Long-term spatio-temporal variation in abundance of the garden tiger moth (Arctia caja) during a population decline

Kelvin Conrad, Ian Woiwod & Joe Perry, Institute of Arable Crops Research, UK
Abstract for Detecting Environmental Change: Science & Society

The garden tiger moth (Arctia caja)  is a well-known and attractive moth that was once regarded as common in the UK. It is polyphagous and overwinters as small larvae. Over the past 35 years numbers of garden tiger moth have declined severely. We examined this decline using almost 2700 trap-years of  data collected at 407 Rothamsted Insect Survey light-traps from all over Great Britain, spanning 1968-98. The annual collated index, commonly used to assess relative changes in lepidopteran abundance, confirms this long-term trend.

However, our examination of the geometric mean abundance across occupied sites has revealed a somewhat different pattern. The annual geometric mean fluctuated around approximately  4.2 individuals/year until 1983, and then fell suddenly to approximately 3.0 individuals/year and continued oscillating near that new, lower level thereafter. In contrast, the proportion of sampled sites occupied (incidence) remained high at approximately 0.60 until 1987-88, when it fell to 0.46 and continued to decline. Thus, garden tiger moth density fell across Great Britain initially in 1983, but the moths did not begin disappearing from individual sites until several years later. Populations may have hovered near some threshold level with local extinctions lagging behind local declines in abundance.

Since 1989, the garden tiger moth has remained at low densities and low incidence. The general trend over time has been for the species to become more restricted to the north and west and almost completely absent from the Southeast.

Multiple regression analysis using monthly mean values from the Central England data set suggest garden tiger moth abundance is adversely affected by warm wet winters and warm springs. However, the sudden collapse in abundance between 1983 and 1984 is more likely associated with an extreme meteorological event.

The sudden drop in abundance and the four to five year lag before the accompanying decrease in incidence underscore the value of long-term monitoring in determining changes in abundance and distribution, even of species considered to be widespread and common.

DETECTING
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Science and Society

17-20 July 2001, London, UK

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