Population dynamics

Abstract: New developments in ecotoxicology are changing the way pesticides and other toxicants are evaluated. An emphasis on life histories and population fitness through the use of demography, other measures of population growth rate, field studies, and modeling are being exploited to derive better estimates of pesticide impacts on both target and nontarget species than traditional lethal dose estimates. We review the state of the art in demographic toxicology, an approach to the evaluation of toxicity that uses life history parameters and other measures of population growth rate. A review of the literature revealed that 75 studies on the use of demography and similar measures of population growth rate in toxicology have been published since 1962. Of these 75 studies, the majority involved arthropods. Recent evaluations have indicated that ecotoxicological analysis based on population growth rate results in more accurate assessments of the impacts of pesticides and other toxicants because measures of population growth rate combine lethal and sublethal effects, which lethal dose/concentration estimates (LD/LC50) cannot do. We contend that to advance our knowledge of toxicant impacts on arthropods, the population growth rate approach should be widely adopted.

Abstract: Life history data for Aphidius gifuensis (Ashmead) and Myzus persicae (Sulzer) were collected in the laboratory. To consider both sexes and variable developmental rates among individ- uals, the raw data were analyzed using the age-stage, two-sex life table. The intrinsic rate of increase (r) for A. gifuensis is 0.264 d 1 . The mean parasitism rate is 92.3 aphids per female. The intrinsic rate of increase for M. persicae is 0.252 d 1 . For applying the female age-speciÞc life table to a female population, we prove that the relationship between the mean female fecundity (F) and the net reproductive rate (R0 )i sR0saF, where sa is the preadult survival rate. When the female age-speciÞc life table is applied to two-sex populations, the relationship between F and R0 is R0 sawF, where sa is the preadult survival rate of females, and w is the female proportion in offspring. This is valid when w is a constant for the age-speciÞc fecundity (mx) of all ages. Because sexing preadult individuals is difÞcult, and obtaining a constant sex ratio in offspring is uncertain, determining preadult mortality of the individual sexes may be problematical. As a result, calculations of the age-speciÞc survival rate (lx) and fecundity and population parameters may be adversely affected. Moreover, if lx and mx are constructed based on adult age, they may also cause errors in population parameters. Because the application of female age-speciÞc life table to stage-structured bisexual population results in inac- curacies, we recommend that the age-stage, two-sex life table should be used in insect demographic studies.

Abstract: Elasticity analysis is a useful tool in conservation biology. The relative impacts of proportional changes in fertility, juvenile survival, and adult survival on asymptotic population growth λ (where ln(λ) = r, the intrinsic rate of increase) are determined by vital rates (survival, growth, and fertility), which also define the life history characteristics of a species or population. Because we do not have good demographic information for most threatened populations, it is useful to categorize species according to their life history characteristics and related elasticity patterns. To do this, we compared the elasticity patterns generated by the life tables of 50 mammal populations. In age-classified models, the sum of the fertility elasticities and the survival elasticity for each juvenile age-class are equal; thus, age at maturity has a large impact on the contribution of juvenile survival to λ. Mammals that mature early and have large litters (“fast” mammals, such as rodents and smaller carnivores) also...

Abstract: A long-standing hypothesis posits that natural selection can favour two female strategies when density cycles. At low density, females producing many smaller progeny are favoured when the intrinsic rate of increase, r, governs population growth. At peak density, females producing fewer, high-quality, progeny are favoured when the carrying capacity, K, is exceeded and the population crashes. Here we report on the first example of a genetic r versus K selection game that promotes stable population cycles in lizards. Decade-long fitness studies and game theory demonstrated that two throat-colour morphs were refined by selection in which the strength of natural selection varied with density. Orange-throated females, r strategists, produced many eggs and were favoured at low density. Conversely, yellow-throated females, K strategists, produced large eggs and were favoured at high density. Progeny size should also be under negative frequency-dependent selection in that large progeny will have a survival advantage when rare, but the advantage disappears when they become common. We confirmed this prediction by seeding field plots with rare and common giant hatchlings. Thus, intrinsic causes of frequency- and density-dependent selection promotes an evolutionary game with two-generation oscillations.