An important goal in ecology is to understand the mechanisms that permit fluctuations in population size yet prevent extinction at low densities. Nevertheless, understanding of how animal populations are regulated is poor, although numerous studies have shown that animals compete for food and breeding sites. Another potential mechanism is predation, and predation by specialist predators has been suggested as a mechanism regulating e.g. small rodent populations. One problem in studies of the temporal variation in population size is that regulation is sometimes confused with limitation, and that these processes may be at work simultaneously. Population size may thus be unexpectedly small because predation limits numbers to an even greater extent than the food supply, alone, would do.

One line of evidence indicating that food may limit population size comes from studies in northern areas, where there are large and regular fluctuations in the numbers of microtine rodents. Furthermore, population sizes of predators, specialists as well as generalists fluctuate in synchrony with small rodent abundance. The cyclic behaviour of small rodent populations mainly occurs with small variations in the period, the length of which appears to depend on species and latitude. Moreover, there is evidence painting to the fact that the fluctuations in the numbers of microtine rodents and their predators are not synchronous over large areas of the Arctic tundra. However, the cause of these fluctuations is not fully understood.

Recently, the role of predators in the population dynamics of their prey has attracted much attention. The predation hypothesis suggests that variations in population size are produced only by the interactions between prey and predator, i.e. predators regulate the prey population size. Some studies have supported the predation hypothesis. Thus, it has been shown that microtine rodent populations experience very large changes in predation intensity during a cycle, from 88% of total mortality at the lowest population level to 25% at the increase stage of the cycle. The high predation rate at low densities suggests that predators can further depress rodent numbers and, consequently, cause an increase in the interval between successive population peaks. Hence, predators may play an important role in the tundra ecosystem by limiting small rodent populations.

The aim of this study was to investigate the abundance and breeding performance of avian lemming predators in the Canadian Arctic, both specialist and generalist predators. Specialist predators in the study comprise: the long-tailed skua, the pomarine skua, the rough-legged buzzard, the northern harrier, the short-eared owl and the snowy owl. Apparently, many predators in the western Arctic depend more on small rodents for breeding than do their conspecifics in other areas of the distributional range. Therefore, the generalist predator category includes: the raven, the gyrfalcon, the peregrine, the merlin, the golden eagle, the Arctic skua and gulls, mainly the glaucous gull.

Field-study

Data were collected from 17 different sites in the Canadian Arctic. The sites were chosen so as to accomplish two transects, one east-west and the other north-south. The easternmost site was Cape Hooper, 68°26,21’N, 66°48,92’W, Baffin Island, and the most westerly situated one was Ivvavik National Park, 69°25,23’N, 139°36,33’W, North Yukon. The site furthest to the north was the Isachsen Peninsula, 78°55,73’N, 104°38,42’W, Ellef Ringnes Island. Cap de Nouvelle-France, 62°20,73’N, 73°40’W, on the Ungava Peninsula was the most southerly situated site.

The expedition route, starting at the Ungava Peninsula on 1 July and finishing at Cape Hooper, Baffin Island, on 1 September, permitted determination of the reproductive performance of several species, particularly rough-legged buzzards, peregrines, the snowy owl, skuas and gulls up until the penultimate site on Devon Island, which was studied on 24 and 25 August. There, rough-legged buzzards, peregrines and glaucous gulls were tending large chicks of near fledging age while two gyrfalcon chicks appeared to be independent of the parents. At the final site on Baffin Island, it was late autumn with the winter migration of birds well in progress. The reproductive performance of ravens, golden eagles and gyrfalcons was difficult to assess at sites visited in late August since their young leave the nesting area early in the summer. However, nest contents and wear could confirm breeding in some of the cases where offspring production could not be assessed.

Each study area was censused by two or three field-observers over a period of 1-2 days using binoculars to spot birds and nests. We recorded the number of birds; the number of pairs of mated birds, the number of nests, the number of resident non-breeding birds and the number of vagrant birds. The census routes were plotted on maps using topographic features and a GPS-system, which calculates co-ordinates, latitude and longitude, using up to 9 satellites. Most study areas were plotted on maps, scale 1:50 000, and the size of each area estimated using the grid-system on the maps. Maps of larger scale were used for site 6, Graham Gore Peninsula, King William Island, site 11, Cape Bathurst, Amundsen Gulf, site 15, between Muskox Fjord and Baad Fjord, Ellesmere Island, and site 17, Cape Hooper, Baffin Island.

Nests of rough-legged buzzards and snowy owls are conspicuous and, thus, easy to locate, even at a distance. Similarly, nesting sites of raven and the gyrfalcon are easily visible due to the white colouring of the nesting ledge. In contrast, nests of skuas are cryptic and difficult to find. However, both long-tailed skuas and Arctic skuas were aggressive near the nest, and thus their behaviour revealed if they were breeding. Nest positions were determined using GPS-coordinates. Nest contents, eggs and chicks, were counted in all cases, with the exception of a few nests that we could not look into. Pellets and prey remains were collected at perch posts and near nests.

The distribution and abundance of birds were also studied on the landscape level using a helicopter with two field-zoologists on board. Animals the size of waders and larger were counted by the two observers, one on the left side and the other on the right side of the helicopter. A distance of 250 m on each side of the helicopter was considered as the area for observation. The helicopter flew at a speed of 50 knots or more and about 50 m above the ground. The time spent on aerial survey and thus the area surveyed, varied between sites, for reasons related to weather and logistics. In many areas, the aerial surveys comprised a square with the campsite as mid-point. In other areas, the survey routes were more irregular due to the non-homogenous topography and the irregular distribution of vegetation. On the first leg, sites 1 to 9, aerial surveys were carried out by Nils Kjellén and Magnus Tannerfeldt, and on the second leg, sites 10 to 17, by Torgny Nordin and Christer G. Wiklund respectively. No aerial surveys were performed at sites 1, 10 and 11. In addition, high winds affected the aerial survey at site 15.

Results and discussion

Five different specialist predators on small rodents were recorded: the long-tailed skua, the snowy owl, the short-eared owl, the northern harrier and the rough-legged buzzard. An unexpected result was the complete lack of  pomarine skuas. Pomarine skuas depend on a high abundance of small rodents for breeding: they arrive at potential breeding areas, stay a couple of days and, if lemmings are numerous, they stay and breed, or, else, they disappear. The highest number of breeding lemming predators was recorded at site 4, where 10 pairs of the long-tailed skua were incubating full clutches. Note, however, that long-tailed skuas may commence breeding in years of low lemming abundance if there are plenty of invertebrates. It is therefore premature to conclude that lemming abundance was sufficiently high for the breeding of other specialist predators too. For instance, 6 snowy owls occupied hunting territories at site 4 but there was no evidence of breeding. Moreover, the absence of pomarine skuas indicates that rodent density was fairly low. Hence, the general picture obtained from the study sites was that there was a low density of specialist predators on lemmings (also see Tannerfeldt).

The distribution of specialist predators across study areas was strongly skewed with no representation of this type of predator in 6 out of 17 study areas. This distribution suggests dumping of species. Moreover, the distribution of specialist predators emphasized the picture of low abundance of specialist predators. The highest number of different specialists was 5, which was recorded in study area 10. In 5 other study areas, there were 2 different species of specialist predators, and in another 5 study areas only 1 species of specialist predator was recorded. Interestingly, in 7 study areas where rough-legged buzzards and/or snowy owls were recorded, both species occurred only in one study area, site 10. Combining the observations from aerial surveys and those conducted on foot shows that both species occurred in only 2 out of 10 study areas. This observation verifies the results of the previous expedition to Siberia, viz. that rough-legged buzzards avoid breeding near snowy owls.

The distribution of generalist predators differed markedly from that of the specialist predators. For instance, the assemblages of generalist predators ranged between 0 and 5 species, and the most common assemblage was 3 species. This data suggest that numbers of species of generalist predators were randomly distributed across study sites. Generalist predators, particularly gulls and peregrines, were more common than specialist predators. Gulls, mainly glaucous gulls, were recorded in all, except two, study areas. Probably due to the fact that the study areas were close enough to the shoreline for gulls to hunt both at sea and on shore. Another, perhaps surprising, result is that the peregrine appears to be fairly common in the western Arctic area, encountered in 10 of the 17 study areas. As waders are a common prey for peregrines, the choice of study sites may, again, have influenced the result. Also, ravens and Arctic skuas appeared to be distributed over a wide range of the study sites being recorded in 9 and 6, respectively, study areas.

The areas covered by the aerial surveys Varied a great deal between the 14 sites that were censused, from 10 to 42 km2. As in the case of the surveys conducted on foot, more generalist predators than specialist predators were recorded during the surveys. Moreover, the distributions of the two types of lemming predators were similar to those obtained during the survey on foot.  The most important difference between the two surveys was that fewer species were recorded in the aerial survey compared with the survey on foot. One likely reason is that much more time was devoted to surveys on foot, and, consequently, larger areas were then censused compared with the aerial surveys.

The observations made in the field-study will be used for further analyses of the distribution and abundance of specialist and generalist predators as well as for biogeographical studies of the distribution of birds in Arctic areas. Moreover, in collaboration with people studying small rodents, the relationships between small rodents and their predators will be examined.