St. Kilda

When it was inhabited, the people on Hirta lived manly on birds (Gannets, Puffins and Fulmars). Steel (1994) reported that when Martin Martin visited the island in 1697, he estimated that in a year 180 islanders consumed 16,000 eggs and ate 22,600 sea birds. From Stac Lee alone, he reckoned, the St. Kildans took between 5 and 7 thousand gannets annually. However the number killed per inhabitant remained fairly steady throughout the island’s history. By 1841, as a result the decline in the number of people on Hirta, the catch had dropped to an average of 1,400 birds a year (Steel, 1994).

The Soay sheep

The Soay sheep (Ovis aries) is a domesticated sheep classified as a rare breed. It is thought to be similar to the domestic Neolithic sheep and they have probably been introduced to St.Kilda at that time (Clutton-Brock, 1999). The sheep were left on Soay island and reintroduced to Hirta at the time of the evacuation. The population is naturally regulated since then (Clutton-Brock et al. 1991).

Soay sheep are relatively small, males (right) and females (left) typically have a distinctive black coat, but some polymorphism in coat colour is present. Polymorphisms on coat and horn types are probably maintained by changes in the direction in which selection operates during population crashes (Pemberton et al. 1996). Mortality occurs in slightly different periods in males and females and is mainly caused by starvation. Males are more vulnerable in the early months of the winter, during or soon after the rut. Females are more likely to die in late winter; just before or after giving birth. Births are generally concentrated in April and weaning is prolonged until the end of the summer. For both sexes the probability of surviving until the next summer is negatively correlated with population size and winter severity (Catchpole et al. 2000).
 

Population dynamics and Mark-Recapture-Recovery data

Since 1955 the total number of Soay Sheep on the whole Island of Hirta has been counted each summer. As found for some other food-limited populations of vertebrates in northern latitudes, the population size of Soay Sheep shows periods of rapid increase to high density, followed by periodic "crashes" when up to 60% of the population can die in one winter. An insight into population dynamics has be obtained by the study of survival and fertility processes using observations on previously marked individuals. A first detailed study on marked individuals was conducted between 1959 and 1967 (Jewell et al., 1974) in the Village Bay area, a part of the Hirta Island (see photograph above). The date of birth and date of death of individually marked sheep were recorded. Although this early study provided a significant insight into the ecology of the Soay sheep, it was ended in 1967 and the causes and the frequency of population crashes remained largely unknown. In the subsequent eighteen years the information collected differed in quantity and quality. For example for 5 years between 1970 and 1985 (shaded bars in the above figure) the census was limited to the total island count without the usual age breakdown recorded during the previous decades. The information collected on marked animals was also less precise. The data on the time of death for example was generally confined to the year only with no specification of the month.

A second detailed study was conducted between 1986 and the present on the Village Bay area where a large number of animals are marked-released and recaptured during the summer (Catchpole et al., 2000). Animals are caught within temporary corals into the Village bay (photos below). Before release, each individual is measured and when absent, a plastic tag with a unique combination of letters and numbers is placed in each ear.
The analysis of the individual-based information collected provided estimates of the demographic parameters underlying population fluctuations and of factors that influence them (Catchpole et al., 2000). A recent study by Coulson et al. (2001) showed that the population crashes observed between 1986 and 2000 result from a complex combination of the effects of extrinsic and intrinsic factors on individual survival and the age composition of the population.
 

Tweaking the time-series
Between 1970 and 1985 population fluctuations in the whole island of Hirta are surprisingly different from other times(shaded bars in the above graph). For example between 1971 and 1978, the population apparently showed a steady decline, which is difficult to reconcile with our current understanding of the way the population fluctuates. There has long been some uncertainty regarding the estimates of the whole island counts in some years of the decade 70-80, suggesting that during this period crashes might have occurred undetected. Population counts have been used in the past in time-series analyses (Grenfell et al. 1998; Stenseth et al. in prep.) and an understanding of the possible errors in the counts is necessary and important.

An insight in the time-series pattern might be obtained by comparing the observed counts with those predicted by a population model. Coulson at al. (2001) showed that the population dynamics of the sheep on Hirta was well predicted by a 5-age class matrix model in which survival and fertility were dependent on two external covariates, weather and previous summer population size, and their interaction. In particular we used the relationship between North Atlantic Oscillation Index and population density on survival and fertility parameters to project the population from 1961 onwards. This model have been parameterised using the estimates of survival and fertility obtained by modelling individual based data collected from 1986 to 2000 (see some of the details of the analysis on Female survival: data 86-00 (.pdf file) and Soay sheep fertility (.pdf file)). The historical data concerning birth and death dates of animals marked between 1961 and 1970 ( Capture-Mark-Recapture-Recovery data from cohorts 61/70) were recovered from electronic and different sources. Recoveries from 1961 to 1984 were analysed to investigate whether the structure of the survival model found for the period 1986-2000 was applicableto the previous decades. This analysis confirmed the age-dependent pattern found for the periods 1961-1984 and 1986-2000 were similar ( Female survival: data 61-84 (pdf file)). The final matrix model included demographic stochasticity and proved to be very successful in describing the population trajectory from 1986 to 2000. This model however sistematically underestimated the population size during boost and overestimates it during crashes. When the observed number of lambs that survived until the summer was plugged into the model at each year, the predicted total number of sheep was very close to the observed one. When the model was projected from 1961 onwards, results suggested that the estimates of the middle period were less accurate (see summary in report Report on BBSRC grant 96/E14253.pdf ).
 

Bibliography

Clutton-Brock, J. (1999). Domesticated Mammals. Cambridge University Press. Cambridge

Clutton-Brock, T. H. Price, O.F. Albon, S.D. and Jewell, P.A. (1992). Persistent instability and population regulation in Soay sheep. Journal of Animal Ecology. 60, 593-608

Coulson, T., Catchpole, E.A., Albon, S.D., Morgan, B.J.T., Pemberton, J.M., Clutton-Brock, T.H., Crawley, M.J., & Grenfell, B.T. (2001) Age, sex, density, winter weather an population crashes in Soay sheep. Science, 292, 1528-1531.

Pemberton, J.M., Smith, J.A., Coulson, T.C., Marshall, J.Slate, Paterson,S. Albon, S.D. and Clutton-Brock, T.H. (1996). The maintenance of genetic polymorphism in small island populations: large mammals in the Hebrides. Philosophical Transactions of the Royal Society of London. Series B. 351, 745-752

Steel, T. (1994) The life and death of St.Kilda. The moving story of a vanishied island community Harper Collins, London.
 
  
 

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