Low abundance of king eider nests during low lemming years in Northeast Greenland.
ABSTRACT. Long-term surveys on Traill Island (Northeast Greenland) show that numbers of king eider (Somateria spectabilis) nests are correlated with high densities of collared lemming (Dicrostonyx groenlandicus). These observations match other zoological records from this region and agree with similar observations recorded elsewhere for arctic geese. Processes at work may be related to the fact that mammalian predators focus on lemmings when lemming numbers are high, but exert higher predation pressure on other prey at low lemming densities. Predators seem to increase their search effort during such low lemming years, thus increasing the likelihood that they will discover eider nests. This study highlights the importance of studying patterns at the community level in any attempt to interpret cyclic fluctuations in northern ecosystems.
Key words: arctic fox, Alopex lagopus, collared lemming, Dicrostonyx groenlandicus, Greenland, king eider, Somateria spectabilis, population cycles, predation
RESUME. Une etude a long terme menee sur l'ile de Traill (nord-est du Groenland) a mis en evidence que le nombre de nids d'eiders a tete grise (Somateria spectabilis) est correle a de fortes densites de lemming a collier (Dicrostonyx groenlandicus). Ces observations sont identiques a celles des autres releves fauniques realises dans cette region et confortent des constats similaires sur le succes de reproduction des oies arctiques. Le determinisme de ces phenomenes semble etre lie au fait que les mammiferes predateurs se concentrent sur les lemmings lorsque ces derniers sont abondants, mais exercent une pression de predation plus forte sur les autres especes lorsque les lemmings se font plus rares. L'effort de recherche des predateurs semble augmenter durant les annees de creux, ce qui accroit la probabilite de decouvrir les nids d'eiders. Cette etude souligne l'importance d'examiner les phenomenes dans l'ensemble de la communaute animale pour tenter d'expliquer les fluctuations cycliques dans les ecosystemes nordiques.
Mots cles: eider a tete grise, Somateria spectabilis, lemming a collier, Dicrostonyx groenlandicus, renard polaire, Alopex lagopus, predation, cycles des populations, Groenland
Cyclic fluctuations of rodents such as lemmings are a common feature of northern ecosystems (Stenseth and Ims, 1993). Associated with these phenomena are the cyclic dynamics of predators such as arctic foxes (Alopex lagopus) and snowy owls (Nyctea scandiaca) (Portenko, 1972; Summers et al., 1998). The mechanisms behind these fluctuations are still largely unknown (Stenseth and Ims, 1993). Recent findings have highlighted the role that specialist predators play in these phenomena, generating a delayed density dependence (Hanski et al., 1993; Norrdahl, 1995; Reid et al., 1995), while the role of food availability and intrinsic factors (i.e., parental care) remains yet to be resolved (Boonstra et al., 1998).
Knowledge of the population dynamics displayed by other species in the community can aid in understanding cyclic population dynamics. If coexisting species competing for the same food resource display fully departing dynamics, then food limitation could not account for cyclic patterns in one species and not in the other. On the other hand, similar population dynamics in species that have different diets but share the same predators could suggest that direct or indirect effects of predator-prey interactions might be involved.
Observations suggest that the breeding success of brent geese (Branta bernicla) is related to the lemming cycles (Summers, 1986; Greenwood, 1987; Underhill et al., 1993; Summers et al., 1998). However, these herbivores share some of the same food resources as microtine rodents. Observations on non-herbivorous species are scarce (Pehrsson, 1986; Martin and Baird, 1988; Spiekman and Groen, 1993; Summers et al., 1998). Our goal is to examine the population dynamics of a non-herbivorous species with respect to lemming population cycles.
Such a candidate species is the king eider, a typical tundra breeder whose diet is almost entirely marine animals (Uspenski, 1972). In summer, females nest on the tundra and hence share the same habitats and predators as lemmings (Uspenski, 1972). The rest of the year, this species uses marine habitat. By determining whether there is a correlation between lemming densities and the breeding of the king eider (a potential alternative prey for predators), one could see whether predator pressure may at times reduce breeding success in king eiders.
To compare the number of king eider nests with lemming abundance in Northeast Greenland, we used two different data sets: observations gathered as part of a long-term study devoted to lemmings on Traill Island and data compiled from the literature.
Investigations reported here were conducted within a study site of about 10 km2 in the Karupelv Valley (75 km2), Traill Island (72 [degrees] 30'N, 24 [degrees] 00'W), Northeast Greenland (Fig. 1). This area is part of the High Arctic breeding range of the collared lemming as well as the king eider and snowy owl. It consists of a wide-open valley on the southern part of Traill Island, facing Kong Oscars Fjord. Numerous raised beaches parallel the present shoreline. Inland elevations do not exceed 100 m a.s.l. Lakes are less numerous and relatively smaller (< 1 ha) than in other tundra areas. At the beginning of the breeding season, open water is limited to shore leads or ice-free areas that result from meltwater discharges. These are generally the places where small groups of king eider first gather in June. This region has polar semidesert vegetation that is discontinuous. The collared lemming is the only small rodent. Its population dynamics display typical cyclic patterns that are also illustrated by the densities of its main predators, arctic fox, stoat (Mustela erminea), long-tailed skua (Stercorarius longicaudus), and snowy owl (Pedersen, 1942; de Korte, 1988; Sittler, 1995).
This study, part of a long-term monitoring of a lemming population initiated in 1988, is based on a systematic survey of winter nests revealed at snowmelt at the end of June (Sittler, 1995). For this purpose, the area is screened by walking strip transects (25 m wide) during the king eider incubation period from the end of June until the beginning of August. The transects, which are walked once, vary in length; therefore, they cover all parts of the study area. Distances of at least 500 km per field season are covered, with every effort to ensure that the census effort is the same every year. While walking these transects, observers mark king eider nests on air photos and destroy lemming winter nests after counting them. These transects also allow us to census breeding birds and larger mammals. Breeding king eider females are more difficult to detect than lemming winter nests and most of the bird nests recorded on these transects because of their silent, secretive behaviour and their reluctance to leave the nest unless closely approached by the observer. Data may therefore be biased because we cannot be sure that all the breeding king eiders were recorded. As the sampling methods remained constant over the 12 years of the study, this bias should be the same in all years. Since snowy owls have larger territories than king eiders, snowy owl nests were censused over the whole Karupelv Valley (7500 ha).
In addition, to get a comparative and larger overview, we compiled all available zoological records from Northeast Greenland that provide information on lemmings and king eiders. In contrast to systematic surveys like the one reported above (same study area over years and same census effort), these data are from various sites and without consistency in sampling procedures. Direct surveys were usually not available to estimate the lemming densities. Authors generally describe the lemming situation either as "bad" or "low" when lemmings were not observed, or as "abundant" or "common" during an increase or peak phase. Therefore, we also included snowy owl breeding records, which are known to be reliable indicators of lemming abundance (Portenko, 1972).
) to compare lemming winter nests with king eider and snowy owl nests in our study area. For the other Northeast Greenland sources presented in Table 2, we used the Pearson's correlation coefficient to compare lemming abundance (0 for low, 0.5 for intermediate, or 1 for high) and king eider breeding (0 when no evidence of breeding and 1 if breeding).
TABLE 2. Breeding records of king eiders in Northeast Greenland obtai ned from various reports. Information is compared to population density o f lemmings and to breeding records of snowy owls. [Part 1 of 2] Year Area Lemming(1) King eider (2) 1900 Mackenzie Bay +/++ B 1900 NE Greenland 0 0 1906 Danmarkshavn ++ B 1907 Danmarkshavn 0 0 1908 Danmarkshavn 0 0 1924 Scoresby Sund 0 0 1925 Scoresby Sund 0 0 1927 Jameson Land + B (n = 3 ) 1928 Jameson Land ++ B (n = 40 ) 1929 Jameson Land 0 0 1930 NE Greenland 73-75 [degrees] 30N 0 0 1933 Hochstetter Forland +/++ B 1933 Jameson Land 0 0 1936 Mackenzie Bay ++ B 1937 Mackenzie Bay ++ B 1938 Mackenzie Bay 0 0 1938 Dovebugt 0 0 1939 Germania Land 0 0 1961 Orsted Dal 0 0 1962 Jameson Land + B (n = 1) 1963 Orsted Dal ++ B 1964 Wollaston Forland 0 B (n = 1, on islet) 1966 NE Greenland 71-75 [degrees] N ? B (n = 1 ) 1969 Scoresby Sund 0 0 1969 NE Greenland 76-78 [degrees] N 0 B 1970 NE Greenland 76-78 [degrees] N + B 1970 Scoresby Land + B 1972 Mestersvig 0 0 1973 Hold with Hope 0 0 1974 Scoresby Sund 0 0 1974 Orsted Dal 0 0 1974 Mestersvig & Traill Island + B (n = 1 ) 1975 Germania Land +/++ B (n = 8 ) 1976 Hochstetter Forland ++ B (n = 18 ) 1979 Mackenzie Bay ++ B (n = 10 ) 1980 NE Greenland ++ 0 1982 Vega Sund 0 0 1984 Vega Sund 0 0 1984 Orsted Dal + B (n = 1 ) 1985 Traill Island 0 0 1987 Germania Land 0 0 1988 Jameson Land 0 0 1988 Ile de France 0 0 1989 Ile de France 0 0 1989 Germania Land + B 1990 Mackenzie Bay ++ B 1990 Kong Oscars Fjord ++ B (n = 4 ) 1992 Kong Oscars Fjord 0 0 1995 Kong Oscars Fjord 0 0 1995 Zackenberg 0 0 1996 Zackenberg +? B (n = 1 ) 1997 Zackenberg + B (n = 1 ) 1998 Zackenberg ++ 0 1998 Kong Oscars Fjord ++ B (n = 2 ) 1999 NE Greenland 72-74 [degrees] 30N + B (n = 1 ) TABLE 2. Breeding records of king eiders in Northeast Greenland obtai ned from various reports. Information is compared to population density o f lemmings and to breeding records of snowy owls. [Part 2 of 2] Year Snowy owl(2) Source(3) 1900 B Kohltoff (1903) 1900 0 Deichmann (1909) 1906 B (?) Manniche (1910) 1907 0 Manniche (1910) 1908 0 Manniche (1910) 1924 0 Pedersen (1926) 1925 0 Pedersen (1926) 1927 B Pedersen (1930) 1928 B Pedersen (1930) 1929 0 Pedersen (1930) 1930 0 Loppenthin (1932) 1933 B Pedersen (1934) 1933 0 Bertram et al. (1934) 1936 B Bird and Bird (1940, 1941) 1937 B Bird and Bird (1940, 1941) 1938 0 Bird and Bird (1940) 1938 0 Pedersen (1942) 1939 0 Pedersen (1942) 1961 0 Marris and Ogilvie (1962) 1962 0(4) Hall (1966) 1963 B Hall and Waddingham, (1966) 1964 0 Rosenberg et al. (1970) 1966 0 Marris and Webbe (1969) 1969 0 Vranes (1971): Smart (1969) 1969 0 Meltofte (1975) 1970 B Meltofte (1975) 1970 0 Smart and O'Brien (1971) 1972 0 O'Brien and Greenwood (1972) 1973 0 Hjort (1976) 1974 0 Meltofte (1976b) 1974 0 Ferns and Mudge (1976) 1974 0 Green and Greenwood (1978) 1975 B Meltofte (1977) 1976 0(4) Meltofte et al. (1981) 1979 B Elander and Blomquist (1986) 1980 B Sellar et al. (1981) 1982 0 B. Sittler (1997, unpubl. data) 1984 0 B. Sittler (1997, unpubl. data) 1984 0 Cabot (1984) 1985 0 Newton (pers.comm. 1998) 1987 0 Cabot et al. (1988) 1988 0 de Korte (1988) 1988 0 Andersen and Berg (1991) 1989 0 Andersen and Berg (1991) 1989 n.d. Boertmann et al. (1991) 1990 B Dornbach (pers. comm. 1990) 1990 B O. Gilg and B. Sabard (1999, unpubl, data) 1992 0 O. Gilg and B. Sabard (1999, unpubl, data) 1995 0 O. Gilg and B. Sabard (1999, unpubl, data) 1995 0 Berg (1996a, b) 1996 0 Berg (1997); Meltofte and Berg (1997) 1997 B Berg (1998); Meltofte and Berg (1998) 1998 0(4) T. Berg (1999, unpubl, data) 1998 B O. Gilg and B. Sabard (1999, unpubl, data) 1999 B O. Gilg and B. Sabard (1999, unpubl, data) (1) 0 = low year; + = intermediate or increase; ++ = peak or very abu ndant. (2) 0 = no breeding record; B = breeding record (either nest or chick s). (3) sources are completed from de Korte and Wattel (1988) and Boertmann (1994).
Because of the small sample size (n = 12 years), we used the Spearman rank correlation coefficient (rs
(4) no breeding recorded but several birds seen.
= 0.87; p < 0.01). A similar pattern was recorded in the study area in 1979, when two king eider nests were found during an exploratory survey in a high lemming year (GREA, 1982). Not a single nest was found in all other years.
TABLE 1. Numbers of king eider and snowy owl nests compared to numbers of lemming winter nests in the lower Karupelv Valley, Traill Island, Northeast Greenland. Year Lemming winter nests(1) King eider nests(1) Snowy owl nests(1, 2) 1979 n.d. (increase/peak) 2 1 (2) 1985 n.d. (low)(3) 0 0 1988 149 0 0 1989 1270 1 1 (6) 1990 3687 2 (+ 1 family outside) 2 (12) 1991 282 0 0 1992 105 0 0 1993 145 0 0 1994 2938 3 (+1 nest on islet) 1 (5) 1995 1007(4) 0 0 1996 195 0 0 1997 311 0 0 1998 3521 3 (+1 family on islet) 1 (13) 1999 1596 1 1 (3) (1) in the restricted study area (1000 ha). (2) Figures in parentheses are for the Karupelv Valley (7500 ha). (3) data from Newton (pers. comm. 1998).
The first pattern that emerged from our 12-year monitoring in the Karupelv Valley (Table 1) was that king eiders in this part of their range may breed only in low numbers, since the maximum number of nests discovered per season was three. King eider nests were discovered only during high lemming years (indicated by lemming winter nests), i.e., when lemmings were either in an intermediate phase (1989 and 1999) or in a peak year (1990, 1994, and 1998), and are significantly correlated with lemming winter nests (rs = 0.88; p < 0.01). This correlation is as strong as that between lemming winter nests and snowy owl nests (rs
(4) rapid lemming decline in spring before king eider breeding.
The other available sources from Northeast Greenland, presented in Table 2, give a similar result. King eider nests are not found when lemmings are at a low, but only when lemming numbers are high. Pearson's correlation coefficient is highly significant (r = 0.75; p < 0.001) when comparing lemming abundance (0 for low, 0.5 for intermediate, or 1 for high) and king eider breeding (0 when no evidence of breeding and I if breeding). Only 5 out of 55 references did not match this general pattern. In one instance in 1964, breeding occurred on an islet (Rosenberg et al., 1970; but see Discussion), in another case in 1966, data on lemming densities were not clearly documented (Marris and Webbe, 1969), and in two instances, in 1980 (Sellar et al., 1981) and 1998 (author's observation), no breeding was observed despite high lemming densities. Clear evidence for king eider breeding during a lemming low was provided only by Meltofte (1975), on Germania Land, close to Danmarkshavn, in 1969.
The failure to detect any king eider nests in our study area in the eight years when lemmings were either at a low or in a pronounced decline, together with positive recordings in all high lemming years (Table 1), suggests that breeding success of king eider in this region is related to processes that also act on lemmings. This assumption is supported by the comparative data presented in Table 2. Available data for the rest of the Arctic suggest similar trends, even if breeding is annual. On southern Victoria Island (Northwest Territories, Canada) for example, Parmelee et al. (1967) found by far more king eider nests in 1960 (25 nests), when lemmings were abundant, than in 1962 (only four nests), during a lemming low. On the same site, O. Gilg, G. Ritter, E. Buchel and R. Sane (unpubl. data) found 16 king eider nests in 1996 during a lemming peak (plus several dozen pomarine skua and snowy owl nests), but only 1-3 nests during lemming lows (two in 1997, one in 1998, and three in 1999, plus one failed snowy owl nest in 1997, but no pomarine skua nests). Similar trends were also reported from Siberia (Stishov et al., 1991; Kondratyev and Sokolov, 1993).
It is unclear whether the failure to detect any nests in Northeast Greenland in low lemming years is due to non-breeding or to nest predation before detection by the observer (since pairs are generally present on open waters in the sea from June until they migrate south). Reasons for non-breeding or low breeding success among Arctic birds include poor weather and delayed snowmelt (Bertram et al., 1934; Sdobnikov, 1971), but there are others (Pehrsson, 1986; Syroechkovskiy et al., 1991; Dickson et al., 1997). In 1994, a summer with an exceptionally late snowmelt in our study area, king eider egg laying was postponed by more than two weeks, but not prevented (three nests were recorded). Hence, predation-related processes may be regarded as the major factors affecting the breeding success of king eider in our study area and in Northeast Greenland. Studies documenting nest predation rates, are rare but Lamothe's (1973) study showed that they can be very high. Of the 25 king eider nests he found on Bathurst Island (Canada), 18 were preyed upon by arctic foxes.
Breeding association (commensalism) with long-tailed jaegers (Blomquist and Elander, 1988), gulls (D. Solovieva, pers. comm. 1997), snowy owls (Litvin et al., 1985; Dorogoi, 1990; Summers et al., 1994; Cotter et al., 1997), and chained dogs (Kaliakin, 1992) also suggests that successful nesting of king eiders is indirectly linked to predation-related processes. Similar nesting associations are known for the three other eider species (Ahlen and Andersson, 1970; Uspenski, 1972; Kistchinski and Flint, 1974; Meltofte, 1978). Such associations, thought to offer protection against mammalian predators, were obvious in our study area (i.e., two king eider nests were found less than 100 m from a jaeger nest). But they are not a prerequisite for king eiders to breed in the tundra, as several lone nests were also discovered.
The results from our study area at Karupelv show that eiders actually bred during a lemming increase (1989) or high situation (1990, 1994, 1998) while mammalian predator densities were still low (see delay in response of stoats in Sittler, 1995). According to Ovsyanikov (1993), during such lemming peaks, foxes take lemmings to the den at least four or five times a day (with records of up to three times per hour), and the hunting range represents only 40% of the whole territory. To get the same result in low years, foxes probably increase time spent in the field and distances covered, thereby also increasing their probability of discovering secretive breeding species. In these low years following the decline of the lemming population, high numbers of predators (foxes and stoats) have to share poor food resources, forcing each predator to increase searching activity and range (Ovsyanikov, 1993), which may increase their probability of detecting breeding eiders. Sparse fox data from our study area support this explanation. In June 1998, during a lemming peak, we counted a mean of only 0.12 fox track/km (snow tracks) in our study area, while in June 1993, during a lemming low, more than 1 track/km was recorded from the same area.
Viewed in a broader context, the unexplained gaps in the distribution range of some arctic shorebirds and wildfowl (i.e., in west and south Greenland) have sometimes been attributed to the lack of rodents and to the increased fox predation pressure faced by these species (Larson, 1960).
If king eider nest abundance is related to fox predation pressure, small islets, supposedly less exposed to fox predation, should host king eider nests even during low lemming years. Reports of breeding records on islets during such low years support this assumption (Salomonsen, 1950; Johnsen, 1953; and Rosenberg et al., 1970). Similarly, in North Greenland, king eiders regularly breed on islets and vegetationless stony sea-slopes and reproduce successfully even in low lemming years (Johnsen, 1953; Just, 1967; Andersen, 1970; Meltofte, 1976a).
Similar effects of fox predation on tundra breeding birds have been reported from several places, mainly in Siberia. Ovsyanikov (1993) reported that on Wrangel Island, in low lemming years, up to 100% of the snow geese (Chen caerulescens) nests may be preyed upon, resulting in losses of 100000 eggs in these seasons. Syroechkovski and Krechmar (1981) likewise noticed that during high lemming years, nearly all snow geese were breeding, in contrast to low lemming years, when up to 86% of the pairs were reported to refrain from breeding. During a lemming low in 1984 on Vaygach Island (northwestern Siberia), Kaliakin (1992) recorded only one instance of successful breeding among 14 pairs of barnacle geese (Branta leucopsis). In 1988, during a lemming high, the same area supported 114 breeding pairs, and only six nests were taken by foxes. Sdobnikov (1959) earlier noticed intensive reproduction among tundra species, such as hares, geese, ptarmigans and other birds, only in years when lemmings were abundant. Uspenski (1972) and Summers et al. (1998) also noticed that on Novaya Zemlya and the Taimyr Peninsula, breeding abundance of birds was correlated with lemming densities and number of predators. Lastly, in Canada (Toker Point, Tuktoyaktuk Peninsula), Dickson (1992) found an obvious relationship between vole abundance and hatched eggs of red-throated loon (Gavia stellata) during a five-year study. The number of eggs that hatched was also negatively correlated with fox abundance.
Our study provides an example of the long-term effort needed to detect population patterns. It also highlights the importance of approaching patterns at the community level when attempting to unravel underlying processes such as cyclic fluctuations in northern ecosystems.
As in any project of this magnitude, a very large number of associates helped in ways far too numerous to detail. The authors owe a particular debt of gratitude to the Danish Polar Center and Greenland Home Rule for permission to carry out this research, while logistics benefited from support granted by the Sledge Patrols Sirius (Commander Norrit). The University of Freiburg (Germany), the Groupe de Recherches en Ecologie Arctique, the Centre d'Etudes et de Documentation sur les Milieux Polaires, the Badische Landesverein fur Naturkunde und Naturschutz, the Ministere Francais de la Recherche et de la Technologie, and the Deutsche Forschungsgemeinschaft contributed in various stages to the achievement of this programme. Further heartfelt thanks go to Mads C. Forchhammer, Knud Falk, Christopher Carcaillet, and three anonymous reviewers for their creative guidance in the preparation of the manuscript.
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|Author:||Sittler, Benoit; Gilg, Olivier; Berg, Thomas B.|
|Date:||Mar 1, 2000|
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