A Highland Deer Herd & its Habitat
Chap 1: Red deer are important to the rural economy of the Scottish Highlands, providing a vital source of income from unimproved hill ground. Furthermore, their management can have conservation benefits compared with more intensive land uses. However, deer numbers in Scotland have risen over the past 50 years, brining deer into conflict with agriculture, forestry and conservation interests. The ‘natural heritage’ is now protected under law, following its inclusion in the Deer (Scotland) Act 1996. Concerns about the impact of deer on the natural heritage are focused on three issues: a widespread lack of regeneration in upland woodlands, declines in heather moorland and the potential loss of biodiversity. In many places, high sheep stocking rates add to the grazing impact. Increasing deer numbers have also raised concerns for deer welfare. Intermittent high winter mortality is often thought to be associated with high deer numbers. High density can also affect other aspects of the condition and performance of deer. There is a need to find an optimal deer management strategy, whereby conflict between land uses is minimised while the stalking interests of the landowner and the local economy are maintained. The optimum is likely to require compromises and will vary depending on individual management objectives and the nature of the area. This study aims to find an optimum for a large area of relatively poor ground in the north-west Highlands, with some areas of high conservation value but no sheep. Letterewe, the Heights of Kinlochewe and Little Gruinard estates in Wester Ross are the focus of the study. They cover an area of 32,000ha of predominantly open moorland with a mosaic of heath vegetation. The land is rugged with many lochs, several peaks over 900m and no roads. Much of the upland flora and fauna characteristic of the Highlands can be found here. While deer are the most numerous large mammal, there are also feral goats and a few roe deer. There are no rabbits. Over 60% of the land area is covered by relatively unproductive wet heath or blanket bog and 6.5% by open water. The plant community with the highest conservation value is the oak woodland along the north shore of Loch Maree. This is one of the most northerly remnants of the Atlantic oak woods. Agrostis-Festuca ‘greens’, the best quality grassland and preferred grazing, account for only 1% of the land area. Letterewe estate and the Heights of Kinlochewe are run as a traditional stalking estate. Ponies are used to extract carcasses. The cull has been very light for many years (about 5%), with milk hinds, calves and old or poor stags being targeted. Since 1998, the cull has been increased to about the level of annual recruitment, with a concurrent increase in the number of yeld hinds taken. About a quarter of the stags are fed in winter, together with a few hinds. Current grazing pressure at Letterewe is lower than its has been throughout much of the 19th and 20th centuries. Large-scale sheep farming began at Letterewe in 1810 and lasted for about 70 years before the estate was classified as a ‘deer forest’ in the 1880s. The extent of sheep grazing in the first half of the 20th century is unclear but sheep were certainly kept on the Home Beat by the previous owner. All sheep were removed from Letterewe estate in the 1970s although about 600 remain at the Heights of Kinlochewe. Deer share their range with sheep on most of the neighbouring estates. In this study, we aim to estimate the size of the deer herd and assess whether current deer densities pose a threat to the environment or the welfare of the deer. We also evaluate the costs and benefits of various management options. We examine the evidence for any adverse effects of the current deer population on vegetation community structure, plant production and plant species diversity. We also look at the effects on deer performance and condition. Finally, we review some of the options for managing both the deer herd and the woodland and discuss the consequences of changes in practice. Chap 2: Our attempts to estimate the deer population size using three different methods are described in this chapter. Average spring deer density was estimated to be 14.3 deer/km2 on Letterewe estate and 15.2 / km2 at the Heights of Kinlochewe, from helicopter counts made by estate staff. Little Gruinard had a density of less than 5 deer/km2. Helicopter counts gave consistently higher population sizes than DCS foot counts for Letterewe estate although there was close agreement between the two methods at the Heights. An estimate of population size and, importantly, accuracy was also made from line transects using ‘Distance sampling’. Transect counts agreed fairly well with helicopter counts but had wide error margins, especially on Letterewe estate. Detectability of deer groups, modelled using line transect data, was much lower on Letterewe than the Heights. We suggest the variation in accuracy between the estates was due to the effect of topography on detectability. Letterewe estate is more rugged and hence is harder to count accurately than the Heights, but the effect is less severe when counting from the air. Although this finding is based on only two estates, it is nonetheless important because no assessment of the factors affecting variation in accuracy of deer counts has yet been made in Scotland. There has been a decline in spring numbers on Letterewe estate since the increased cull in 1998. By contrast, the population at the Heights has remained stable. Total deer density varied between stalking beats, being consistently higher on the Home Beat. However, hind density was similar on all beats. There were more stags than hinds on the Home Beat, similar proportions at the Heights and a female biased sex ratio at Carnmore and Larachantivore. On Letterewe estate as a whole, there has been an increase in the relative number of stags in the last 3 years, concurrent with the decrease in numbers. The average stag:hind ratio across Letterewe and the Heights was 0.75. Sex ratios agreed fairly well between foot and helicopter counts. We estimated the average calf: hind ratio was 0.26 from censuses carried out on foot. No differences were found between beats. Estimates of calf:hind ratios from helicopter counts in spring appeared to over-estimate the number of calves. The current hind density on Carnmore and Larachantivore beats represents an increase of about 50% since Fraser Darling’s study of red deer in the area in the 1930s. While some of the difference may be due to different counting methods, this rise is in line with the increase in deer numbers across Scotland as a whole. Darling recorded a higher ratio of stags to hinds in the 1930s, as expected under lower density conditions. Current deer density at Letterewe is similar to other parts of the West Ross DMG and within the range of other areas of the west Highlands. However, total herbivore density is likely to be lower than in many other places because of the absence of sheep. Herbivore numbers on Letterewe and the Heights are equivalent to about 7000 sheep, including the small flock of sheep at the Heights. This corresponds to a density of approximately 0.25 sheep-equivalents/ha. By comparison, the average density of sheep in Gairloch parish is 0.5 ewes/ha. If deer numbers are added, the total herbivore density of neighbouring estates ranges from 0.5-0.9 sheep-equivalents/ha, compared with a recommended stocking density for the north and west Highlands of 0.5 ewes/ha. Chap 3: Deer habitat use and the impact of deer on woodland regeneration, plant production and species diversity are described in this chapter. Deer are widely distributed across Letterewe estate and the Heights of Kinlochewe as well as across the range of vegetation communities. Deer used the Festuca-Agrostis greens, dry heath and coarse Nardus grassland more often than would be expected from their relative availability. These were also amongst the most productive plant communities. By contrast, wet heath is less productive and although it is widely used because of its extensive availability, it is not actively selected by deer. Deer are having a marked effect on the plant community structure at Letterewe, by inhibiting the spread of woodland and scrub. This may in turn affect biodiversity if the range of species in other groups such as invertebrates is limited. While regeneration of oak, birch and rowan was good in the absence of grazing, few tree seedlings or saplings were found outside fenced exclosures. There was no oak regeneration under the shade of a closed canopy either inside or outside the fences. A controlled experiment showed that removing competing ground vegetation, providing a seed source and excluding deer all improved the establishment of native trees. Exclusion of voles had no effect. The altitudinal limit to oak establishment was about 300m, but no effect of altitude was found on pine and rowan. The effect of excluding deer was more pronounced at low than high altitudes. Vegetation productivity at Letterewe was relatively low, averaging about 110 g/m2/yr (1.1 t/ha/yr). However, estimates varied between communities, ranging from over 300 g/m2/yr on Agrostis-Festuca greens to about 40 g/m2/yr in the woodland, with heaths producing 90-140 g/m2/yr of edible biomass. These estimates are in line with expectation for upland communities in north-west Scotland. The proportion of production removed by grazing was estimated to be between 9-13%, based on measurable offtake and approximate daily food intake of deer. Grazing offtake of heather and the less productive communities was difficult to measure because it was low relative to the spatial variation in biomass. Although no long-term data were available to assess changes in heather cover, heather condition was generally good. Plant species diversity at Letterewe is not unusually low as a result of grazing by deer at current densities. A total of 391 plant species were found across Letterewe estate, Heights of Kinlochewe and Little Gruinard. Species diversity was close to the national average for most vegetation communities, compared with figures in the National Vegetation Classification. With the exception of woodland regeneration, we have found no evidence of adverse effects of deer at their current densities on plant communities. Chap 4: The current performance and condition of the Letterewe herd is investigated in this chapter and compared with other Highland populations. Survival and recruitment rates are measures of the performance of a population. These depend on individual condition which can be assessed from body weights and fat reserves. Spring calf:hind ratios were relatively low but typical of the north-west, despite heavy calf culling. Natural mortality varied between years from 6-14% of the pre-winter population, being highest in 1998/99. Mortality at Letterewe fell within the range observed on Rum and at Invermark although it was higher than at Glenfeshie. Comparing current performance with that in the time of Fraser Darling, we found a similar recruitment rate despite apparently higher summer calving rates in the 1930s. Larder weights and mass of kidney fat suggested that deer were in better condition in the 1999/00 season than in 2000/01, and in consistently better condition on the Home Beat and at the Heights of Kinlochewe than at Carnmore and Larachantivore. Differences in condition between beats were more likely to be due to variation in the availability of high quality vegetation, low ground or shelter than to population density since hind densities varied little between beats. Variation in kidney fat was a more sensitive indicator of condition than body weight, showing greater variation both between individuals and beats. The timing of antler cleaning and casting supported conclusions reached from the larder data but no differences in the timing of hind coat change were detected between beats or years. There was no evidence of an improvement in the average carcass weight or number of antler points of stags shot since 1981. In fact there was a slight trend towards decreased carcass weights, suggesting that selective culling of stags has not had the desired effect. Stag carcass weights appear to have changed little since the 1940s. Comparisons with other Highland populations suggest that the current Letterewe herd is not unusual in terms of the adult sex ratio or carcass weights, despite its deer density. The aspect of performance that compared least favourably with other populations was the reproductive output. Calf:hind ratios, and in particular the proportion of milk hinds becoming pregnant, were at the lower end of the range of values observed. This is an important means of natural population regulation. Spatial variation in performance occurred both within the Letterewe area, and between estates across the Highlands. Variation in many aspects of performance seemed to be more closely related to differences in habitat quality than deer density. Chap 5: Previous chapters have shown that current deer densities are having no adverse effects on most of the vegetation communities at Letterewe. Nor is condition or survival of the deer herd unusual, although the reproductive performance is relatively low. However, the lack of regeneration in the highly valued Atlantic oak woodland means that some management changes should be considered, especially on the Home Beat where the woodland occurs. The main management decisions required at Letterewe concern woodland regeneration and what level of culling to maintain. In this chapter, we present three options to promote regeneration of the oak woodland; heavy culling, diverting grazing pressure and fencing. We then use a computer simulation model to predict the consequences for deer population dynamics of four different levels of culling. These are a zero cull, a low hind cull (equivalent to the cull before 1998), a moderate hind cull (equivalent to the current cull) and a high hind cull (double the current level). We believe a long-term (20-30yr) rotation of 12 to 18 small exclosures (2ha), sited along the woodland margins and in large gaps, is the most realistic option to ensure regeneration. The tendency of deer to aggregate on low ground in winter means that a cull of about 85% of the lochside population (over 500 animals) would be required for regeneration to have a chance of occurring without fences. Goat numbers would also need to be controlled. Not only would a cull of this scale be difficult to implement, but it is also incompatible with the stalking interests of the estate in the longer term. In some instances it may be possible to use supplementary feeding to divert grazing pressure away from sensitive areas, but at Letterewe there are no suitable areas for feeding away from the woodland on the Home Beat. Of the four deer culling options explored, maintenance of the current cull (11% hind cull) appears to provide the best compromise between costs and benefits, while allowing the desired offtake of 100 stags per year. Simulation of the herd under a zero cull shows the population quickly reaching a carrying capacity of about 16-17 deer/km2. This is a heavily female biased herd with low productivity. As the size of the hind cull increases so population size decreases and becomes more male biased. This allows a larger stag cull, until low hind numbers constrain the number of stags born. A high hind cull option (22% of hinds culled) reduces the average population density to about 6.5 deer/km2 in 20-30 years. However, local densities are likely to be far higher than average on the low ground in winter, making natural regeneration unlikely. Furthermore, stag offtake is lower than under the current cull. Therefore the additional benefits over the current cull option are limited, while the financial cost would be considerable. In general the costs of carrying out the hind cull are greater than the revenue from venison sales. Increasing the hind cull has implications for cull selectivity, culling standards, and the levels of disturbance to deer. The past increase in the size of the hind cull at Letterewe led to a decrease in selectivity with respect to age but no detectable change in the condition of shot animals, age for age. The number of yeld hinds shot increased, as did the number of damaged carcasses sold to the game dealer. No relationship was found between the intensity of culling and the accuracy with which stalkers were able to correctly identify hind/calf pairs. Accuracy was high (>90%) throughout the period September to January. For the hind cull to increase, efficiency or the number of days stalked need to increase. Efficiency could be improved by leaving some carcasses on the hill which also has some benefits for biodiversity and the nutrient content of vegetation. However, there would be strong opposition from stalkers and the public to this practice. Alternatively the number of stalking days could be increased by bringing the hind season forward to the start of September, so avoiding the poorest weather and shortest days. Although this prolongs the period of disturbance to the deer, disturbance can be minimised by targeting small groups. Chap 6: Concern that widespread, uncontrolled increases in deer would pose a threat to the environment and result in high winter mortality has led to estates, including Letterewe, being put under increasing pressure to raise the size of their culls. This study was set up to estimate the number of deer at Letterewe, assess the level of their impact on the habitat and deer performance, and to explore future management options. We have shown that density at Letterewe is comparable with many other areas of the West Highlands and because of the absence of sheep, the combined sheep and deer grazing pressure is relatively low. Deer impact is beneficial in maintaining the plant species diversity of highly productive vegetation communities, but has little effect on plant biomass or diversity of the low productivity communities. However, deer are currently preventing the natural regeneration of the oak woodland. Most aspects of growth, condition and performance of the deer herd fall within the expected range. At first sight, a dramatic reduction in deer population size at Letterewe appears to be beneficial. However, closer examination shows that even doubling the current cull is unlikely to allow tree regeneration, but could cause a loss of species diversity of the grasslands. By contrast, ceasing culling is unlikely to severely affect the survival of herbs, sedges and grasses although grazing pressure may be sufficient to affect heather cover. Benefits to the deer herd of reducing numbers are likely to include increased reproductive output and reduced emigration of stags but adult mortality is unlikely to be affected. Our calculations suggest that prior to 1998, the annual cull could not have constrained population growth. This suggests that the population had been at a level at which it was limited by natural factors, including the effects of density and climatic fluctuation, for a long time. This is also likely to be the case in many other parts of the West Highlands. However, in the central and eastern Highlands, where productivity is much higher, culling is probably important in controlling deer numbers. The bare landscape of the Highlands today is largely attributable to the effects of historical grazing pressure rather than recent rises in deer numbers. There are no obvious changes to the landscape at Letterewe since the 1930s, despite the increase in the number of deer. However, relatively low densities of deer and sheep are sufficient to prevent regeneration of woodland. Only a very large reduction in deer numbers (about 80%), combined with a similar reduction in sheep stocks, is likely to have a substantial effect on woodland cover. Such a move would have dramatic implications for the livelihoods of people in the rural communities. On the other hand, in areas where deer do not constitute a threat to other interests, ceasing culling may be seen as the most economic option. It is not clear from our results that this would lead to substantial changes in deer condition or survival or to a substantial increase in their impact on woodland in north-west Scotland. Naturally regulated deer populations may not be incompatible with the maintenance of biodiversity in the open landscapes that have characterised the Highlands for the last few centuries.