Wild Boar
Wild boar (Sus scrofa) were once common throughout Britain, and played a unique role in ground disturbance and the numerous ecological processes associated with it. We can’t be certain how long boar have been absent, but habitat loss and hunting seem to have made them extinct in Scotland by the 17th century at the latest (along with their natural predators) (1).
In recent years, however, boar have been making something of a comeback. European boar have escaped from farms or been released illegally, and have begun to re-hybridise with their domesticated cousins. The resulting ‘feral pigs’ have spread into several areas of Scotland, including the Bunloit estate. Due to their murky background, these new arrivals are officially considered an Invasive Non-Native Species (2).
This population around Bunloit is of uncertain origin, but has been working its way up the northern side of the Great Glen for many years. We don’t know when they first arrived in Bunloit, but they have been a fairly common sight for those of us on the estate over the past three years.
Wild boar near oak woods at Bunloit
Boar are large animals that cause obvious – and often very unwelcome – ground disturbance. They’re particularly contentious as a result, and most of our neighbours cull them heavily (with the result that we’ve seen fewer, in-person or on our camera traps, as time has gone on). We haven’t yet done anything to affect their numbers, but are monitoring their impacts in as much detail as possible.
There are a number of reasons to reserve judgement and gather evidence in this way. Firstly, most of the animals here behave like wild boar. This doesn’t mean that they are wild boar; their genetics are probably a mixture (3). Unlike domestic stock, those on the estate breed just once a year with relatively small litter sizes, which limits population growth. But most importantly, they disturb the soil in ways very similar to their native ancestors.
Rooting by boar and pigs is obviously destructive in the short term. This can be good where plants such as bracken, which can otherwise dominate large areas, are broken up. But it can also be beneficial even in species-rich areas, where it enables rare species that are normally outcompeted to spread to new ground, potentially increasing diversity (4–6). There’s also evidence that the consequent changes in plant communities favour other native species that require some level of disturbance for their life cycles and food sources (7–9).
Similarly, impacts on soil carbon may be less problematic than they appear. Rooting exposes soil and can increase soil carbon emissions, although it is unclear whether the general effect is positive, negative or neutral (10, 11). But rooting also mixes surface organic matter into the soil, and has been found to increase the amount of stable mineral-associated carbon (12, 13).
Of course, even beneficial effects can become negative at high intensities (14). Determining the number of boar on Bunloit is extremely difficult because the animals are hard to spot and distinguish from one another, and move far and fast (though we’re working with neighbours to estimate numbers across large scales). Even knowing numbers, however, it would be very hard to set a target density, and this would vary between, for example, working pasture and native woodland.
Nevertheless, we can check numbers approximately, and assess impacts precisely. We’re doing this through a growing programme of monitoring, involving:
Counting the number of boar recordings on our camera traps (which show a large decrease between 2022 and 2023, although we’re cautious about interpreting this as solid evidence of population loss because our network of cameras is small and other factors could affect recordings);
Hosting an independent project on interactions between deer and boar;
Walking transects across the estate to record the extent of boar disturbance
Placing quadrats to record long-term vegetation change along these transects
If we can, we will add long-term soil sampling to these quadrats, to track soil carbon and other chemical change related to boar activity
Establishing exclosure plots to test the effects of removal of grazing pressure from deer and boar activity.
Wider monitoring, sharing information between neighbouring landowners.
Figure 1: Locations of transects and quadrats within different habitats on the estate
We don’t yet have final results from this work, but our transects are already producing interesting data. We have 43 transects on the estate (plus a few on neighbouring land, with the landowner’s permission), each of which is 250 metres long. These transects were first walked in spring 2023, and within each 5-metre section we recorded the percentage of ground disturbed by rooting out to 2 metres either side of the transect line. Figure 2 shows the results, with a clear gradient of intensity that is greatest in the oak and birch woods on the lower slopes of Bunloit (right-hand side of the image), reaching up to 30% of ground disturbance on one transect (with an average of 7.15% across them all).
Figure 2: Mean rooting disturbance on each transect
We’ll repeat these and record change over time, but the spatial patterns along transects are also interesting, with clear signs that different habitats are being used differently. As Figure 3 shows, we’re finding intensive but patchy disturbance in birch woods, much more extensive in oak and pasture, and limited but highly localised disturbance in pine woods. Our juniper woods are not strongly affected.
Figure 3: Percentage of disturbed ground within each 5m segment (columns) of each transect (rows), across the habitats surveyed.
Our intention now is to expand this research and collaborate with neighbours to assess numbers and, where possible, monitor impacts on other land. Where boar effects are too intense or too damaging we will consider controlling their numbers (with the caveat that we don’t want to push animals off onto neighbours’ land where they are not wanted). Conversely, we recognise that wild boar were once, and can again become, a key ecosystem engineer. As a nature-recovery company, Highlands Rewilding aims to restore natural processes, including disturbance. If we find that over time our resident boar are in balance with their ecosystem, or that their numbers are indeed declining systemically with negative impacts on that ecosystem, we will seek to provide them a safe haven, just as we try to do for many other species of wild animals on our land.
References
1. Yalden, D. The History of British Mammals. (A&C Black, 2010).
2. NatureScot. Managing feral pigs in Scotland. NatureScot https://www.nature.scot/professional-advice/land-and-sea-management/managing-wildlife/managing-feral-pigs-scotland.
3. Goulding, M. Native or alien? The case of the wild boar in Britain. Invasive and Introduced Plants and Animals: Human Perceptions, Attitudes and Approaches to Management 289 (2012).
4. Horčičková, E., Brůna, J. & Vojta, J. Wild boar (Sus scrofa) increases species diversity of semidry grassland: Field experiment with simulated soil disturbances. Ecol. Evol. 9, 2765–2774 (2019).
5. Sims, N. K. E. The ecological impacts of wild boar rooting in East Sussex. https://britishwildboar.org.uk/The%20ecological%20impacts%20of%20wild%20boar%20rooting%20in%20East%20Sussex.pdf (2006).
6. Milton, S. J., Dean, W. R. J. & Klotz, S. Effects of small-scale animal disturbances on plant assemblages of set-aside land in Central Germany. J. Veg. Sci. 8, 45–54 (1997).
7. Labadessa, R. & Ancillotto, L. Beauty and the beast: multiple effects of wild boar rooting on butterfly microhabitat. Biodivers. Conserv. 32, 1189–1204 (2023).
8. Cabon, V. et al. Endangered animals and plants are positively or neutrally related to wild boar (Sus scrofa) soil disturbance in urban grasslands. Sci. Rep. 12, 16649 (2022).
9. de Schaetzen, F., van Langevelde, F. & WallisDeVries, M. F. The influence of wild boar (Sus scrofa) on microhabitat quality for the endangered butterfly Pyrgus malvae in the Netherlands. J. Insect Conserv. 22, 51–59 (2018).
10. O’Bryan, C. J. et al. Invasive wild pigs (Sus scrofa) as a human-mediated source of soil carbon emissions: Uncertainties and future directions. Glob. Chang. Biol. 28, e1–e3 (2022).
11. Don, A. No threat to global soil carbon stocks by wild boar grubbing. Global change biology vol. 28 685–686 (2022).
12. Don, A., Hagen, C., Grüneberg, E. & Vos, C. Simulated wild boar bioturbation increases the stability of forest soil carbon. Biogeosciences 16, 4145–4155 (2019).
13. Wirthner, S. et al. Do changes in soil properties after rooting by wild boars (Sus scrofa) affect understory vegetation in Swiss hardwood forests? Can. J. For. Res. 42, 585–592 (2012).
14. Brunet, J., Hedwall, P.-O., Holmström, E. & Wahlgren, E. Disturbance of the herbaceous layer after invasion of an eutrophic temperate forest by wild boar. Nord. J. Bot. 34, 120–128 (2016).