortality factor is illustrated with some examples taken from the literature. Also discussed is a more complicated case, in which the annual fluctuation in the proportion of animals killed by a given factor is correlated with those killed by other contemporaneous mortality factors. Concluding remarks include some notes on the division of stages in a life table so as to facilitate the evaluation of the killing powers of as many mortality factors as possible.