SWITCHING LIFE-HISTORY SENSITIVITIES TO STRESS IN SOIL INVERTEBRATES

Abstract

Over the last decades ecologists have expressed a growing interest in studying the life-cycle responses of animals and plants to stress following global threats of climatic change and pollution. Subsequent reductions in population growth rate (λ) are the result of stress-affected changes in various vital rates. At present it is unknown whether a stressful environment increases the sensitivity of λ to changes in vital rates other than the ones which have been affected. In this paper we explore whether organisms exposed to anthropogenic pollutants are more sensitive to other environmentally induced life-cycle changes than prior to exposure. If so, this may enhance or reduce the direct effect of toxic substances at the population level. Since many soil organisms are continuously exposed to pollutants in the field we investigate the contributions of vital rates to λ under different pollutant concentrations. Soil invertebrates with divergent life cycles were selected (the mite Platynothrus peltifer and the isopod Porcellio scaber exposed to cadmium, and the nematode Plectus acuminatus exposed to pentachlorophenol). Stage-structured matrix models were constructed based on experimental data sets and used to perform a perturbation analysis of λ, i.e., to measure a proportional change in λ resulting from a proportional change in vital rates (elasticity analysis). For Platynothrus peltifer cadmium had the largest effect on reproduction and a small impact on the survival over all stages except for the egg stage resulting in a diminished λ. However, λ was most sensitive to changes in the duration of the egg stage followed by survival of the second nymph stage at all concentrations compared to the other stages. The elasticity of both stages increased at the highest cadmium concentration. For Porcellio scaber cadmium reduced the length of the juvenile period as well as reproduction. Under control conditions λ was most sensitive to changes in the duration of the oocyte stage, followed by the duration of the first reproductive stage and survival of juveniles. At the highest cadmium concentration the duration of the oocyte stage was most important. For Plectus acuminatus pentachlorophenol strongly affected λ, and the largest effect was found on egg survival. In the control treatment λ was most sensitive to changes in juvenile survival, and the elasticity decreased with increasing pentachlorophenol concentrations in contrast to elasticity for the other traits. However at the highest concentration, when egg survival was nearly zero, juvenile survival was the most important trait. From these results it can be concluded that: (1) although there were differences in life cycles between species and types of stressors, in general λ was most sensitive to changes in survival or duration of early-life stages, (2) the analyses of stress-induced life-cycle alterations indicate that the elasticity of λ to changes in a particular rate was not related to the sensitivity of that rate to a toxicant, and (3) for all three species it appeared that a toxic environment switches the sensitivity of λ to changes in vital rates. The results may have consequences for ecological risk assessment of pollutants. Our results imply that exposure to toxic compounds increases the sensitivity of organisms to stressors that affect vital rates other than the ones that have been affected by the toxicant. This aspect has largely been overlooked and requires more attention when the ultimate goal is to protect biodiversity from the impact of anthropogenic pollution.