The effects of the reintroduction of wolves in Yellowstone National Park, USA, have been widely studied – the prolonged absence and then reintroduction of wolves at the top of the food chain makes this case study a great natural experiment into ecosystem dynamics. This post will collate the interesting bits from the research, and look at the public perceptions of such a reintroduction, to see if it could be done elsewhere.
Predation and Competition
Grey wolves (Canis lupis) were completely removed from Yellowstone National Park, northwest USA during the mid-1920s, until being re-introduced in 1995-1996 (Ripple & Beschta, 2012). In the 70 years without wolves, populations of elk and bison thrived, whilst vegetation was far less common due to unregulated grazing. The simplest food web to visualise this would have wolves at the top, bison and elk below, then vegetation such as aspen, willows and cottonwoods at the bottom (Ripple & Beschta, 2012):
Browsing (tree eating) across the north of Yellowstone National Park was at 100% in 1998, shortly after wolf reintroduction – but by 2010, it had reduced to 25% in upland areas and 10% in riparian (riverside) areas (Ripple & Beschta, 2012). This reduction was due to the wolves controlling the population of deer, which allowed the growth of aspen and other plants to be restored. In turn, this allowed beaver and bison populations to increase, with an increase in vegetation reducing the need for competition between species.
“During the seven-decade wolf-free period, the collapse of a tri-trophic cascade allowed elk to significantly impact wildlife habitat, soils, and woody plants. For example, species such as aspen and willows were generally unable to successfully recruit young stems.” – Ripple & Beschta, 2012
The reintroduction of wolves also changed the distribution of elk across the park, whose new priority was to avoid getting eaten. In fact, since the reintroduction of wolves bison and elk have changed their foraging behaviour, often reducing foraging time in exchange for increased vigilance (Laundré, Hernández & Altendorf, 2001). Because wolves so effectively control elk population, culling seen from the 1920s-1960s is no longer required (Beschta & Ripple, 2016). As well as direct predation, wolves affect other large mammals through competition: since the reintroduction of wolves, a reduction in coyotes has also been recorded. This has increased the number and diversity of small mammals, which has benefitted birds and other mammals like badgers and red foxes (Ripple & Beschta, 2012).
Improving habitat quality
The restoration of plants has greatly benefitted those reliant on the habitat they provide. For example, songbird richness has greatly increased across Northern areas of Yellowstone (Ripple & Beschta, 2012), and the number of beaver colonies has increased from 1 in 1996 to 12 in 2009 (Ripple & Beschta, 2012). Beavers are good news for rivers: they reduce streambank erosion, improve nutrient cycling and improve both vegetation and animal biodiversity within the rivers themselves (Beschta & Ripple, 2016). For example, streams with beaver ponds in Wyoming had 75x the amount of wildfowl than those without dams (Ripple & Beschta, 2012).
“In the first two decades following the 1995–1996 reintroduction of wolves, all but two of the 24 northern range studies of deciduous woody species found young woody plants increasing in size or frequency.” – Beschta & Ripple, 2016
Before wolves were reintroduced to Yellowstone, most people (from a survey of over 1000 visitors) favoured the return of wolves, with 80% agreeing that wolves had a place in modern-day Yellowstone, and over 70% agreeing that wolves would help maintain balanced wildlife in the park (McNaught, 1985).However, there were still concerns from some concerning the safety of people and of livestock, and on the high cost of the reintroduction.
A study conducted much after the reintroduction of wolves found that these concerns were actually unfounded: Duffield, Neher & Patterson (2008) estimated that the annual number of visitors to the region increased by 1.5% in the spring and 5% in autumn directly because of the presence of wolves – people who go to Yellowstone specifically to see wolves spend $35.5 million every year.
The evidence from Yellowstone points to the potentially widespread ecological and economic success that controlled reintroduction of large mammals can have on an area. But could it happen anywhere else? In the Scottish Highlands, wolves were almost completely wiped out by 1769, allowing deer populations to get out of hand. The Highlands have been assessed as a potential location for reintroduction in the UK, since the human population there is low, and the wolves’ main prey would be deer (Landon, 2020). And though some concerns have been voiced about predation on livestock, general public opinion seems to be favourable. So perhaps wolves might be seen around the UK a lot more frequently soon….
“Results from Yellowstone, other areas in western North America, and around the world increasingly point to a need for recovering ecologically effective populations of large predators to help recover or maintain biodiversity in ungulate populated landscapes” – Beschta & Ripple, 2016
Beschta, R.L.& Ripple, W.J. (2016) Riparian vegetation recovery in Yellowstone: the first two decades after wolf reintroduction. Biological Conservation, 198, pp.93-103
Duffield, J.W., Neher, C.J.& Patterson, D.A. (2008) Wolf recovery in Yellowstone: park visitor attitudes, expenditures, and economic impacts. The George Wright Forum, Vol. 25, No. 1, pp. 13-19
Landon, R. (2020) Exploring the Possibility of Reintroducing Wolves to the Scottish Highlands as a means of Ecological Conservation. The Irvine Atlas, p.71
Laundré, J.W., Hernández, L. & Altendorf, K.B. (2001) Wolves, elk, and bison: reestablishing the” landscape of fear” in Yellowstone National Park, USA. Canadian Journal of Zoology, 79(8), pp.1401-1409
McNaught, D.A. (1985) Park visitors attitudes towards wolf recovery in Yellowstone National Park
Ripple, W.J. & Beschta, R.L. (2012) Trophic cascades in Yellowstone: the first 15 years after wolf reintroduction. Biological Conservation, 145(1), pp.205-213
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