Long term monitoring of the moth populations at Broom's Barn

LONG TERM MONITORING OF MOTH POPULATIONS AT BROOM'S BARN I. P . WOIWOD

In 1960 a light trap was set up at Rothamsted Experimental Station at H a r p e n d e n , Hertfordshire, on an agricultural site where an identical one had b e e n operated experimentally in the 1930's and 1940's. A comparison of the n u m b e r s of the larger moths caught showed that there had been a significant change in population levels in the intervening years. This change, although not unexpected with the large changes in agricultural practice that had also occurred, highlighted the lack of quantitative information in insect populations in Great Britain as no comparable data were available from elsewhere even in such a relatively well recorded group as the moths. Partly as a result of this it was decided to establish a network of light traps throughout Great Britain to provide a baseline for future comparisons, to develop methods of analysis of populations change and to provide data on several species simultaneously at different places.

T h e network built up gradually and there are at present over 120 light traps in operation, one of which is situated at Broom's Barn Experimental Station at Higham near Bury St. Edmunds (TL 752656), an outstation of Rothamsted specialising in sugar beet research. This trap has been in continuous operation since 1968, the only one in Suffolk to have been operated systematically for a long period, and some results from it will be described later.

O t h e r traps are operated on a voluntary basis at sites throughout Great Britain ranging from farmland to open moorland and ancient woodland to u r b a n gardens. All traps are of the Standard Rothamsted design which have a 200 Watt tungsten bulb, covered by an opaque roof, at 4 feet above the g r o u n d . The traps are emptied daily throughout the year and the catch is killed so that it can be sent elsewhere for identification or checking. The type of trap used was chosen partly for historic reasons but also because it has been f o u n d to give a more consistent catch than other designs of light traps in c o m m o n use (Taylor and French, 1974). Another important feature of the trap is that it takes only a small sample and detailed analysis of catches from traps operating in stable environments show that this sample has no effect on populations, an important feature from a conservation point of view and also necessary if the results of such monitoring are to have scientific validity as m e a s u r e m e n t s of population change. The macrolepidoptera, the larger moths, are the only group of insects identified from all of the traps but other groups are identified when possible or for particular analyses. Moths were chosen for these studies because they can be identified relatively easily and quickly, often with amateur help, and yet there are enough species for useful comparisons.

T h e monitoring of significant population trends in insects is essentially a long term project but the simultaneous measurements of moth population densities at different places over several years has already provided data for many useful ecological analyses. The Broom's Barn light trap, because of its

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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

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Population parameters of the macrolepidoptera from Broom's Barn light trap 1968-1979. a) Total n u m b e r of moths per year (N) Logarithmic scale. b) Total n u m b e r of moth species per year (S). c) Diversity of moth population per year ( a ) .

32.1 for all the Survey traps (Taylor, French and Woiwod (1978)). This higher than average diversity at Broom's Barn is probably related to the variability of soil type within the area and the proximity of the trap to the B r e c k l a n d s which are noted for their entomological interest, and the richness of their invertebrate f a u n a .

A l t o g e t h e r 296 species of the larger moths have been caught at Broom's Barn in the twelve years it has been in operation. This is a high value considering the small sample of moths caught each year and is a reflection of the high diversity of the site. A s the maximum number of species caught in any o n e year is 151 many of these species are only being caught occasionally.

T h e r e are too many species to list or analyse in detail in this paper but, as might be expected f r o m the overall decline in total numbers, the majority of

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species show some reduction in numbers during the twelve years. However, this is not invariably so and Figure 2a shows Luperina testacea, the Flounced Rustic, a species whose numbers have increased from 26 in 1969 to a peak of 271 in 1977. This is one of the few species that can survive in heavily managed grassland areas, such as lawns and playing fields, as its larvae feed on the roots of grasses and it is probably breeding successfully in lawns surrounding the research Station at Broom's Barn. Figure 2b shows a species Lithosia lurideola, the C o m m o n Footman, whose population has no obvious trend, fluctuating around a constant mean value. This is mainly a lichen-feeding species and its food supply may not be under such pressure from intensive agricultural practice as other species. Figure 2c shows a species with a characteristic decline in population. This is Amathes c-nigrum, the Setaceous H e b r e w Character, a species which feeds on a variety of low growing annual herbs which are the first casualties of increased efficiency in agricultural practice. In 1969, 433 individuals of this species were recorded, whereas in 1979 only 6 turned up in the trap.

Because the sample from the Rothamsted light-trap is small most species caught are the more common and widespread ones and the Survey was primarily instigated to monitor this group of species. However, with any intensive sampling at a single site over a long time the more local and interesting species will turn up occasionally. In the case of Broom's Barn these are often species characteristic of or, particularly associated with, the nearby Breckland. Agrotis vestigialis, the Archers Dart, has been regularly recorded. This is a mainiy coastal species of sandhills, but the Breckland is a noted inland site. Other species typical of Breckland or sandy heaths which have occurred occasionally are Heliothis viriplaca, the Marbled Clover, Paradisia glareosa, the Autumnal Rustic, and Lithostege griseata, the Grey Carpet. This last species is found nowhere eise in the British Isles. One only has been caught, in 1979, probably a wanderer from nearby Breckland.

A n o t h e r group of moths that turns up sporadically includes some of the rarer migrants. Notable catches at Broom's Barn have been Lamphygma exigua, the Small Mottled Willow, one of which was caught in 1976, and Nycterosea obstipata, the Gern, two in 1969. Both of these species are noted migrants which only turn up in favourable years and are never very common.

T h r e e other species that are widespread but rarely recorded due to the difficulty of identification by wing pattern have been identified at Broom's B a r n , by examination of the genitalia, which are unique to each species.

T h e s e are Procus versicolor, the Rufous Minor, Oporinia autumnata, the A u t u m n a l Moth, and Oporinia christyi, the Pale November Moth.

O t h e r species of interest to Suffolk lepidopterists are Acontia luctuosa, the Four Spotted, which used to be common around Stowmarket but has been getting noticeably rarer in Suffolk in recent years. A Single specimen turned up at B r o o m ' s Barn in 1977. Ennomos autumnaria, the Large Thorn, is caught regularly. This is a species which was once very rare but is extending its ränge in the Eastern Counties and has spread eastwards through Suffolk in recent years. O t h e r Broom's Barn records which are of interest because they are rarely recorded in Suffolk or rarely caught in Insect Survey light traps are Aporophyla nigra, the Black Rustic, Griposia aprilina, the Merveille-du-

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Year Fig. 2

L o g i 0 annual totals for three species from Broom's Barn 1968-1979.

a) Luperina testacea. b) Lithosia lurideola. c) Amathes c-nigrum.

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Jour, Bomolocha crassalis, the Beautiful Snout, Plusiapulchrina, the Beautiful Golden Y and Lithophane ornitopus, the Grey Shoulder Knot.

Acknowledgements I am grateful to Mr. H. E. Chipperfield for useful Information on Suffolk lepidoptera and Dr. G. D. Heathcote for ensuring that the Broom's Barn trap has been run so consistently over the years.

References Taylor, L. R. and French, R. A. (1974). Effects of light trap design and illumination on samples of moths in an English woodland. Bulletin of Entomological Research, 63, 583.

Taylor, L. R., French, R. A. and Woiwod, I. P. (1978). The Rothamsted Insect Survey and the urbanisation of land in Great Britain. In Perspectives in Urban Entomology, ed. G. W. Frankie and C. S. Koehler, pp.

31-65, Academic Press, New York.

Taylor, L. R., Kempton, R. A. and Woiwod, I. P. (1976). Diversity statistics and the log-series model. Journal of Animal Ecology, 45, 255.

Taylor, L. R. and Woiwod, I. P. (1980). Temporal stability as a densitydependent species characteristic. Journal of Animal Ecology, 49, 209.

Taylor, L. R., Woiwod, I. P. and Perry, J. N. (1980). Variance and the large scale spatial stability of aphids, moths and birds. Journal of Animal Ecology, 49, 831.

I. P. Woiwod, B.Sc. Rothamsted Experim. ntal Station, Harpenden, Herts.

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