We study the vegetation at two periods each year: in March, before the grass has started to grow, and when things are as hard for the sheep as they get; and in August when the food supply is as good as it ever gets. We measure the dry weight of all of the different plant species separately, so most of our research time is spent separating different blades of grass into neat miniature haystacks, prior to drying each sample in the oven, then weighing them. This work is carried out at 6 locations on each of 5 transects that run uphill from the shore, through the inbye grasslands, and into the heath above the Head Dyke (30 sampling locations in all).
Sheep numbers have been consistently high for the last 9 years, and this is reflected in the vegetation. In 2011 we saw total biomass at its second-lowest ever in both March and August: 7.2 g per 0.04 m2 in March (long-term mean = 10.7; only March 2009 was lower at 6.8) and 9.3 g per 0.04 m2 in August (long-term mean = 15.6; only August 2008 was lower at 8.9). Despite the high sheep population in 2010, the population increased slightly to 2,141 in August 2011, preserving the pattern that we have never seen two population declines in a row since the study began in 1985.
Figure 1. Glen Bay in March when there is precious little green grass left for the sheep to eat.
The in-bye grassland on Hirta is the most productive plant community used by the Soay sheep. At a scale of 10cm or so, the sward consists of a mosaic of gaps and tussocks. The gaps are closely-grazed, remain bright green all year round, and typically contain lots of clover (Trifolium repens). The tussocks are much taller (up to 20cm sward height), more lightly grazed, and in winter are made up largely of dead organic matter (uneaten grass production left over from the previous summer). For both gaps and tussocks, there is a very strong seasonal cycle in their biomass and botanical composition. Green biomass is high in summer and low in winter, with dead organic matter showing the opposite pattern. Because they can grow at very low temperatures, bryophytes (mosses and liverworts) benefit from reduced light competition resulting from die-back of the grasses, and peak in abundance in winter. As is so often the case in studies of climate change, the size of these within-year fluctuations is large compared to the long-term trends in average plant biomass (Figure 2.) There is a significant downward trend over time in green biomass in tussocks (top right; p < 0.01) but not in gaps (top left)
Figure 2. Time series for March and August (1993 to 2011) green biomass (dry mass of green grass plus herbs per 0.04m2; top row), dead organic matter (DOM mostly dead grass; centre row), and bryophytes (mosses plus liverworts; bottom row), in gaps (left column) and tussocks (right column) from the inbye grassland on Hirta (this is the Agrostis-Holcus grassland between the Head Dyke and the Shore; under the National Vegetation classification, it is U4 Festuca ovina – Agrostis capillaris – Galium saxatile grassland: Holcus lanatus – Trifolium repens sub-community). Note the differences in scale on the y-axes between rows and columns (e.g. there is much more green biomass and DOM in tussocks than in gaps, and more bryophyte biomass in gaps than in tussocks).
Fluctuations in the sheep population are reflected by changes in the distribution, abundance and botanical composition of gaps and tussocks. In the year following a population crash, grazing pressure is relatively light, offtake is low, and tussocks are larger and more numerous. The result is that dead organic matter builds up to a peak in the winter following the crash. When sheep numbers are high (as they have been for the last 9 years), then tussocks are rare (especially in winter) and gaps are numerous and very closely grazed. Figure 1 shows the very clear log-term downward trend in tussock cover. It will be interesting to see if tussock cover ever returns to its pre-2005 levels.
Figure. 3. The clearest consequence of increasing sheep numbers (left; whole island count in August showing more than 39 extra sheep per year on average; r2 = 0.48) has been a decline in the mean cover of tussocks in the following March (right; p < 0.001). Note that the slight increase in whole island count in August 2011 (top right of the sheep time series), preserved a pattern that we have never yet seen two years of population decline in a row. Tussock cover (scored on a scale of 1 to 5; right) is an average score out of 30 across 14 stratified random sampling stations within the Head Dyke each year. A score of 5 equates to tussock cover of 5/30 = 16.7%. Clearly this linear trend cannot continue (it predicts negative tussock cover by 2015), and we expect that tussock cover would increase in the year following a crash (if we ever see one again).
The Soay sheep system on Hirta represents one of the longest-running detailed studies of a plant-herbivore interaction anywhere in the world. From a botanical perspective, the system is ideal because the Soay sheep are the only large herbivores on the island (there are no rabbits and no voles), so all grazing impacts can be attributed unequivocally to the sheep. Similarly, the Soay sheep have no predators, so their population is clearly food-limited, and hence must be regulated by competition for food.