HELGOLANDER MEERESUNTERSUCHUNGEN Helgol~inder Meeresunters. 43,357-365 (1989)
Long-term population dynamics of breeding bird
species in the German Wadden Sea area*
G o t t f r i e d V a u k 1, J o h a n n e s Prfiter 1 & E i k e H a r t w i g 2
i Norddeutsche Naturschutzakademie; Hof M6hr, D-3043 Schneverdingen, FRG 2 Institut ffir Naturschutz- und Umweltschutzforschung des Verein Jordsand; Wulfsdorf,
D-2070 Ahrensburg, FRG
ABSTRACT: For no other group of organisms in coastal areas are there so exact and long-term data available as there are for seabirds. Since the beginning of the 20th century, documentation of population size, especially for species breeding in colonies from the groups gulls, terns and auks, is almost complete. These species act as bio-indicators, and data on fluctuations in their population size are useful as they reflect changes in the state of the marine ecosystem. The population development of some of these seabird species (Herring Gull, Guillemot, Common, Arctic and Sandwich Tern) from the German North Sea coast, which primarily feed on fish, is given, Common to all these species is an exponential increase in numbers in recent years (1970-1985). Possible causes for this develop- ment, e.g. pressure from enemies or competitors, availability of breeding places, anthropogenic stress and mortality factors, as well as the direct and indirect anthropogenic-influenced changes in the trophic system due to the increasing-eutrophication of coastal waters, are evaluated. Signs of a collapse in the stocks of seabirds resulting from environmenta~ pollution are discussed. Conse- quences resulting from the ecosystem changes, such as reduction of nutrient discharge into the North Sea and the expansion of biological monitoring, are described.
I N T R O D U C T I O N
O n e w e l l - t r i e d m e t h o d of r e c o r d i n g a n d c o m p r e h e n d i n g l o n g - t e r m d e v e l o p m e n t s in the m a r i n e e c o s y s t e m is the continuous m o n i t o r i n g of the p o p u l a t i o n d y n a m i c s of i n d i v i d u a l species or of entire b i o c o e n o s e s as bioindicators. Indicators n e e d to b e well- d o c u m e n t e d as to their position in the e n v i r o n m e n t a n d it m u s t b e possible to carry out l o n g - t e r m m o n i t o r i n g of their p o p u l a t i o n d y n a m i c s w i t h r e l a t i v e l y little t r o u b l e (Ellen- berg, 1982). In this respect, the bird p o p u l a t i o n s i n h a b i t i n g the shallow w a t e r s of the s o u t h e r n North S e a coast a n d the a d j a c e n t - s e a a r e a s h a v e traditionally p l a y e d a p r i m e role (Tasker & Pienkowski, 1987).
No other g r o u p of o r g a n i s m s in coastal a r e a s has b e e n as t h o r o u g h l y i n v e s t i g a t e d a n d as m u c h t u r n e d to a c c o u n t from this particular a n g l e as h a v e the s e a b i r d s (Schulz, 1947; H~ilterlein, 1986; Taux, 1986; B e c k e r & Erdelen, 1987; V a u k & Prfiter, 1987). L o o k e d at from this point of view, the fact that m a n y of the i m p o r t a n t s e a b i r d b r e e d i n g g r o u n d s h a v e b e e n u n d e r the scrutiny of n a t u r e p r o t e c t i o n associations since t h e turn of
" Presented at the VI International Wadden Sea Symposium (Biologische Anstalt Helgoland, Wattenmeerstation Sylt, D-2282 List, FRG, 1--4 November 1988)
358 Gottfried Vauk, J o h a n n e s Priiter & Eike H a r t w i g
the c e n t u r y is of i n e s t i m a b l e value. This h a s m a d e it p o s s i b l e to t r a c e t h e p o p u l a t i o n c h a n g e s p a r t i c u l a r l y of colony b r e e d i n g s e a b i r d s b a c k to the b e g i n n i n g of the p r e s e n t century.
The p o p u l a t i o n figures a v a i l a b l e for the 19th c e n t u r y are s p o r a d i c a n d c o m p a r a t i v e l y inexact; y e t t h e y suffice to s h o w t h a t the o v e r - e x p l o i t a t i o n a n d ruthless p e r s e c u t i o n of t h e p o p u l a t i o n s for c o m m e r c i a l p u r p o s e s h a d b y the t u r n of the c e n t u r y l e d to t h e d e c i m a t i o n of most s e a b i r d p o p u l a t i o n s on t h e G e r m a n coast (Thiessen, 1986; V a u k & Prfiter, 1987). Since t h a t time, as a r e s p o n s e to this d e v e l o p m e n t , a n i n c r e a s i n g n u m b e r of s e a b i r d c o n s e r v a t i o n a r e a s h a s b e e n set up at the m o s t i m p o r t a n t colony sites. As a c o n s e q u e n c e , t h e o v e r - e x p l o i t a t i o n of t h e p o p u l a t i o n s w a s c u r b e d a n d t h e n u m b e r s r e c o v e r e d in v a r y i n g d e g r e e s . O v e r a n d a b o v e this, the t w e n t i e s a n d thirties w i t n e s s e d t h e s e t t l e m e n t of n e w a n d t h e r e o c c u p a t i o n of f o r m e r colony sites b y s e v e r a l species, e.g. L e s s e r Black- b a c k e d Gull
(Larus fuscus),
B l a c k h e a d e d Gull(L. ridibundus)
a n d K i t t i w a k e(Rissa
tridactyla)
(Goethe, 1969; Priiter, 1983; Fleet, 1984).Yet one c a n n o t s a y that their d e v e l o p m e n t since that time has b e e n u n d i s t u r b e d . The i n c r e a s e in the H e r r i n g GuLl
(Larus argentatus),
for instance, v e r y s o o n c a m e to b e c o n s i d e r e d too rapid, so that control m e a s u r e s w e r e t a k e n w h i c h r e a c h e d a c h m a x in the sixties a n d e a r l y seventies. At that time, t h e s e d i r e c t m e t h o d s of s e a b i r d p o p u l a t i o n m a n a g e m e n t w e r e also e m p l o y e d at a n u m b e r of B l a c k - h e a d e d Gull c o l o n i e s on the G e r m a n N o r t h S e a coast (Goethe, 1964; V a u k & Prfiter, 1987),T h e i n t r o d u c t i o n of toxic s u b s t a n c e s (e.g. pesticides) into c o a s t a l w a t e r s of the s o u t h e r n N o r t h S e a w a s c e r t a i n l y a c o n t r i b u t i n g factor in the r e n e w e d p o p u l a t i o n d e c l i n e s of a n u m b e r of s e a b i r d s p e c i e s in the fifties a n d sixties e s p e c i a l l y in terns (Koeman, 1975; B e c k e r & Erdelen, 1987; S c h u m a n n , 1987). T h e s a m e is true of t h e e x p a n s i o n of tourism a n d o t h e r i n d u s t r i e s a l o n g the coast, w h i c h e n c r o a c h e d on p o t e n t i a l n e s t i n g - s i t e s a n d e v e n to s o m e e x t e n t on e s t a b h s h e d p r o t e c t e d a r e a s too.
I n f l u e n c e d b y the b e n e f i c i a l effects of s e a b i r d p r o t e c t i o n efforts on t h e o n e h a n d a n d the p e r n i c i o u s w o r k i n g s of m a n on the other, p o p u l a t i o n sizes of c o l o n y - b r e e d i n g s e a b i r d s p e c i e s on the G e r m a n N o r t h S e a coast in the m i d d l e of the c e n t u r y s h o w e d a n u m b e r of different reactions. In the gulls the o b s e r v e d t r e n d s w e r e of s t a g n a t i o n or s l i g h t i n c r e a s e (Becker & Erdelen, 1987). T h e s a m e w a s n o t e d in the g u i l l e m o t s ([fn'a
aalge)
on H e l g o l a n d ( V a u k - H e n t z e l t et al., 1986). In contrast, d r a m a t i c p o p u l a t i o n c o l l a p s e s occur- r e d a m o n g the terns (Thiessen, 1986).RESULTS
In the following, w e shall confine our a t t e n t i o n to a n u m b e r of c o l o n y - b r e e d i n g , p r i n c i p a l l y f i s h - e a t i n g s p e c i e s of s e a b i r d s from the g r o u p of families i n c l u d i n g t h e gulls, the terns a n d the auks. W e shall d e s c r i b e their p o p u l a t i o n d e v e l o p m e n t s u s i n g s e l e c t e d e x a m p l e s from a l a r g e n u m b e r of s t u d i e s (Vauk et ai., 1989; H a r t w i g et al., 1989) a n d p r e s e n t p o s s i b l e e x p l a n a t i o n s for the o b s e r v e d p h e n o m e n a .
Our e x a m p l e s w e r e c h o s e n from t h e s e t h r e e g r o u p s b e c a u s e
- t h e y are r e l a t i v e l y c o m m o n a n d constitute b e t w e e n t h e m t h e b u l k of t h e b r e e d i n g birds in the s o u t h e r n N o r t h Sea,
Population d y n a m i c s of b r e e d i n g b i r d s p e c i e s 359
W h a t t h e e x a m p l e s p e c i e s also h a v e in c o m m o n is a s u d d e n e x p o n e n t i a l l y i n c r e a s i n g p o p u l a t i o n b o o m in the late sixties a n d seventies.
In the H e r r i n g Gull
(Larus argentatus),
a n a c c e l e r a t i n g g r o w t h in t h e total p o p u l a t i o n o c c u r r e d in spite of m a s s i v e control o p e r a t i o n s i n v o l v i n g poisons, w h i c h r e a c h e d a p e a k on t h e G e r m a n N o r t h S e a coast in the sixties a n d s e v e n t i e s (Fig. 1). D e s p i t e s u c h drastic m e a s u r e s , t h e existing l a r g e colonies (e.g. Mellum, M e m m e r t , Trischen) c o n t i n u e d to e x p a n d a n d n e w r a p i d l y g r o w i n g s e t t l e m e n t s w e r e e s t a b l i s h e d (e.g. Juist, S p i e k e r o o g ) . W h e r e a s in 1969 the H e r r i n g Gull p o p u l a t i o n of the G e r m a n North S e a coast stood at a b o u t 17 000 b r e e d i n g pairs, b y 1984 this figure h a d r e a c h e d 45 000. In t h a t year, n e a r l y 90 % of the total p o p u l a t i o n b r e d in no m o r e t h a n s e v e n s u p e r - c o l o n i e s of m o r e t h a n 1000 pairs. In the s a m e period, the p o p u l a t i o n in the N e t h e r l a n d s g r e w from 17 000 to a b o u t 90 000 p a i r s ( V a u k & Pr~iter, 1987; Spaans, pets. comm.).T h e p o p u l a t i o n d y n a m i c s of the K i t t i w a k e
(Rissa tridactyla)
on H e l g o l a n d , its sole b r e e d i n g g r o u n d in the s o u t h e r n North Sea, can b e t r a c e d with a h i g h d e g r e e of a c c u r a c y : w h e r e a s the a v e r a g e g r o w t h in the sixties w a s 14 % p e r a n n u m , b e t w e e n 1970 a n d 1980 the f i g u r e i n c r e a s e d to 22 %. The b r e e d i n g p o p u l a t i o n e x p a n d e d d u r i n g this t i m e from 400 to 2350 pairs a n d b y 1986 it h a d r e a c h e d a total of 3310 pairs. J u s t recently, h o w e v e r , t h e r e h a v e b e e n signs that a n u p p e r limit is b e i n g a p p r o a c h e d (Fleet, 1984; Prfiter, 1986; Voss et aL, 1987).Similarly, t h e G u i l l e m o t s
(Uria aalge)
b r e e d i n g on H e l g o l a n d h a v e d i s p l a y e d a n a c c e l e r a t e d p o p u l a t i o n i n c r e a s e in the last few y e a r s (Fig. 2). B e t w e e n 1956 a n d 1969, the c o l o n y c o n s i s t e d of a r o u n d 1000 pairs. A drop to a b o u t 800 pairs w a s n o t e d in the m i d - s e v e n t i e s b u t b y 1985 the p o p u l a t i o n h a d t r i p l e d its size to n e a r l y 2500 b r e e d i n g pairs ( V a u k - H e n t z e l t et al., 1986).Finally, t h e p o p u l a t i o n d y n a m i c s of C o m m o n T e r n
(S~erna hirundo)
a n d Arctic Tern(St. paradisaea)
on the G e r m a n North S e a coast d i s p l a y a v e r y similar p a t t e r n (Fig. 3): a s t e e p fall-off in the fifties a n d sixties followed b y a r a p i d i n c r e a s e (Vauk & Priiter, 1987). This p h e n o m e n o n of e x p a n s i o n d u r i n g the last few y e a r s has also b e e n n o t e d in the G e r m a n N o r t h S e a p o p u l a t i o n of the S a n d w i c h T e r n(St. sandvicensis)
(Fig. 3). In the mid- e i g h t i e s , this s p e c i e s actually r e a c h e d its h i g h e s t n u m b e r of b r e e d i n g p a i r s so far this c e n t u r y ( S c h u m a n n , 1987). On the coast of the N e t h e r l a n d s too, after the low p o i n t in the sixties c a u s e d b y h u m a n interference, o b s e r v e r s r e p o r t a s t e a d y i n c r e a s e in p o p u l a t i o n o n c e m o r e (Bourne & Vauk, 1988).W h a t e m e r g e s as a c o m m o n factor in all t h e s e e x a m p l e s is b a s i c a l l y a v e r y s u r p r i s i n g p h e n o m e n o n : t h e b r e e d i n g p o p u l a t i o n s of gulls, terns a n d a u k s on the G e r m a n N o r t h S e a coast h a v e for the p a s t 15 y e a r s all b e e n following a r e m a r k a b l y similar p a t t e r n of s t e e p to e x p o n e n t i a l growth. W h a t is more, this is h a p p e n i n g i r r e s p e c t i v e of w h e t h e r a d e p l e t e d p o p u l a t i o n is on the w a y to r e c o v e r y or a p r e v i o u s l y a p p a r e n t l y s t a b l e p o p u l a t i o n s p o n t a n e o u s l y b e g i n s to e x p a n d .
D I S C U S S I O N
360 Gottfried Vauk, Johannes Prater & Eike Hartwig
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P o p u l a t i o n d y n a m i c s of b r e e d i n g bird species 361
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2000
1500-
1000-
700
60
65
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75
80
Jeh"
Fig. 2. Population changes of Guillemot
{Uria aalge)
on Helgoland from 1956 to 1985 (from Vauk- Hentzelt et aL, 1986; completed by reports from Verein Jordsand)coasts the living conditions for p r i m a r i l y fish-eating seabirds are at p r e s e n t a p p a r e n t l y excellent. E v e n the rapid i n c r e a s e i n n u m b e r s of G a n n e t s
(Sula bassana)
on p a s s a g e off the G e r m a n a n d D a n i s h North S e a coasts c a n b e i n t e r p r e t e d in this h g h t (Brunckhorst, 1985; Meltofte & Faldborg, i987).Looking b e y o n d the piscivorous seabirds, it m a y b e as well to m e n U o n at this point that the b r e e d i n g p o p u l a t i o n of the Eider
(Somateria mollissirna),
f e e d i n g exclusively on mussels, has also s h o w n a r a p i d i n c r e a s e on the G e r m a n North Sea coast since the seventies, a d e v e l o p m e n t b e a r i n g a s t r i k i n g r e s e m b l a n c e to the p a t t e r n s d e s c r i b e d a b o v e (Nehls, unpubl.).3 6 2 Gottfried V a u k , J o h a n n e s Prfiter & Eike Hartwig
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Population d y n a m i c s of b r e e d i n g b i r d s p e c i e s 363
after t h e r a p i d i n c r e a s e in s e a b i r d losses at the b e g i n n i n g of the e i g h t i e s d u e to the p e r s i s t e n t oil-pollution of the coastal s e a areas, for e x a m p l e , it w o u l d h a v e b e e n far m o r e p l a u s i b l e t O e x p e c t the exact o p p o s i t e d e v e l o p m e n t in the p o p u l a t i o n s of at l e a s t the most s e r i o u s l y a f f e c t e d s p e c i e s of s e a b i r d s (Vauk et al., 1987).
O t h e r factors m u s t b e at w o r k here, factors w h i c h w i t h i n a v e r y short t i m e r a i s e d the c a p a c i t y of t h e e n v i r o n m e n t for t h e s p e c i e s in question.
In the past, food availability h a s often e n o u g h b e e n i d e n t i f i e d a n d d e s c r i b e d as a v e r y effective - a n d quick r e a c t i n g - r e g u l a t o r of s e a b i r d p o p u l a t i o n s (Furness & Hislop, 1981; M o n a g h a n & Zonfrillo, 1986; Meltofte & F a l d b o r g , 1987; P e d e r s e n & Chri- s t o p h e r s e n , 1987). T o d a y it m u s t b e c o n s i d e r e d h i g h l y p r o b a b l e t h a t t h e e x p l a n a t i o n of this s i m u l t a n e o u s d e v e l o p m e n t in the W a d d e n S e a r e g i o n of the s o u t h - e a s t e r n N o r t h Sea a n d in the s e a a r e a s b e y o n d is to b e f o u n d - in p a r t at l e a s t - in a t r e m e n d o u s l y i m p r o v e d f o o d - s u p p l y .
L o n g - t e r m i n v e s t i g a t i o n s into the f e e d i n g h a b i t s of l a r g e gulls in t h e H e l g o l a n d r e g i o n h a v e r e v e a l e d that c e r t a i n s p e c i e s of small fish (e.g.
Ammodytes, Sprattus,
Agonus)
a r e p r e s e n t in t h e s e w a t e r s t o d a y in far g r e a t e r q u a n t i t i e s t h a n t h e y u s e d to b e e v e n in the l a t e sixties (Prfiter, 1988, 1989). N e t - c o n t e n t a n a l y s e s c a r r i e d out b y the G e r m a n s h r i m p fisheries confirm t h a t in p a r t i c u l a r parts of the s a n d flats the p o p u l a t i o n s of c e r t a i n s p e c i e s of fish i m p o r t a n t for s e a b i r d nutrition are on the i n c r e a s e (Tiews, 1983). As to the p o s s i b l e c a u s e s of t h e s e d e v e l o p m e n t s , it m u s t o n c e a g a i n b e c o n s i d e r e d v e r y p r o b a b l e t h a t h u m a n i n t e r f e r e n c e in the m a r i n e e n v i r o n m e n t c o n t r i b u t e - directly or i n d i r e c t l y - to t h e s e c h a n g e s in the t r o p h i c s y s t e m (van Impe, 1985). T h e i m p a c t of the fishing i n d u s t r y on the q u a n t i t a t i v e b a l a n c e b e t w e e n fish s p e c i e s in a p a r t i c u l a r r e g i o n c a n b e e n o r m o u s (Hempel, 1978). E v e n m o r e i m p o r t a n t in this respect, h o w e v e r , s e e m s to b e t h e p r o b l e m c o m p l e x a s s o c i a t e d with t h e a d v a n c i n g e u t r o p h y of c o a s t a l w a t e r s (Gerlach, I987).C h a n g e s w h i c h h a v e b e e n r e c o r d e d in the q u a n t i t y a n d n a t u r e of p h y t o p l a n k t o n in the s o u t h e r n N o r t h S e a must b e s e e n in a s s o c i a t i o n w i t h t h e i n c r e a s i n g a m o u n t s of n u t r i e n t s i n t r o d u c e d into t h e s e w a t e r s (Radach & Berg, 1986). It m u s t b e a s s u m e d that t h e s e c h a n g e s set up reactions w h o s e effects e x t e n d to the h i g h e s t levels of t h e trophic system.
If it is i n d e e d the c a s e . t h a t the o n - g o i n g c h a n g e s in s e a b i r d p o p u l a t i o n s on the G e r m a n North S e a coast are the result of a b a s i c a l l y u n n a t u r a l e x p a n s i o n of capacity, t h e n t h e r e are definite r e a s o n s for c o n c e r n w i t h r e s p e c t to future d e v e l o p m e n t s . A c o l l a p s e of the p o p u l a t i o n s r e s u l t i n g from o v e r l o a d i n g t h e m a r i n e e c o s y s t e m c a n n o t a p p a r e n t l y b e r u l e d out, e v e n for t h e n e a r future.
E x a m p l e s of such d e v e l o p m e n t s h a v e b e c o m e i n c r e a s i n g l y c o m m o n recently. T a k e for i n s t a n c e the d r a m a t i c d r o p in t h e p o p u l a t i o n s of c h f f - b r e e d i n g s e a b i r d s in North N o r w a y a n d on t h e S h e t l a n d s ( H e u b e c k , 1988; Prfiter & V a u k , 1988) - in this c a s e the p r i m e c a u s e is p r o b a b l y over-fishing of the f e e d i n g g r o u n d s - or t h e s u d d e n r a p i d d e c h n e in t h e p o p u l a t i o n s of l a r g e r gulls in the n o r t h of G r e a t Britain (Lloyd, 1987).
W h a t action is called for, then, on t h e b a s i s of t h e s e c o n s i d e r a t i o n s ?
First a n d foremost, of course, t h e u n n a t u r a l l y h i g h l e v e l of n u t r i e n t s b e i n g i n t r o d u c e d into the North S e a m u s t b e r e d u c e d a n d if p o s s i b l e cut out a l t o g e t h e r , w i t h o u t w a i t i n g for t h e definitive proof of w h e t h e r t h e s e p r e s e n t p r o g n o s t i c a t i o n s a r e i n d e e d correct!
364 G o t t f r i e d V a u k , J o h a n n e s P r f i t e r & E i k e H a r t w i g
p r e v e n t , a s f a r a s p o s s i b l e i n t h e f u t u r e , a r e p e t i t i o n of t h e p r e s e n t s i t u a t i o n w h e r e b y s u d d e n d e v e l o p m e n t s l i k e t h o s e i n t h e c a s e s d e s c r i b e d a b o v e t a k e e v e r y b o d y u n a w a r e s a n d a l m o s t t o t a l l y u n p r e p a r e d .
A f i n e n e t w o r k of r e s e a r c h p r o j e c t s is n e e d e d i n o r d e r to t r a c e l o n g - t e r m c h a n g e s a f f e c t i n g m a r i n e o r g a n i s m s . F u r t h e r , m u c h c l o s e r c o l l a b o r a t i o n is c a l l e d f o r b e t w e e n t h e d i f f e r e n t d i s c i p l i n e s of m a r i n e e c o l o g y . T h e t r a d i t i o n of v e r y n a r r o w s p e c i a l i z a t i o n a n d c o m p a r t m e n t a l i z a t i o n m u s t g i v e w a y t o a m o r e c r i t i c a l overall v i e w . I n t h e m a r i n e e c o s y s t e m , b i r d s a r e f r e q u e n t l y a t t h e e n d of f o o d - c h a i n s a n d a g l a n c e a t t h e i r p o p u l a t i o n f i g u r e s m a y v e r y w e l l p r o v i d e t h e m a r i n e e c o l o g i s t s w i t h v a l u a b l e l e a d s . E q u a l l y , w i t h o u t d e t a i l e d k n o w l e d g e of c h a n g e s i n t h e l o w e r l e v e l s of t h e t r o p h i c s y s t e m , it is h a r d l y p o s s i b l e to u n d e r s t a n d f l u c t u a t i o n s i n s e a b i r d p o p u l a t i o n s .
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