MONITORING OF T H E MOTH POPULATION AT BROOM'S BARN
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l o n g a n d consistent r u n , has been particularly i m p o r t a n t in much o f this w o r k . T h i s was c e r t a i n l y so in an extensive study of the measurement o f diversity. D i v e r s i t y is a very i m p o r t a n t concept in ecology and many methods have been p r o p o s e d f o r measuring the p r o p e r t y in biological populations f r o m sample data. T h e r e p l i c a t i o n p r o v i d e d by the light trap data was successfully used to d e t e r m i n e the most suitable o f the proposed indices ( T a y l o r , K e m p t o n and W o i w o d , 1976) a n d w i l l be described in more detail and used later t o describe changes at B r o o m ' s B a r n . O t h e r studies have investigated the analysis o f spatial a n d t e m p o r a l stability ( T a y l o r , W o i w o d & Perry (1980); T a y l o r & W o i w o d ( 1 9 8 0 ) ) , the effect o f land use o n diversity, and the effect o f urbanisation o n m o t h p o p u l a t i o n s ( T a y l o r , French & W o i w o d (1978)). T h e data have also m a d e possible the p r o d u c t i o n by Computer of density d i s t r i b u t i o n maps o f i n d i v i d u a l m o t h species w h i c h have considerably increased our u n d e r s t a n d i n g o f patterns o f p o p u l a t i o n change. L e t us n o w l o o k at some o f the more interesting results f r o m the B r o o m ' s B a r n light t r a p . F i g u r e 1 shows changes in the three m a i n p o p u l a t i o n parameters o f the t r a p f o r the twelve years 1968 t o 1979. These parameters are t o t a l n u m b e r o f m a c r o l e p i d o p t e r a per year, N, t o t a l n u m b e r o f species per y e a r , 5 , a n d the diversity o f m o t h p o p u l a t i o n for each year, a . T o t a l numbers N are p l o t t e d o n a l o g a r i t h m i c scale as changes in this tend to be m u l t i p l i cative. T h e g r a p h shows a general decline f r o m about 2000 individuals in 1968 t o 770 in 1979, w i t h a m i n i m u m in 1974 and a peak o f 1500 in the exceptional year o f 1976. D e s p i t e the 1976 peak the general p o p u l a t i o n t r e n d is d o w n . T h e exact reason for this decline is not k n o w n but the t r a p is in an agricultural s e t t i n g o n the edge o f an e x p e r i m e n t a l f a r m , and it may be typical of a general decline in n u m b e r s in such habitats in recent years, f o l l o w i n g changes in a g r i c u l t u r a l practice such as hedgerow r e m o v a l , use o f herbicides and insecticides a n d generally m o r e intensive c u l t i v a t i o n . T h e average value for N f r o m all B r i t i s h sites is a b o u t 2500 so that B r o o m ' s B a r n has a m o t h popul a t i o n b e l o w average t h r o u g h o u t the p e r i o d . T h e n u m b e r o f species, 5 , tends to f o l l o w l l u c t u a t i o n s in n u m b e r s o f individuals, N , f r o m a m a x i m u m o f 151 species in 1969 a n d m i n i m u m o f 98 in 1974. T h e mean value over 12 years is 129 species per year a n d there were 122 species recorded in 1979. D i v e r s i t y has already been m e n t i o n e d briefly. It is best measured by a statistic, a . o b t a i n e d f r o m the characteristic shape o f the species frequency d i s t r i b u t i o n ( t h e n u m b e r o f species w i t h d i f f e r e n t n u m b e r s of individuals). T h i s statistic is o b t a i n e d f r o m the log series d i s t r i b u t i o n , and has several desirable p r o p e r t i e s . U n l i k e S, the n u m b e r o f species, it is independent o f sample o r p o p u l a t i o n size ( i n thiscase N), it i s a m e a s u r e o f the richness o f the p o p u l a t i o n in species, a l l o w i n g for the changes f r o m year to year in general level o f i n d i v i d u a l s . It is very consistent at a site w h i c h is k n o w n t o have a stable e n v i r o n m e n t b u t not in unstable o r rapidly changing ones. It discriminates w e l l b e t w e e n sites w h i c h are k n o w n t o be d i f f e r e n t and it corresponds w e l l t o e x p e c t e d patterns o f diversity. A s can be seen f r o m Figure 1. a . is m u c h m o r e consistent t h a n N OT S a n d i f a n y t h i n g is slightly higher f r o m 1976 t o 1979, a l t h o u g h not significantly so. T h i s indicates that despite changes in t o t a l p o p u l a t i o n size the u n d e r l y i n g structure has not been affected. T h e mean value o f a is 37.8 Âą 1.13, w h i c h is significantly higher than the average o f
Trans.
Suffolk
Nat. Soc. IS pari
3.