Sammendrag
Abundances of 12 laboratory populations of the greenbottle blowfly
(Lucilia sericata) were recorded every two days for 776 d, with
separate counts for larvae, pupae, and adults. Half of the populations
were exposed to sublethal dosages of the toxic compound cadmium
acetate; the remaining populations were considered controls. Initial
density was low for half of the populations in each group, and high for
the other half. In all populations, the adult abundance underwent
sustained fluctuations. However, cadmium-exposed populations had
smaller mean larval and adult densities, and fluctuations in adult
abundance were less regular than for controls. Data from the first and
the second half of the experimental period were analyzed separately in
order to assess the effects of possible long-term changes in the
dynamics on the estimates. Nonparametric (generalized) additive
modeling (GAM) was used to investigate time series dynamics and, in
particular, to explore the density-independent components and the
structure of the density-dependent components of the system. Overall,
cadmium populations had larger larva-to-adult survival rate and smaller
adult survival rate than control populations, and for the second half
of the experimental period the reproductive rate was smaller for
cadmium populations than for control populations. Estimation of the
density-dependent components suggested that survival from larva to
adult depended nonlinearly on larval density and that increased larval
density had a positive effect on larval survival at low densities.
Furthermore, cadmium generally decreased vital rates. However, the
analysis suggested that most of the observed differences in dynamical
behavior between control and cadmium populations are related to
differences in the density-independent components of the demographic
rates, rather than differences in the density-dependent structure.
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