Effects of dieldrin on population growth rates of sparrowhawks 1963-1986.

Abstract

A recent reappraisal of the evidence of the effects of cyclodiene insecticides on sparrowhawks Accipiter nisus and kestrels Falco tinnunculus in Britain suggested that during 1963-75 at least 29% of sparrowhawks found in an eastern area of high cyclodiene use died directly from dieldrin poisoning, compared with only 6% in a more western area of low cyclodiene use. Here, these data are used in a life-history analysis of the quantitative effects of dieldrin on some British sparrowhawk populations. Where necessary, life-history parameters were taken from two British study populations, one stable and one increasing. Four scenarios were analysed. In scenario 1, reflecting high cyclodiene use, the effect of dieldrin poisoning was to increase the instantaneous mortality rate by 0.20 year^-1. Life-history modelling revealed that this would have produced a population decline of 20% year^-1 (λ=0.82 year^-1). Adding in possible sublethal effects of dieldrin produced an even more serious decline of 60% year^-1 (scenario 2). Data suggested that population growth rate increased linearly as population density decreased. A simple analysis omitting age structure suggested that density-dependence would not save the scenario 2 population from extinction, but would hold the scenario 1 population at 64% of its former value (scenario 3). The available data suggest the former is correct, i.e. sublethal effects were important during the population decline. During 1976-82, when use of cyclodienes was reduced, the effect of dieldrin poisoning on sparrowhawks in eastern Britain was estimated as an increased instantaneous mortality rate of 0.05 year^-1. This is low enough to allow population recovery from low density, at a rate of 13% year^-1 (scenario 4). Another way to analyse the effects of dieldrin (or any other additional density-independent mortality agent) is to ask what reductions in fecundity or survival could be sustained long-term by an otherwise increasing population. The data suggested that the population could withstand a halving of fecundity (from 2.07 to 1.035) or a reduction of adult survival from 0.74 to 0.55. The analyses presented are based on field data, and are thought to be reasonably realistic of the true impacts of dieldrin and other cyclodienes on British sparrow-hawks.

Key words

• age structure
• agricultural entomology
• cyclodiene insecticides
• dieldrin
• fecundity
• insecticides
• life history
• models
• mortality
• natural enemies
• nontarget effects
• nontarget organisms
• pesticides
• poisoning
• population density
• population dynamics
• population growth
• predators
• predatory birds
• sublethal effects
• survival
• toxicity
• wild animals
• wild birds