Irreplaceable Mortality: Is It?

Abstract

Peterson et al. (2009) presented an analysis of mortality rates of herbivorous insects using published life tables. For each mortality factor in each life table, they estimated the marginal attack rate (hereinafter, “marginal mortality rate”), i.e., the mortality rate expected in the absence of contemporaneous mortality factors assuming independence of occurrence and making some assumption concerning the outcome of their joint occurrence. Using these marginal mortalities, they calculated rates of irreplaceable mortality (“indispensable mortality”, Southwood 1978), i.e., the decrease in total mortality when a given mortality factor or group of factors is excluded from the life table. They compared irreplaceable mortalities for six categories of factors and found that irreplaceable mortality because of non-natural enemies (primarily abiotic factors) was significantly higher than that because of natural enemies. Characterizing irreplaceable mortality because of natural enemies as “very low”, they concluded that natural enemies typically have little impact on insect populations at temperate latitudes and in agricultural systems. They suggested that the impact of natural enemies may be higher in ecosystems with higher stability. They also presented an example based on Kuhar et al. (2002) of using irreplaceable mortality to assess the value of biological control agents. Unfortunately, problems with estimation and application of irreplaceable mortality weaken or invalidate their conclusions. It is important to consider proper usage of irreplaceable mortality because its misuse can result in misleading generalizations concerning insect ecology and in poor pest management decisions. ### Irreplaceable Mortality: Suitable Measure of the Effect of Mortality Factors on Population Growth Rate? Peterson et al. (2009) used irreplaceable mortality as a measure of the effect of mortality factors on population growth rate in comparisons among life tables. However, as pointed out by Bellows and Van Driesche (1999) and discussed in this section, irreplaceable mortality is generally inappropriate for such comparisons. For discrete generations, population growth rate is measured as the net multiplication rate per generation ( R …