Soay sheep: an island survivor

Abstract

Many of the best field studies in evolutionary ecology are located on remote islands. Therefore, when listening to talks that are accompanied by slides of sun-drenched sandy beaches, the thought that some substitute motif has been involved in the choice of study site is unavoidable. However, no one can accuse the researchers who study the Soay sheep on the St Kilda archipelago off the northwest coast of Scotland of having a study site that offers the opportunity to spend leisure time in the sun. Soay Sheep. Dynamics and Selection in an Island Population, edited by Tim Clutton-Brock and Josephine Pemberton, demonstrates clearly that this study site was chosen for purely scientific reasons, because it offers a rare opportunity to study fundamental ecological and evolutionary processes.The Soay sheep was introduced to the study island (Hirta) from the neighbouring island of Soay in 1932 and their population was subsequently allowed to develop freely. The large-scale study of the sheep was launched in 1985 by a team from the University of Cambridge. Most of the sheep in the study area were individually marked and, since then, a unique database has accumulated on individual variation in reproductive success and survival. This has enabled the study to be expanded to cover a wide range of general-interest questions with a unique cross-disciplinary focus integrating several research fields. Thus, several research groups are now involved in the project and, accordingly, Soay Sheep has 25 coauthors.The book begins with a description of the general biology of the sheep, followed by a chapter outlining their population dynamics. From these chapters, we learn that the large fluctuations in the population levels of the sheep are caused by a combination of density dependence and stochastic variation in climate, as well as being strongly influenced by temporal variation in the age and sex composition of the population. In the two subsequent chapters, the mechanisms that might cause this large variation in population growth rates over time are examined. Described for the first time is an important characteristic of the ecosystem on Hirta; that is, that the plant communities appear to be extremely resilient to grazing, but that the grazing pressure towards the end of winter is extremely high and will probably result in die-offs at high population densities. However, the effects of parasites are more subtle. I found Chapter five to be an excellent illustration of how difficult it is to assess how host–parasite interactions affect the population dynamics of large herbivores. Chapters 6–8 detail how the huge effort invested by the St Kilda research group in developing advanced genetic tools has provided a unique understanding of the social organization and genetic structure of the population. The final chapters describe the life history and population dynamics of the Soay sheep. In particular, I think that the last chapter about regulatory mechanisms will become a classic in population ecology because its non-quantative approach will attract a wide audience.Is there anything new in Soay Sheep? Yes, there is. The book has merit that is far greater than the sum of its parts. For example, the authors and editors provide a balanced treatment of the various mechanisms that influence population biological processes of the sheep. Even for researchers who are familiar with the individual papers, it is often difficult to get an overall view of how different factors affect the sheep. This is hardly surprising, given that the reference list of the book includes 50 papers published about the Soay sheep since 1985. In particular, the balanced treatment afforded in Chapter five to parasites and their impact on the population dynamical processes fills a long-awaited need. Similarly, Chapter eight, concerning genetic variation, provides a useful overview by presenting evidence that is often hidden in technically orientated genetics papers that can be difficult to access. Furthermore, to my great surprise, Soay Sheep contains previously unpublished data that enables us to better put the data resulting from the studies of the sheep in a more general context. The description of the vegetation–sheep interactions is especially welcome.Soay Sheep will undoubtedly become a classic text in population biology literature. The editors should be acknowledged for producing a book that, even with so many coauthors, follows a coherent style and contains careful cross-referencing across its chapters. The book also serves as a tribute to the British research system, which willingly provides continuous support to such important long-term studies.