Predation may alter vulnerability of Daphnia populations to insecticides by suppressing the development of peak densities

摘要

Cladoceran Daphnia are key species in lakes and ponds because they are both effective grazers of phy-toplankton and a favorite food of many predators (Hanazato 2001). Furthermore, they are highly sensitive to toxic chemicals that contaminate surface water bodies (Hanazato 1998). The effects of a toxic chemical on any population may differ depending on timing of application. In a laboratory population of Daphnia pulex, Hanazato & Hirokawa (2004) demonstrated that application of the insecticide car-baryl at different population phases induced different recovery patterns, and the impact was strongest for the population at peak density, indicating the population was most sensitive to the chemical at that stage. They then hypothesized that food condition, which varies with population density, affects vulnerability of populations to insecticides. In lakes and ponds, however, not only food condition but also predation are important factors controlling population dynamics of Daphnia, and therefore may affect vulnerability of the population to chemicals. We have assumed that predation interferes with development of a sharp and large density peak of Daphnia populations and it alters predation vulnerability to insecticides. We tested this assumption in the present study, where Daphnia magna populations were established in beakers, peak densities were controlled by artificial removal of adult individuals to imitate fish predation, and populations were exposed to the insecticide carbaryl to assess population vulnerability to the chemical.