Spatial pattern complements unusual changes in abundance and occupancy during the national population decline of a common Arctiid moth.
Conrad, Kelvin*,1, Perry, Joe1, Woiwod, Ian1, 1 Rothamsted, Harpenden, Hertfordshire, UK
ABSTRACT- 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. An association matrix was calculated for all pair-wise spatial associations between years. The association indices showed serial autocorrelation of spatial structure between years, but association declined quickly with lags greater than one. A principal co-ordinate analysis of the association matrix revealed three spatial phases during the population decline: an early period (1969-78) with highly-structured spatial pattern and limited occupancy and abundance in the north; a middle period (1979-90) with nearly random distribution overall and declining abundance in the south; and a recent period (1991-99) with highly structured spatial pattern and small abundance overall. The changes in spatial structure in the early 1980's accompanied rapid changes in abundance and preceded a sharp decline in occupancy. The appearance of a random spatial pattern in the early 1980's confirms that the sharp decline in abundance at that time included decreases from relatively high-density sites spread throughout Great Britain. Spatial structure returned in the 1990's after widespread low-density populations began disappearing in the late 1980's. The areas showing the greatest variation in abundance displayed the greatest range in spatial structure over time, but also the greatest stability of spatial structure, resulting from a gradual change between extremes of spatial pattern. The distribution of abundance and occupancy moved northward and westward while retreating to coastal areas over time. SADIE techniques are a powerful method to quantify temporal changes in spatial pattern and relate them to temporal changes in abundance.
Key words: spatial pattern, spatial association, SADIE, spatio-temporal dynamics
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