Comparative Ungulate Dynamics: the Devil Is in the Detail
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Published online 23 August 2002 Comparative ungulate dynamics: the devil is in the detail T. H. Clutton-Brock* and T. Coulson Large Animal Research Group, Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK Attempts to relate species differences in population dynamics to variation in life histories rely on the assumption that the causes of contrasts in demography are sufficiently simple to be derived from first principles. Here, we investigate the causes of contrasts in dynamics between two ungulate populations on Hebridean islands (red deer and Soay sheep) and show that differences in stability, as well as in the effects of variation in density and climate, are related to differences in timing of reproduction relative to seasonal variation in resource abundance. In both populations, attempts to predict changes in population size sufficiently accurately for the results to be useful for management purposes require a knowledge of the responses of different age and sex categories to changes in density and climate, as well as of population structure. Keywords: dynamics; demography; sheep; deer 1. INTRODUCTION 1977). Another possibility is that contrasts in population dynamics are a consequence of detailed differences in the Recent increases in the number of time-series long enough demographic processes affecting dynamics, driven by spe- to provide an adequate description of population fluctu- cific interactions between breeding systems and life-his- ations clearly show that population fluctuations vary tory parameters and the distribution of resources. If so, widely among animals with similar longevities and rates of current attempts to predict variation in population dynam- reproduction, as well as between species with contrasting ics using general models may meet with little success until life histories (Caughley & Krebs 1983; Gaillard et al. we have a better understanding of the specific causes of 2000). For example, among grazing ungulates, popu- contrasts in dynamics (Sutherland 1996). lations may either show little variation in size across years, In this paper, we compare the dynamics and demogra- irregular oscillations, semi-regular oscillations resembling phy of two populations of food-limited ungulates (red deer the stable limit cycles found in some smaller mammals Cervus elaphus L. and Soay sheep Ovis aries) on different or dramatic oscillations occasionally leading to extinction Hebridean islands over the same years (figure 1). We show (Peterson et al. 1984; Fowler 1987; Coulson et al. 2000). that a detailed knowledge of demographic processes and While many ecological differences probably contribute to population structure is necessary to predict changes in these differences (including predation, disease and human population size successfully and to explain the contrasts interference), the fact that stability varies widely among in population dynamics between the two populations. naturally regulated ungulate populations living in environ- Research on the red deer population of the north Block ments where human intervention is minimal and predators of Rhum has continued since 1972, when the annual 14% are absent (Boyd 1981a,b; Bousse`s et al. 1991; Clutton- cull of the population of around 200 deer was terminated Brock et al. 1997a), suggests that variation in population (Clutton-Brock et al. 1985b, 1997a). After 1972, numbers dynamics may often be caused by interactions between rose rapidly to around 300, stabilizing by 1980 although herbivore populations and their supplies. the adult sex ratio continued to change in favour of Theoreticians have explored the possibility that con- females (see figure 2a). Demographic processes varied trasts in population dynamics may be consistently related between the initial period of population growth and the to differences in life histories or in the temporal or spatial subsequent years when deer numbers had reached eco- distribution of resources (e.g. Peterson et al. 1984; Sinclair logical carrying capacity (Albon et al. 2000); so, to maxim- 1989; Sæther 1997; Illius & Gordon 2000; Owen-Smith ize comparability with the sheep (see below), we have 2002). While it is likely that both these differences con- restricted our analysis of dynamics in the deer to the per- tribute to variation in dynamics, attempts to explain iod between 1985 and 2001. observed variation with general models assume that the Since 1985, we have also monitored the dynamics of causes of contrasts are sufficiently simple to be explained the Soay sheep on Hirta, the largest island of the St Kilda by general models derived from first principles (Caughley archipelago, ca. 120 km to the northwest (figure 2b). The sheep population was originally introduced from the neighbouring island of Soay and has been naturally regu- * Author for correspondence ([email protected]). lated since 1932 (Grubb 1974a–c; Grubb & Jewell 1974; One contribution of 15 to a Discussion Meeting Issue ‘Population growth Clutton-Brock et al. 1991; Grenfell et al. 1992). Soay rate: determining factors and role in population regulation’. sheep are derived from domestic stock that were probably Phil. Trans. R. Soc. Lond. B (2002) 357, 1285–1298 1285 2002 The Royal Society DOI 10.1098/rstb.2002.1128 1286 T. H. Clutton-Brock and T. Coulson Ungulate dynamics Cervus elaphus Ovis aries births (June) births (April) singletons only ca.15% twinning mortality (Feb – April) mortality (March – April) rut (October) rut (November) females sexually females sexually mature at mature at age 2 years age 6 months maximum longevity maximum longevity males: 17 years males: 11 years females: 24 years St Kilda females: 16 years Rhum Figure 1. Summaries of the life cycles of red deer and Soay sheep. (b) 700 700 600 600 500 500 N 400 400 300 300 200 200 100 100 0 0 1973 1981 1989 1997 1984 1992 2000 year year Figure 2. Time-series for (a) deer and (b) sheep numbers. Filled circles, total; open circles, total females; open triangles, total males. The ranges on the y-axes for sheep and deer are identical to allow comparison of the relative size of fluctuations in population size. introduced to the Hebrides over 2000 years ago and have of our datasets are unusual in that virtually all individuals remained on Soay since then (Clutton-Brock 1981). Com- in both populations are recognizable as individuals and pared with most other time-series for large mammals, both their life histories have been monitored from within a few Phil. Trans. R. Soc. Lond. B (2002) Ungulate dynamics T. H. Clutton-Brock and T. Coulson 1287 (b) 0.8 70 60 0.6 50 40 0.4 30 20 proportion fecund 0.2 10 0 0 deer sheep max. growth rate (% annual increase) juveniles yearlings 2–3 years4–6 years7–11 years 12+ years (c)(d) 1.0 0.20 0.8 0.15 0.6 0.10 0.4 twinning rate proportion fecund 0.05 0.2 0 0 deer sheep deer sheep Figure 3. Fecundity in deer (black bars) and sheep (white bars). (a) Maximum population growth measured as the maximum percentage annual increase in population size. (b) Proportion of animals conceiving offspring at different ages. (c) Proportion of milk (hatched bars) and yeld (grey bars) females giving birth. ‘Milk’ are those that reared an offspring successfully until (at least) the onset of the winter in the previous year; ‘yeld’ are those that failed to do so either because they did not give birth or because their calf died during the summer. (d) Proportion of individuals bearing twins—note that red deer never twin on Rhum. days of birth, when most lambs and calves are caught, between successive years. For example, while deer num- weighed, sexed and skin-sampled for genetic analysis bers have never declined by more than 17% in a single (Clutton-Brock et al. 1982b, 1991; Pemberton et al. winter (figure 2a), over 60% of the sheep in autumn can 1996). As a result, we are able to identify the contributions die in the course of 2 months in late winter (see figure 2b). of specific demographic changes to variation in population When high mortality occurs in the sheep, this not only size with unusual accuracy. removes the increment in population size that has The habitats occupied by the two populations at the two occurred in the course of the last year, but, on average, sites are broadly similar, with areas of herb-rich or reduces the population to less than 65% of the maximum Agrostis-dominated grassland at sea level grading into number that has been known to survive the winter. By heather-dominated communities interspersed with flushes contrast, in the deer, winter mortality never reduces spring on the slopes of the surrounding hills (Jewell & Grubb numbers much below 90% of observed maximum winter 1974; Jewell et al. 1974). Densities of sheep reach higher numbers. These contrasts in winter mortality are associa- levels than those of the deer, rising to 25 kmϪ2 compared ted with differences in growth rate between the two popu- with ca.15kmϪ2 in years of high density. Compared with lations. In the deer, numbers rarely increase by more than deer, the sheep show relatively high population growth 10% per year; for example, it took the population 7 years rates (figure 3a), partly because many females conceive to rise by 50% following the termination of culling in 1972 for the first time at 7–8 months instead of at 2–3 years (see figure 2a). Sheep numbers, on the other hand, can (Clutton-Brock et al. 1997a; figure 3b), partly because increase by over 50% in the course of a single season and most females over a year old conceive each year (figure commonly double in the course of 2 years (figure 2b). 3c) and partly because, on average, ca. 15% of females In both species, high winter mortality affects some sex produce twins (figure 3d).