Introduction

Sheep were domesticated in the Near East around 10 000 years ago and spread into Western Europe from there (J. Clutton-Brock 1981). Sheep similar to Soays had reached the Orkneys by 4000 bc and the sheep population of St Kilda may have originated around that date. In many aspects of their anatomy and physiology, they appear to be intermediate between contemporary domestic sheep and wild sheep (Boyd and Jewell 1974; Jewell 1986).

To understand the unusual dynamics of Soay sheep and their consequences for selection and adaptation, it is important to know something of their history as well as of the human inhabitants of St Kilda. The first two sections of this chapter describe the islands of St Kilda (section 2.2) and their history (section 2.3). Subsequent sections describe the appearance and anatomy of Soay sheep (section 2.4), their feeding ecology (section 2.5) and their reproductive system (section 2.6). Since variation in fecundity and neonatal survival affect the growth rate of the population, we describe the factors affecting the early development of lambs (section 2.7) as well as the factors affecting winter survival in juveniles and yearlings (section 2.8). Finally section 2.9 reviews the costs of reproduction and other factors affecting mortality in adults.

The islands of St Kilda

The four main islands of the St Kilda archipelago lie 160 km to the north-west of the Scottish mainland (Fig. 1.1).

Introduction

A conspicuous feature of many naturally limited populations of long-lived vertebrates is their relative stability. Both in populations that are regulated by predation or culling and in food-limited populations, population size can persist at approximately the same level for decades or even centuries (Runyoro et al. 1995; Waser et al. 1995; Clutton-Brock et al. 1997a; Newton 1998). The persistent fluctuations shown by Soay sheep and by some other island populations of ungulates (Boyd 1981; Leader-Williams 1988; Boussès 1991) raise general questions about the causes and consequences of variation in the stability of populations (see section 1.2). How regular are they? How are they related to population density? What are their immediate causes? To what extent do fluctuations in food availability, parasite number or predator density contribute to them? And what are their effects on development and on the phenotypic quality of animals born at contrasting population densities? And how much do changes in phenotype contribute to changes in dynamics?

As yet, there are very few cases where we understand either the ecological causes or the demographic consequences of persistent fluctuations in the size of naturally regulated populations of mammals (Hanski 1987; Saether 1997). Since we are able to monitor the growth, movements, breeding success and survival of large samples of individuals as population density changes, the Soay sheep offer an opportunity to investigate the causes and consequences of changes in population size with unusual precision (see Chapter 1).

Sheep on St Kilda

Off the north-west coast of Scotland, beyond the protective chain of the Outer Hebrides, lie the islands of St Kilda (Fig. 1.1). The fragmented ring of a tertiary volcano, the four main islands are steep and craggy, their low ground green with lush grass (Fig. 1.2). The steep sea-cliffs of the islands are streaked with the droppings of seabirds for whom the islands are a major breeding base. Over many centuries, the birds have enriched the islands' soil and their eggs and young have attracted humans for more than four thousand years. The earliest hunters left few visible marks on the landscape, but the dry-stone walls built by the farmers that followed still bisect the low ground and the ruins of their cottages are scattered across the lower slopes of the hills.

Through these ruins, Soay sheep wander (Fig. 1.3a, b). Small, horned and mostly brown or black, they are the survivors of the earliest domestic sheep that spread through Europe in the Bronze Age, reaching Britain's remotest islands between three and four thousand years ago. In the course of time, they were replaced by larger, more productive breeds, but a remnant population of original sheep was abandoned on Soay, a 99-ha island where their existence was protected by the difficulty of access (Fig. 1.4).

Introduction

This final chapter reviews our understanding of the causes and consequences of fluctuations in sheep numbers on St Kilda and sets our results in the context of other studies. It is structured around the three main questions that we have tried to answer (see section 1.1): how are sheep numbers regulated and what physical and biological processes are responsible for changes in population size? How do changes in population density affect selection? And how do they affect the optimal reproductive strategies of individuals? The first section (10.2), examines the causes of death and the reasons why, unlike many other ungulate populations, the population of Soay sheep on Hirta shows such large fluctuations in size. In addition, it reviews the demographic consequences of changes in population size, including the effects of variation in density on development. Section 10.3 examines the impact of fluctuations in sheep numbers on the opportunity for selection and the intensity of selection on particular traits and speculates on the role these changes may play in maintaining genetic and phenotypic diversity. Fluctuations in sheep numbers also affect the costs and benefits of reproduction: section 10.4 reviews evidence of these changes and examines their consequences for the optimal breeding strategies of the sheep. Finally, section 10.5 examines the relevance of work on St Kilda to understanding the dynamics of other resource-limited populations of mammals, arguing that the demographic processes operating in the sheep are probably found in many other resource-limited populations.

Introduction

Recent increases in the number of time-series long enough to provide an adequate description of population fluctuations clearly show that population stability varies widely among animals with similar longevities and rates of reproduction, as well as between species with contrasting life histories (Caughley & Krebs 1983; Gaillard et al. 2000). For example, among grazing ungulates, populations may either show little variation in size across years, irregular oscillations, semi-regular oscillations resembling the stable limit cycles found in some smaller mammals or dramatic oscillations occasionally leading to extinction (Peterson et al. 1984; Fowler 1987b; Coulson et al. 2000). While many ecological differences probably contribute to these differences (including predation, disease and human interference), the fact that stability varies widely among naturally regulated ungulate populations living in environments where human intervention is minimal and predators are absent (Boyd 1981a,b; Boussès et al. 1991; Clutton-Brock et al. 1997a), suggests that variation in population dynamics may often be caused by interactions between populations and their food supplies.

Theoreticians have explored the possibility that contrasts in population dynamics may be consistently related to differences in life histories or in the temporal or spatial distribution of resources (e.g. Peterson et al. 1984; Sinclair 1989; Sæther 1997; Illius & Gordon 2000; Owen-Smith 2002). While it is likely that both these differences may contribute to variation in dynamics, attempts to explain observed variation mostly assume that the causes of contrasts are sufficiently simple to be explained by general models derived from first principles (Caughley 1977).