Large-scale temporal changes in spatial pattern during declines of abundance and occupancy in a common moth

We examined the changes in spatial pattern that accompanied the population decline of the garden tiger moth, Arctia caja (L.) in Britain between 1968 and 1999 using Spatial Analysis by Distance Indices (SADIE) techniques. A principal co-ordinate analysis of all pair-wise spatial associations between years indicated three groupings of years: 1969-1978,1979-1990,1991-1999, which revealed three phases during the population decline: an early period with highly structured spatial pattern; a middle period with nearly random distribution overall; and a recent period with highly structured spatial pattern but small abundance overall. The change in spatial structure in the early 1980's accompanied rapid changes in abundance but preceded a sharp decline in occupancy and confirms that the sharp decline in abundance included decreases from widespread high-density sites. Perhaps unusually, A. caja varied from spatially aggregated to randomly distributed and back to spatially aggregated, all while its abundance declined sharply. Present distribution pattern may reflect past abundance changes poorly in this species. Areas showing the greatest variation in abundance displayed the greatest range in spatial structure, but also the greatest stability of spatial pattern, indicating changes between extremes of spatial pattern occurred slowly. SADIE techniques are a powerful method to quantify temporal changes in spatial pattern and relate them to temporal changes in abundance.