Can the ecological functions of wolves be substituted?
About six years ago, I wrote about the ecological incompleteness of Britain arising from the loss of its top predators (1). It was annoying then that the prevailing dead hand of the conservation industry denied the biophysical and ecological reality of the wildness that those predators imbue, so reducing our horizons and fatally restricting us to an inherited incompleteness. I made the case that we could learn from the example of contemporary research in America and elsewhere knowing that there would be those naysayers who would say that that America has no lessons for the ecology of Britain, that wilderness has no relevance here. I gave robust examples from contemporary research in America that had resonance with Britain, the intra-guild predation of the mesopredator coyote (Canis latrans) by wolves that resulted in a four-fold higher survival of pronghorn antelope (Antilocapra americana) fawns, the prey of the coyote. The parallel here was with the expected effect of reinstatement of lynx leading to intra-guild predation of foxes, so reducing this mesopredator’s impact on ground nesting birds. There was also the potential for lynx reinstatement to be beneficial for woodland regeneration through reducing the impact of roe deer, its main prey, a trophic cascade between lynx, roe deer and trees.
I gave another example of a trophic cascade initiated by the reinstatement of the wolf to Yellowstone National Park in Wyoming, where the regeneration of willow species and aspen was not just due to the fall in the number of elk (Cervus Canadensis – the N. American red deer) but also because the elk were avoiding places or browsing less where there was a higher risk of wolf predation, such as along riparian corridors. This was demonstrating a behavioural effect of the predator, modifying the browsing location of the elk through fear of predation, and thus adding another dimension to trophic cascades other than just lethal control of prey (2,3). Like the elk in Yellowstone, burgeoning red deer populations in the Scottish Highlands were blamed for significant ecological impact, especially in holding back woodland regeneration, leading to interest in the reinstatement of wolves to control deer and reverse the ecological degeneration. I gave the findings of a study that predicted that the non-lethal behaviourally-mediated effects of wolves would also have a profound effect on deer behaviour in Scotland, and consequently on the ecosystems in which they lived, the message being that it was important to learn the lessons from analogous ecosystems where the wolf is extant, and that fewer wolves may be needed than indicated by predator–prey modelling to have significant positive impacts on ecosystems in the Scottish Highlands.
Imagining lynx adding to the atmosphere of wildness
It seemed to me important that if we were to progress with reinstating the ecological richness of wildness in Britain, then we needed to become familiar with ecological concepts like mesopredators and trophic cascades, and how they applied here. What set me on that course was an article that I had written a few months before about the impact of rabbits on the woody vegetation of the Craven limestone in the southern Yorkshire Dales (4). I had been watching roe deer in the woodland of this limestone pavement country for a number of years, but it was the plague of rabbits there that gave rise to the most extensive damage seen on tree saplings in the nibbled stems and debarking holding back their growth. The damage was mostly in the larger openings in woodland, where the rabbits had a clear view around them and could flee if danger presented, and not at the woodland edge. My conjecture was that foxes as a predator were an important influence on where the rabbits fed, and were therefore in effect protecting the young tree growth of the woodland and woodland edge through inducing fear of predation. I also speculated about whether lynx, as an ambush hunter, had historically clung on much later in this rocky landscape and, as I watched roe deer in this limestone landscape, I tried to imagine lynx being reinstated there, adding to the atmosphere of wildness. If the lynx got tired of eating roe deer, then it could begin to make a dent in the rabbit population instead, all the while protecting new tree growth from browsing and nibbling.
It was about two and half years ago that I gave up writing on what often was the more popular theme of a critique of the conservation industry, and focused instead on the ecological restoration of Britain. Since then I have considered the ecological consequence of predator removal in the mass culling of badgers, and the process of voluntary reinstatement of top carnivores in the return and legal protection of the wolf in Denmark and the Netherlands (5) the lack of natural control mechanisms in relation to deer browsing pressure, and where lynx could be reinstated (6) habitat fragmentation and how that affects the ecology of mammals (7) the role of large carnivores as the focal species for reinstatement of natural processes in Britain and the positive ecological relationships associated with them (8) the ecological requirements of focal species and their need for complex, vegetated landscapes (9) the return of the wolf (10) and lynx (11) to Britain, giving the experience of France; the impact of the reinstatement of beaver (12) and the range expansion of wildcat and pine marten (9). However, ecological restoration is not just about the reinstatement of the animal kingdom, and must be accompanied by the reinstatement of natural vegetation (13) so that it leads to spontaneous woodland perpetuation and wildwood structure (14) old growth woodland and the processes of rotting and decay (7). Thus it is about restoration of completely functional ecological systems at all trophic levels (9,15).
For some of the ecology of natural systems that
I described in those articles, I used examples from
continental Europe where wolf, lynx, bear and bison roam, and where there are
still old growth forests and the many species that inhabit its range of
ecological niches, from fungi and beetles to bats, woodpeckers, owls, eagles
and small mammals. I learnt of this through writing a chapter on the
ecological values of Europe’s wilderness for a forthcoming book (16). In its
most evolved state to date, I gave in that chapter my description of the
dependence of the natural processes of a wilderness on the presence and
abundance of organisms with particular functional traits:
Rumblings over the recovery of vegetation in Yellowstone
The publication of the book is still some months
away, and I fear that my chapter will be sorely out of date when it is finally
out, given that new research is coming out of locations such as the forests of
Białowieża National Park, Poland, where there is scope to research the
interaction of wolves, lynx, red and roe deer and wild boar, and the fear of
predation (17,18). In spite of this, there is of yet no studies that I know of
from Europe that show the effect of predators on vegetation through the
mediation of herbivore pressure, and thus I have still to turn to America. The
recourse is often to Yellowstone where wolves were reinstated in 1995 as part
of a recovery program for this species following their extirpation from the
area in the early 1900s (19). Their reinstatement in Yellowstone had been
foreseen in the late 1960s when the National Park Service wildlife management
policy changed to allow populations to manage themselves, many suggesting at
the time that the regulation would only succeed in the presence of the wolf
(20). There are, however, continuing rumblings over whether there is a real
recovery of vegetation in Yellowstone, and if it can be ascribed to a
behaviourally mediated trophic cascade initiated by the wolves, or if there
are other factors involved (21,22,23,24). George Monbiot invested his own
personal kudos in this, when his description of the trophic cascades in
Yellowstone in a TED talk filmed in Edinburgh in 2013, was subsequently
excerpted and remixed with video footage of Yellowstone
in How wolves change rivers (25). This
video drew the ire of one of the co-authors of a paper reporting 30 years of
willow-monitoring data in Yellowstone’s Northern Range, and which found there
was no difference in willow height after reinstatement of the wolf, ascribing
it to a continuing low water table due to lack of beaver dams arising from the
absence of beaver (26). Hobbs was unflattering about the video (27):
The discrepancies have arisen because different studies have measured different things (average aspen sucker heights compared to tallest five sucker heights) and in different places (riparian, non-riparian) or have had too many variables (willow, water, elk, wolves) and few have had follow up studies that tracked changes as long as possible. Partly, it’s a question of hysteresis, the imperfect expectation that returning one thing should return the system to original conditions. However, wolves have only been back since 1995, and so most studies of effects have only had a few years and none are published yet that cover the last five years, giving the potential of 20 years rather than the 10 (or even three) years of most studies. Thus for some of the rumblings, like the apparent lack of aspen regrowth despite a 60 percent decline in elk numbers (28) that elk rarely changed their feeding behaviour in response to wolves in Yellowstone (29) and that scientists, wolf advocates and the news media alike were painting an overly positive image of the wolf in the ecological restoration of Yellowstone (30) I was able to assuage Monbiot’s anguish by showing that there were counter arguments published (31,32,33). On the issue of beavers and willow, I think the linking of the need for the return of beavers before the effect of wolves will become apparent is a bit of a stretch, since most streams in Yellowstone lack either suitable vegetation or a sufficiently low gradient to provide beavers with habitat (34). It is also a bit of an endless circular argument to say that beaver decline between 1921 and 1953 resulted from an elk explosion due to extirpation of wolves, leading to over browsing and loss of willow, the return of willow after reduction in elk by reinstatement of wolf forestalled because of the lack of wetness due to a lack of action of beaver because of a lack of willow (35,36).
As it was, the example of Yellowstone was cited in a draft reintroductions policy that was circulated around the advisory group of Rewilding Britain, drawing forth the comment that it should be included, but that it was important to recognise the difficulties with it, that there were accusations that some ecologists were over stating the importance of the wolf driving trophic cascades, and which was raising too high an expectation for the impact of returning wolves. Reading this comment prompted me to find and circulate amongst the group a photo of an interpretation board that I took in Yellowstone in 2008, and which explained the ecological or life zones in the Greater Yellowstone Ecosystem, the range of open landscape types shown alongside the various forest types. My first impression on arriving was surprise at the extent of open space in the National Park, but this proved to be an illusion, since 80% of the park is forested (37). The grasslands of the Hayden and Lamar Valleys were typically illustrative of the park because of the presence of buffalo, but I thought the group would be surprised to see how sparsely colonised they are by grassy vegetation.
I explained that the interpretation board reinforced my observation over the two weeks I was there, that edaphic and climatic factors were the major drivers of the vegetation in the eight ecological zones identified in the park, including the Hydrothermal Zone of this caldera (a cauldron-like volcanic feature formed by the collapse of land, following a volcanic eruption) and which encompasses a moisture spectrum from dry sagebrush shrublands to wet alpine meadows. There is a Humboldt-like graphic for zonation of plant life with altitude and associated climate for Yellowstone (38) which in combination with a map of vegetation communities of the National Park (39) illustrates this point very well, conifer woodland dominating away from the open areas above the tree-line and at lesser altitudes in the park, with Douglas-fir forests at lower elevations and Lodgepole pine, Engelmann spruce, subalpine fir, whitebark pine, and limber pine at higher elevations (37). I doubted that trees would pop up generally in the grasslands of the Hayden and Lamar valleys if the grazing pressure was reduced (I am not aware that it did when buffalo there fell to less than 25 in the early 1900s (40,41)) because the low fertility and moisture is against tree growth. However, it is where the moisture is greater, the palatability of woody species to elk higher, that tree growth likely waxes and wanes with herbivore pressure, and where an influence of wolves is seen. This is the small clones of aspen interspersed among the sagebrush/forest transition zone along the major rivers drainages, the streamside vegetation of cottonwoods, willows, and dwarf birch, and the willow flats of wetland areas. These habitats constitute a small area in Yellowstone: wetlands are only 10.3% of the total area, and of that 51.8% is wet meadows, swamps and marshes, 44.1% is lakes and ponds, and only 4.1% is riparian wetland (42). Thus spatially, the impact of the wolf on vegetation was always going to be far less overall than the impact of climate and soils. While I saw wolves in Yellowstone, and a distinctly polarised distribution of elk away from where there were wolves, I don’t remember seeing any major engineering evidence of beaver (I was not apparently in the right places to see this) but I did see regenerating aspen and lots of willow flats. I speculated that given the potential natural vegetation of Britain based on climate and soils, and its palatability to native herbivores, that the spatial impact of behaviourally mediated trophic cascades instigated by either wolf or lynx would be far greater here, as will be the impact of beaver (12,13).
A process of public engagement that offers lessons
In another sense, Yellowstone may not be the right example for us because of its spatial heterogeneity at a landscape scale, the 20% of open landscape interleaved around and above the forests. Due to the high availability of refuges and foraging sites in such landscapes, prey are able to reduce predation risk by shifting to different locations and habitats. Thus a recent paper contrasted the situation in Yellowstone with that in the Białowieża Primeval Forest in Poland where deer focus their foraging activity on small food-rich forest gaps, which turn out to be “death traps” as the gaps are primarily targeted by the predators lynx and wolf (43). The structure and extent of the forest leaves little opportunity for moving to low predation risk areas, as a result leaving them exposed to constant high predation pressure. I suppose a similar argument about availability for habitat shift could be said about the elk population in the Rocky Mountain National Park (RMNP) in Colorado, but the deliberations about reinstatement of the wolf, because of the degradation of vegetation in the park by elk, reveal a process of public engagement that offers us lessons.
I first visited RMNP in 2003, and it was during a National Park Service wildflower walk around Lily Lake that the Ranger brought up the high rate of degradation of the aspen and willow in the park from elk browsing, and that wolf reinstatement was being considered to reverse it. He noted that there was some predation of elk in the park, but he did not go into details. I asked whether the evidence for this was from the discovery of elk carcasses (I had seen them in Canada) and a few had been found. It was only afterwards when I was walking some of the Open Space Parks in the Front Range of Colorado that I realised he was alluding to elk predation by mountain lion (Puma concolor). The Colorado Division of Wildlife notes that much of Colorado, including the Front Range, has a presence of mountain lion (44) and it is the largest predator currently in RMNP, although sightings of these solitary, non-pack mammals is rare (45). It must be presumed that the numbers of mountain lion in the park area are insufficient to limit the population of elk.
Elk were extirpated from the area that is now RMNP by 1870 through intensive and unregulated hunting, the meat being sold in Denver (46). Wolves were also hunted out around 1900. In 1913 and 1914, just before the national park's establishment, 49 elk were transplanted from Yellowstone National Park. In the absence of a significant predator, the population flourished to the point in the early 1930’s that concern was expressed about deteriorating vegetation conditions on the elk winter range. From 1944, elk were culled by shooting, maintaining a population between 350 and 800 animals, but the implementation of the natural regulation policy of the National Park Service in 1969 put a bar on culling (and see above for Yellowstone) the hope being that hunting in the national forests around the perimeter of the park would control the elk population in and near the park. However, elk numbers continued to increase, from 500–600 animals to a high point ranging from 2,800 to 3,500 between 1997 and 2001 (47). Changes in vegetation were observed, particularly a decline in willow and aspen on the elk's primary winter range, leading to a questioning of the appropriate elk population size, and associated effects on plant communities and biodiversity (46). The elk winter range is focussed in the eastern side of RMNP and encompasses the aspen and willow vegetation of the wet meadows and long, thin riparian strips of the Montane zone, especially in Moraine and Horseshoe Parks (see Fig. 2 in (47)(48, 49)). Like Yellowstone, there is a correlation between altitude and moisture in the distribution of tree growth, described as a topographic-moisture gradient for the forests of the Rocky Mountains where grassland is found at both low and alpine elevation with varying conifer forests laying in between, these giving way to deciduous trees and shrubs at most altitudes where there are wet valley bottoms (see Fig. 3.3 in (50)).
By the time I revisited RMNP in 2008, the decision had been taken not to reinstate wolves and instead elk would be culled, the meat being distributed by the Colorado Division of Wildlife to local residents on a lottery basis (51). The Elk Vegetation Management Plan that is being implemented now relies on a variety of approaches, including fencing, vegetation restoration, culling, and elk redistribution to reduce the impacts of elk on vegetation and restore the natural range of variability in the elk population and affected plant and animal communities. The fencing is expected to be temporary, and will be in place until monitoring results, assessed every 5 years (consumption will be measured annually (47)) indicate that willow, aspen and other plants can sustainably withstand browsing. Just over 5% of the open habitat types (non-forested) on the elk winter range (3,400 acres) had fencing installed during 2008-2011, protecting 145 acres of willow habitat and 45 acres of aspen habitat on the elk winter range on the east side of the park. Fencing was also installed in autumn 2011 to restore about 16 acres of riparian habitat on the summer range in the Kawuneeche Valley on the west side of the park. Thus far, this represents just over one-third of the 600 acres identified in the management plan to be fenced. The exclosure fencing is just over 6 feet high, enough to keep out elk but allowing smaller animals to enter under the 16 inch gap at the bottom (52,53). The hope is that once willow vegetation is restored to an acceptable level, beaver populations may increase or recolonise riparian areas on the elk range, which could improve hydrologic conditions that support additional restoration (51). It is estimated that a minimum of approximately 10 acres of restored tall willow would support a beaver family over the long term. If natural beaver recolonization does not occur, reintroduction would be considered.
Much as I was disappointed, it is worth reviewing the process with which the National Park had explored the alternatives with the local community, and then made its decision, for the lessons that can be learnt. In 2003, as part of a scoping process, public meetings had taken place in communities to the E and W of the park (Estes Park, Loveland, Boulder, and Grand Lake) where issues, concerns and ideas for potential management actions were identified before beginning the planning process (51,54). Collectively, 1,137 comments were received by letter, fax, and electronic mail; through the Internet; from public workshop results; on comment forms distributed via postal mail, and at visitor centres in the park. In 2004, the public meetings focused on reviewing and commenting on six management alternatives that were a mix of various levels of lethal control by shooting, exclosure fencing, fertility control agents, redistribution by herding, hazing (e.g., rubber bullets, cracker shot, or other dispersal actions) and other aversion techniques, the reintroduction of 14-20 wolves into RMNP, and an option to do nothing (55). None of the six alternatives clearly stood out as receiving only favourable or unfavourable comments. The Pros and Cons for “Alternative D - Wolf Reintroduction in Rocky Mountain National Park” make for interesting reading, particularly the Pros grouped under “Naturalness or ecological integrity” and “Practicality, efficiency, or efficacy”. Proponents of Alternative D felt it assisted in elk dispersal, had the greatest long-term result, and would restore a missing component of the ecosystem. Opponents felt it "created more problems than it solved" was "impractical" and that RMNP was too small for wolf reintroduction. A recurring question that was asked was whether funding to mitigate impacts to livestock producers would be available.
Five alternatives were taken forward into a Draft Elk and Vegetation Management Plan and Environmental Impact Statement: no management; rapid lethal reduction combined with redistribution; gradual reduction of the elk population by lethal reduction combined with fences and redistribution methods; gradual reduction of the elk population by lethal reduction and fertility control combined with fences and redistribution methods; reduction of the elk population by lethal shooting combined with release of two pairs of wolves to begin the “natural redistribution” of elk, the population of wolves being allowed to gradually increase to a maximum of 14 over the 20 year life of the plan (pub). The draft plan/EIS was released for public review and comment in 2006, public meetings were again held in local communities, and over 2,600 responses were received that contained about 3,146 comments in the form of letters, emails, internet submissions, comment forms at public meetings, and petitions (56). On receipt and analysis of the public comments, the National Park Service re-evaluated the alternatives and came up with the decision in 2008 that its preference was for Alternative 3, the gradual reduction of the elk population by lethal reduction combined with fences and redistribution methods (56).
A natural means of elk redistribution could still be considered
The decision was made on the basis that the
choice of that alternative reflected public opinion as well as meeting the
general management objectives of the National Park Service for protecting park
resources and values while being consistent with the park's enabling
legislation, purpose, mission, and goals. In a somewhat confusing section of
the Record of Decision, Alternative 5 with the reinstatement of wolves is
shown to the best at meeting the criteria required of Federal Plans for the
choice of environmentally preferred alternative that will best promote
national environmental policy, thus making it the Environmentally Preferred
Alternative under those criteria. It is perhaps the recognition that this
alternative would present “logistical challenges and require significantly
higher levels of park resources to continuously monitor and manage a wolf
population that would be maintained within the park boundaries” and that
instead the chosen Alternative 3 would have a “high degree of certainty of
being successful, and implementation will be less complex compared to
Alternative 5”. As noted above, monitoring of vegetation condition, as
well as annual consumption through offtake, is a key part of the
implementation of the plan, and the frequency and intensity of the use of
redistribution methods could be increased as needed to disperse elk or move
them to the primary summer range. The decision document holds out some hope
that a natural means of elk redistribution could still be reconsidered (56):
I would not want to necessarily pre-empt that reconsideration, but a study in 2011 by college students from nearby Denver on the effectiveness of the elk and vegetation management plan compared the size and number of willow, and whether it had been browsed, in sample plots along riparian habitat in Moraine and Horseshoe Parks – half of the plots were within the fenced elk exclosures, and the other in unfenced areas (57). Total measurements of individual willows showed unsurprisingly that in the fenced areas new growth, height, circumference, and plot density was greater than in the unfenced areas, which they took to mean that culling was not an effective management tool, whereas fencing in the long term would be a block on the natural and necessary component of deer browsing. They also did not find any noticeable signs of beaver re-establishment within Moraine and Horseshoe Parks. Their conclusion – “From our research, it appears that in order to successfully revive a healthy and natural ecosystem there needs to be a top predator introduced which will likely alter not only elk population but also elk behaviour”
It is perhaps too much to expect to have gotten clear evidence of the effectiveness of the Elk and Vegetation Management Plan after only two years of its implementation, especially since the yearly culling program for reduction of elk was only a few seasons in on the 20 year program when the study was carried out, and just 131 elk had been removed in that period (51). A more persuasive argument was made in a paper that looked at the ethical and philosophical arguments about the choice of culling and fencing over the reinstatement of wolves (58). The authors assert that elk culling practices seek not to preserve, conserve or restore, but rather to manage the health and functioning of the ecosystem through the replacement of the function of one ecological component with another. This is a “functionalist" view of management of the degradation rather than restoration. They argue that it is wrong to assume that the ecological function of wolves can be substituted by surrogates that serve the same function, in this case the human culling of elk (predation) and the fencing exclosure of willow and aspen habitat (redistribution). They point out that many of the arguments against returning wolves to RMNP were neither functional nor ecological, but were instead social and political, and were rooted in the legal liability, and arguably moral culpability, that RMNP would assume if wolves were to prey upon livestock or humans. Thus in that respect, the functional substitution, a “Replacement Restoration” which operates under a presumption that there is no return to the past, was chosen because it could be achieved in a socio-politically acceptable manner. In an irony that the authors noted, RMNP had welcomed the voluntary re-establishment of wolves to the park, but the park cannot initiate this re-establishment, lest RMNP be strictly liable for negative outcomes following from the wolves’ reintroduction. As they explain, RMNP’s naturalness, and historical continuity, were both threatened by its lack of wolves. Reintroducing wolves to RMNP, a “Reparative Restoration” would allow the park to address directly the threat of trophic downgrading, and thus better meet its mandates and preserve natural conditions and processes – “Wolf reintroduction will be hard to justify in many other places, but RMNP is one of few potential havens”
Fences have no place in truly wild systems
I noted last time that fencing is an acknowledgment that we are failing to coexist with and successfully protect wild nature (15). I am thus thankful that the exclosure fencing in RMNP went up in the years after my last visit so that I did not come across it. My observations of elk in RMNP have anyway been when they are in their summer range on the open alpine meadows around the Ute Trail, and where there are no deciduous trees that could be damaged. In early 2009, within a few months of that last visit, the extensive wild backcountry of RMNP, including those alpine meadows, was finally and quite rightly designated as wilderness, encompassing 94% of the park area. (59). Although I can’t be wholly certain of this, the areas with fencing exclosures in RMNP are mostly in the parts of the park that were not included in the designated wilderness area (compare Figs. 4 and 5 in (47) with the wilderness map in (60)). This is also a relief since fences have no place in truly wild systems like a designated wilderness. It is the case that ecological restoration in Britain often has to start with fencing to exclude the centuries of unrestrained herbivore pressure that degraded and simplified the vegetation cover and, sometimes, the return of vegetation species has to be given assistance when there is no nearby source from which it can return (i.e.(14)). However, at some point, that fencing becomes the discussion about whether the only path forward is a thoroughly managed future, increasingly devoid of the naturalness of what should be a functional consumptive pressure that is restrained by predation. If we take the fences down, then the risk is that the degradation cycle of the vegetation from unrestrained herbivore pressure begins again, unless we do something about the herbivores. The arch-grazers and their herbivore-driven landscapes will say that fencing gives them control, that they can regulate the grazing within fenced enclosures in a way that substitutes for the absence of top predators by “pulse” grazing (61). However, that really does tie the future of an increasingly less wild nature to human influence and control. The awfulness of that prospect can be no better argument for reversing the ecological incompleteness of Britain.
Mark Fisher 5 September 2015
(1) Fisher, M. (2009) Ecological incompleteness and our missing top predators: learning the lessons from abroad. Wilder Horizons 1: 14-16
(2) Lima, S.L., and Dill, L.M. (1990) Behavioral decisions made under the risk of predation: a review and prospectus. Canadian Journal of Zoology, 68: 619—640
(3) Brown, J. S., Laundré, J. W., & Gurung, M. (1999). The ecology of fear: optimal foraging, game theory, and trophic interactions. Journal of Mammalogy, 80(2), 385-399.
(4) Reintroducing lynx – sensing an atmosphere of wildness, Self-willed land February 2009
(5) Ecological consequence of predator removal, Self-willed land July 2014
(6) Lack of natural control mechanisms - the missing lynx, Self-willed land June 2014
(7) Habitat fragmentation and the ecology of artefacts, Self-willed land January 2015
(8) Large carnivores as the focal species for reinstatement of natural processes in Britain, Self-willed land November 2014
(9) The third dimension is the last refuge of the wild, Self-willed land December 2014
(10) Cry wolf - the return of Britain's top predator, Self-willed land February 2015
(11) Lynx UK Trust lets the cat out of the bag, Self-willed land April 2015
(12) Coastal temperate rainforest - in Britain?!, Self-willed land June 2015
(13) The natural vegetation of England, Self-willed land August 2014
(14) Watching the naturalness return to the Carrifran Valley, Self-willed land April 2015
(15) A challenge to Rewilding Britain, Self-willed land August 2015
(16) Fisher, M., Ecological values of wilderness in Europe, in: K. Bastmeijer (ed.), Wilderness Protection in Europe: The Role of International, European and National Law,
Cambridge: Cambridge University Press 2016 (forthcoming)
(17) Kuijper DPJ, Verwijmeren M, Churski M, Zbyryt A, Schmidt K, et al. (2014) What Cues Do Ungulates Use to Assess Predation Risk in Dense Temperate Forests? PLoS ONE 9(1): e84607 http://www.plosone.org/article/fetchObject.action?uri=info:doi/10.1371/journal.pone.0084607&representation=PDF
(18) Wikenros, C., Kuijper, D. P., Behnke, R., & Schmidt, K. (2015). Behavioural responses of ungulates to indirect cues of an ambush predator. Behaviour 52: 1019 - 1040
(19) Wolf Restoration, Yellowstone National Park, National Park Service
(20) Wolf Restoration Continued, Yellowstone National Park, National Park Service
(21) Are Wolves Saving Yellowstone’s Aspen Trees from Elk? USGS News 1 September 2010
(22) Rethinking predators: Legend of the wolf, Emma Marris, Nature News Feature 7 March 2014
(23) Is the Wolf a Real American Hero? Arthur Middleton, New York Times 9 March 2014
(24) Yellowstone wolves take a blow to their rep, Liza Lester, Ecological Society of America News 11 March 2014
(25) How Wolves Change Rivers, Sustainable Human Transition Consultancy14 February 2014
(26) Marshall KN, Hobbs NT, Cooper DJ. (2013) Stream hydrology limits recovery of riparian ecosystems after wolf reintroduction. Proc R Soc B 280: 20122977.
(27) Wolf-driven trophic cascade is overblown, Mike Koshmrl, Jackson Hole News and Guide 14 May, 2014
(28) Kauffman, M. J., Brodie, J. F., & Jules, E. S. (2010). Are wolves saving Yellowstone's aspen? A landscape-level test of a behaviorally mediated trophic cascade. Ecology, 91(9), 2742-2755.
(29) Middleton, A. D., Kauffman, M. J., McWhirter, D. E., Jimenez, M. D., Cook, R. C., Cook, J. G., & White, P. J. (2013). Linking anti-predator behaviour to prey demography reveals limited risk effects of an actively hunting large carnivore. Ecology letters, 16(8), 1023-1030
(30) Mech, L. D. (2012). Is science in danger of sanctifying the wolf? Biological conservation, 150(1), 143-149
(31) Beschta, R. L., & Ripple, W. J. (2013). Are wolves saving Yellowstone's aspen? A landscape-level test of a behaviorally mediated trophic cascade: comment. Ecology, 94(6), 1420-1425
(32) Creel, S., Winnie, J. A., & Christianson, D. (2013). Underestimating the frequency, strength and cost of antipredator responses with data from GPS collars: an example with wolves and elk. Ecology and evolution, 3(16), 5189-5200.
(33) Bruskotter, J.T. (2013) To the editor: If science is “sanctifying the wolf” the news media is not complicit. Biological Conservation 158: 420
(34) Beavers, Yellowstone National Park, National Park Service
(35) Wolf, E. C., Cooper, D. J., & Hobbs, N. T. (2007). Hydrologic regime and herbivory stabilize an alternative state in Yellowstone National Park. Ecological Applications, 17(6), 1572-1587
(36) Beaver Habitat and Population, Yellowstone National Park, National Park Service
(37) Forests, Yellowstone National Park, National Park Service
(38) Ecological Zones and climatic data in Yellowstone National Park
(39) Vegetation Communities, Yellowstone National Park, National Park Service
(40) Yellowstone Bison, Yellowstone National Park, National Park Service
(41) Greater Yellowstone Ecosystem – the island of hope, Self-willed land August 2008
(42) Elliott, C. R., and M. M. Hektner. 2000. Wetland Resources of Yellowstone National Park. Yellowstone National Park, Wyoming
(43) Schmidt, K., & Kuijper, D. P. (2015). A “death trap” in the landscape of fear. Mammal Research, 1-10
(44) Lion Country: Part 2, Colorado Division of Wildlife
(45) Mountain Lions, Rocky Mountain National Park, National Park Service
(46) Background: Elk Vegetation Management Plan, Rocky Mountain National Park, National Park Service
(47) Zeigenfuss, L., Johnson, T., and Wiebe, Z., 2011, Monitoring plan for vegetation responses to elk management in Rocky Mountain National Park: U.S. Geological Survey Open-File Report 2011–1013
(48) Rocky Mountain National Park, Colorado 2001-2005 Vegetation Classification and Mapping. Final Report 2005
(49) Graphic showing the distribution of vegetation communities from the ROMO vegetation inventory, Vegetation Inventory Project for Rocky Mountain National Park. 2001-2006. Project-1047727
(50) Peet, R. K. (1988) Forests of the Rocky Mountains. Pages 73-101 in M. G. Barbour and W. D. Billings, editors. North American terrestrial vegetation. Cambridge University Press, New York.
(51) Elk & Vegetation Management Plan Fact Sheet, Rocky Mountain National Park, National Park Service August 2012
(52) VerCauteren, K. C., N. W. Seward, M. J. Lavelle, J. W. Fischer, and D. L. Phillips. 2007. A fence design for excluding elk without impeding other wildlife. Rangeland Ecology and Management 60: 529-532
(53) Gage, E. & Cooper, D. (2009) A Study of the Effectiveness of a Fence Design in Excluding Elk and Moose but Allowing the Movement of Other Wildlife. Final Research Report RM-CESU Cooperative Agreement Number: H1200040001
(54) Public Involvement: Elk Vegetation Management Plan, Rocky Mountain National Park, National Park Service
(55) Draft Alternatives Report for the Rocky Mountain National Park Elk and Vegetation Management Plan/Environmental Impact Statement, Department of the Interior National Park Service. Prepared by Parsons Denver, Colorado November 2004
(56) Record of Decision. Final Environmental Impact Statement/Elk and Vegetation Management Plan, Rocky Mountain National Park, National Park Service February 2002
(57) Arellano, M., Brunner, N., Geuder, S. and Shoppell, K. (2011) A Study of the Effectiveness of Rocky Mountain National Park Elk and Vegetation Management Techniques. Environmental Field Studies, Fall 2011, Metropolitan State College of Denver
(58) Hermans, A. P., Lee, A., Dixon, L., & Hale, B. (2014). Wolf Reintroduction: Ecological Management and the Substitution Problem. Ecological Restoration, 32(3), 221-228
(59) Omnibus Public Land Management Act of 2009 - Public law 111-11 (3/30/2009) An act to designate certain land as components of the National Wilderness Preservation System, to authorize certain programs and activities in the Department of the Interior and the Department of Agriculture, and for other purposes.
(60) Rocky Mountain National Park Wilderness Map
(61) Whitbread, T. (2014) Can we rewild Britain? Living Landscapes, Natural World, News from the Wildlife Trusts across the UK, Summer 2014 pg. 10-13