Long-term population dynamics of breeding bird species in the German Wadden Sea area

Helgoland Marine Research, May 2019

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 development, 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 seabrids resulting from environmental pollution are discussed. Consequences resulting from the ecosystem changes, such as reduction of nutrient discharge into the North Sea and the expansion of biological monitoring, are described.

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Long-term population dynamics of breeding bird species in the German Wadden Sea area

HELGOLANDER MEERESUNTERSUCHUNGEN Helgol~inder Meeresunters. L o n g - t e r m 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 b i r d s p e c i e s in the G e r m a n W a d d e n S e a area* 0 i Norddeutsche Naturschutzakademie; Hof M6hr, D-3043 Schneverdingen, FRG 2Institut ffir Naturschutzund Umweltschutzforschung des Verein Jordsand; Wulfsdorf , D-2070 Ahrensburg, FRG 1 9 Biologische Anstalt Helgoland , Hamburg 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 development, 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. Consequences 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 the marine ecosystem is the continuous monitoring of the population d y n a m i c s of individual species or of entire biocoenoses as bioindicators. Indicators n e e d to b e welld 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 and it must be possible to carry out l o n g - t e r m monitoring of their population dynamics with relatively little trouble (Ellenberg, 1982) . In this respect, the bird populations inhabiting the shallow w a t e r s of the southern North Sea coast and the adjacent-sea areas h a v e traditionally p l a y e d a prime role (Tasker & Pienkowski, 1987) . No other group of organisms in coastal areas 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 and as m u c h t u r n e d to account from this particular angle as h a v e the seabirds (Schulz, Looked at from this point of view, the fact that m a n y of the important seabird b r e e d i n g grounds h a v e b e e n u n d e r the scrutiny of nature protection associations since the turn of Gottfried Vauk, J o h a n n e s Priiter & Eike H a r t w i g the century is of i n e s t i m a b l e value. This has m a d e it possible to trace t h e p o p u l a t i o n c h a n g e s particularly 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 sporadic a n d c o m p a r a t i v e l y inexact; yet t h e y suffice to show that 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 the 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 turn of the century 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 the G e r m a n coast (Thiessen, 1986; V a u k & Prfiter, 1987) . Since that time, as a r e s p o n s e to this d e v e l o p m e n t , an i n c r e a s i n g n u m b e r of s e a b i r d conservation a r e a s has b e e n set up at the most 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 was c u r b e d a n d the 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 the r e o c c u p a t i o n of former colony sites b y s e v e r a l species, e.g. Lesser Blackb 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 cannot say 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, very 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 direct 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 North Sea coast (Goethe, 1964; V a u k & Prfiter, 1987) , The introduction of toxic s u b s t a n c e s (e.g. pesticides) into coastal w a t e r s of the southern N o r t h Sea was certainly 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 declines 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) . The s a m e is true of the e x p a n s i o n of tourism a n d other industries a l o n g the coast, w h i c h e n c r o a c h e d on potential nesting-sites a n d e v e n to some 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. Influenced b y the b e n e f i c i a l effects of s e a b i r d protection efforts on the o n e h a n d a n d the pernicious 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 species on the G e r m a n North Sea coast in the m i d d l e of the century 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 trends 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) . The s a m e was n o t e d in the guillemots ([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 occurr e d a m o n g the terns (Thiessen, 1986). RESULTS In the following, w e shall confine our attention to a n u m b e r of c o l o n y - b r e e d i n g , principally fish-eating 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 the gulls, the terns a n d the auks. We 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 studies (Vauk et ai., 1989; H a r t w i g et al., 1989) a n d p r e s e n t possible 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 groups b e c a u s e - they 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 the b u l k of the b r e e d i n g birds in the southern North Sea, - their total p o p u l a t i o n s are the most t h o r o u g h l y d o c u m e n t e d , and - they are i n t e g r a t e d in the m a r i n e e c o s y s t e m at the e n d of food-chains. W h a t the e x a m p l e species 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), an a c c e l e r a t i n g g r o w t h in the total p o p u l a t i o n o c c u r r e d in spite of massive control operations involving poisons, w h i c h r e a c h e d a p e a k on the G e r m a n North Sea coast in the sixties a n d s e v e n t i e s (Fig. 1). Despite such drastic m e a s u r e s , the existing large 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 settlements w e r e e s t a b l i s h e d (e.g. Juist, Spiekeroog). W h e r e a s in 1969 the Herring Gull p o p u l a t i o n of the G e r m a n North Sea 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 (Vauk & 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 Kittiwake (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 southern 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 growth in the sixties was 14 % p e r annum, b e t w e e n 1970 a n d 1980 the figure 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 time from 400 to 2350 pairs a n d by 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 an 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 Guillemots (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 an 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 colony 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 was n o t e d in the mids 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 tripled 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, the 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 Tern (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 by 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 Tern (St. sandvicensis) (Fig. 3). In the mideighties, 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 pairs 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, observers report 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 once 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 basically a v e r y surprising p h e n o m e n o n : the 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 North Sea 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 irrespective 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 stable 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 expand. D I S C U S S I O N T h e r e are o t h e r e x a m p l e s - for i n s t a n c e the g r o w t h of the C o r m o r a n t (Phalacrocorax carbo) p o p u l a t i o n in the North G e r m a n coastal region (Knief & Witt, 1984) or the s e t t l e m e n t of t h e F u l m a r (Fulmarus glacialis) a n d Razorbill (A/ca torda) of the H e l g o l a n d s e a b i r d colonies in the seventies (Moritz, 1980; Vauk, 1985) - w h i c h s u g g e s t that on our Gottfried Vauk, Johannes Prater & Eike Hartwig Q O O9 u U tO ~ ca r ,.jcq o9 25002000 15001000700 coasts the living conditions for primarily fish-eating seabirds are at p r e s e n t a p p a r e n t l y excellent. Even the rapid increase in 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 Danish North Sea coasts can be interpreted in this hght (Brunckhorst, 1985; Meltofte & Faldborg, i987) . Looking b e y o n d the piscivorous seabirds, it m a y be as well to menUon at this point that the b r e e d i n g population of the Eider (Somateria mollissirna), feeding exclusively on mussels, has also shown a rapid increase 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 striking r e s e m b l a n c e to the patterns described above (Nehls, unpubl.). E v e n today w h e n it comes to questions as to the causes of these r e m a r k a b l e population dynamics, explanations g o i n g b e y o n d mere suppositions are not forthcoming. In the period of time in question, n e i t h e r the pressure of n a t u r a l enemies, competition, nor the availability of n e s U n g sites or the disturbances a n d losses due to h u m a n interference have altered in a n y w a y which would explain this process 9 O n the contrary, i 0 I"I (Z3 rn 1.0 ,,F' ,o" 0 ~D r ~,-~176 "..~, %-. v after the 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 eighties d u e to the p e r s i s t e n t oil-pollution of the coastal sea areas, for e x a m p l e , it w o u l d h a v e b e e n far more p l a u s i b l e tO e x p e c t the exact opposite d e v e l o p m e n t in the populations of at least the most seriously affected species of s e a b i r d s (Vauk et al., 1987) . Other factors must b e at w o r k here, factors w h i c h within a very short time r a i s e d the c a p a c i t y of the 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 has often e n o u g h b e e n identified a n d d e s c r i b e d as a v e r y effective - a n d quick reacting - r e g u l a t o r of s e a b i r d populations (Furness & Hislop, 1981; M o n a g h a n & Zonfrillo, 1986; Meltofte & Faldborg, 1987; P e d e r s e n & Christophersen, 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 that the e x p l a n a t i o n of this simultaneous 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 North Sea a n d in the sea a r e a s b e y o n d is to b e found - in part at least - in a t r e m e n d o u s l y i m p r o v e d food-supply. L o n g - t e r m investigations into the f e e d i n g h a b i t s of l a r g e gulls in the 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 certain species of small fish (e.g. Ammodytes, Sprattus, Agonus) are p r e s e n t in these w a t e r s t o d a y in far g r e a t e r quantities t h a n t h e y u s e d to b e e v e n in the late sixties (Prfiter, 1988, 1989). N e t - c o n t e n t a n a l y s e s carried out b y the G e r m a n shrimp fisheries confirm that 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 certain species 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 causes of these d e v e l o p m e n t s , it must once 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 that h u m a n interference in the m a r i n e e n v i r o n m e n t contribute - directly or indirectly - to these c h a n g e s in the trophic s y s t e m (van Impe, 1985) . T h e i m p a c t of the fishing industry on the quantitative b a l a n c e b e t w e e n fish species in a p a r t i c u l a r region can b e enormous (Hempel, 1978) . Even more 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 the 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 the a d v a n c i n g e u t r o p h y of coastal w a t e r s (Gerlach, I987). C h a n g e s w h i c h have 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 southern North Sea must b e s e e n in association with the i n c r e a s i n g a m o u n t s of nutrients i n t r o d u c e d into these w a t e r s (Radach & Berg, 1986) . It must 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 the 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 Sea 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 there are definite reasons for c o n c e r n with r e s p e c t to future d e v e l o p m e n t s . A collapse of the populations resulting from o v e r l o a d i n g the 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, even for the 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 instance the d r a m a t i c drop in the 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 and on the Shetlands (Heubeck, 1988; Prfiter & Vauk, 1988) - in this case the p r i m e cause 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 the s u d d e n r a p i d d e c h n e in the populations of l a r g e r gulls in the north of G r e a t Britain (Lloyd, 1987) . W h a t action is called for, then, on the basis of these considerations? First and foremost, of course, the u n n a t u r a l l y h i g h level of nutrients b e i n g i n t r o d u c e d into the North S e a must b e r e d u c e d a n d if p o s s i b l e cut out altogether, without 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! A n o t h e r u r g e n t r e q u i r e m e n t is the intensification of biological monitoring, so as to p r e v e n t , as f a r as 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 to 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 . LITERATURE CITED Fig. 2. Population changes of Guillemot {Uria aalge) on Helgoland from 1956 to 1985 (from VaukHentzelt et aL, 1986 ; completed by reports from Verein Jordsand) Becker , P. H. & Erdelen , M. , 1987 . Bestandsentwicklung von Brutv6geln der d e u t s c h e n Nordseekfiste 1950- 1979 . - J. Orn. 128 , 1 - 32 . Bourne , W. R. P. & Vauk , G. , 1988 . H u m a n impact upon North Sea birds . 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Gottfried Vauk, Johannes Prüter, Eike Hartwig. Long-term population dynamics of breeding bird species in the German Wadden Sea area, Helgoland Marine Research, 357, DOI: 10.1007/BF02365896