Mid-Late Holocene Extinctions in the Yukon and Alaska & Implications for Future Restoration
This is a guest article contributed by Rhys Lemoine, a postdoctoral researcher from Canada studying megafauna extinctions at Aarhus University. He also writes articles on megafauna and rewilding - check them out on his Linkedin!
Fig 1. American Bison (Bison bison) in the Yukon Wildlife Preserve.
Terms of use: This image is licensed under a Attribution 2.0 Generic. It is attributed to Gloran Vaclic, and the original can be found here. The image is unedited
The quintessential “ice age” mammal that almost anyone could name is probably the woolly mammoth (Mammuthus primigenius). This animal has dominated our collective imagination since people first entered glacial Eurasia, roughly 40,000 years ago, as recorded in numerous cave paintings (Luzan et al., 2020). What always strikes me as odd (and rather sad) is how recently mammoths went extinct. A common but fun piece of trivia is that mammoths were still around when the ancient Egyptians were building pyramids, in reference to the population from Wrangel Island, off the coast of Siberia, which went extinct roughly 4,000 years ago (Vartanyan et al., 1995). We have been aware of this fact since 1995, but recently it has been determined via environmental DNA that other populations survived on the mainland just as long, i.e. to the mid-late Holocene transition. These include populations on the Taymyr Peninsula in Russia (Wang et al., 2021), but also in the Yukon Territory in Northwestern Canada (Murchie et al., 2022; Murchie, Monteath, et al., 2021). In this article, I will be discussing this and other large mammal extinctions that occurred very recently in the Yukon and adjacent Alaska, and their implications for ecosystem function and restoration in the future.
The latest findings using environmental DNA, i.e. genetic material collected from samples of soil or other substrates, suggest that mammoths survived in the Yukon (specifically near Dawson) until as late as 3800 years ago (Murchie et al., 2022). The same site also produced DNA from wild horses (Equus caballus lambei) for the same period (Murchie et al., 2022). Earlier eDNA studies also show later survival of mammoths and horse in the Yukon and Alaska, though only into the early Holocene (Haile et al., 2009; Murchie, Kuch, et al., 2021). Although a matter of debate, the most recent horse populations from North America are now widely accepted to have belonged to the same species as living horses, including domestic horses (E. c. caballus) and Przewalski’s horses (E. c. przewalskii), with which they cluster genetically (Barrón-Ortiz et al., 2017). At the very least, they were inter-fertile, as studies show that there was genetic exchange between populations on each side of the Bering land bridge during glacial periods, suggesting that living horses are simply the last remnants of a once Holarctic meta-population (Vershinina et al., 2021). The fact that wild horses of the same species that is extant today existed in the Yukon so recently (at least from an eco-evolutionary perspective), is very relevant when we consider that free-living horses are also found in the Yukon today, likely descendants of working horses set loose during colonization (Jung et al., 2015). While generally treated as non-native or even invasive, I would argue that this is a reintroduced population, albeit a very small and localized one, of a native species.
The same is true of another reintroduction to the Yukon, i.e. the wapiti/elk (Cervus canadensis). Although introduced for hunting in the 1950s (Jung et al., 2015), this species was naturally present until very recently. While the youngest dated remains from the Yukon are about 3000 years old, some remains from Alaska near Fairbanks date to only around 200-500 years ago, suggesting that they may have survived much longer (Conroy et al., 2020). Something made clear by the environmental DNA findings for mammoth and horse is that, since fossilization is such an unlikely event in the first place, the youngest fossils may not actually represent the date at which a species went extinct, but merely the point at which it started to become less common. Relict populations can hold on for far longer, allowing for the possibility that the disappearance of wapiti in the north of their range only just happened. Genetics do not really support the idea of distinct subspecies for wapiti in North America and this applies to genotyped specimens from prehistoric Alaska and the Yukon as well, which cluster with modern samples from both North America and the Altai region of Eurasia (Meiri et al., 2018; Meiri et al., 2014). As such, wapiti living in the Yukon today are essentially a reintroduced population of the same taxon that lived there previously.
Fig 2. The Wapiti or American Elk (Cervus canadensis) are present in the modern Yukon, but constitute a reintroduction to the area
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Bison (Bos bison), which have also been reintroduced, are a strange case. Indigenous accounts and dated remains suggest they were present until roughly two centuries ago, and it is common to describe these bison as wood bison (the northern ecotype “athabascae” of American bison, Bos bison bison) but the evidence for that assignment is unclear, and based mostly on skeletal comparisons (Gardner & DeGange, 2003). All wood bison, however, belong to the same mitochondrial group as other American bison, which originated south of the ice sheets via an earlier, larger form (Bos bison antiquus) (Heintzman et al., 2016; Wilson et al., 2008). By contrast, all genotyped Late Holocene bison, some as young as 200-500 years old, from Alaska and the Yukon belong to a mitochondrial group associated with a later migration of steppe bison (Bos bison priscus) from Asia as well as their smaller Holocene descendants (Bos bison occidentalis) (Heintzman et al., 2016; Wilson et al., 2008). Skeletons of these smaller Beringian bison are often confused with modern American bison and transitional forms, leading to the widespread and now erroneous attribution of fossil bison to this type in the lower 48 states (Zver et al., 2021). The genetic data would imply that Beringian-type rather than living American bison were the native type in Alaska and the Yukon until the late Holocene. This would in turn suggest either that wood bison replaced them in a very short time period over a wide distribution, presumably under the same pressure that removed their predecessors, or that wood bison were never present in these regions at all and that indigenous and archaeological accounts are actually of occidentalis-type bison. The possibility for hybridization exists, but we only have genetic evidence of occidentalis moving south into the bison range, not the other way around, and even this seems to have been very limited, with mountains and dense forests preventing much gene flow (Heintzman et al., 2016; MacDonald & McLeod, 1996). Nevertheless, the possibility remains that wood bison retain meaningful introgression from more northern bison, being nearest to the region of overlap, but that this does not reflect in their mitochondrial DNA due to bottlenecks or a bias in hybridization towards male occidentalis and female bison. This might explain some of the unique phenotypic characters of wood bison, though some of these might be epigenetic to a degree (Geist, 1991). None of this is to say that the widespread reintroduction of bison to the Yukon and Alaska using wood bison is not a very good thing, as it is still the best-suited living ecotype for boreal climates. However, if neither American type is original to Alaska/Yukon, and given the genetic bottlenecks in both types (Hedrick, 2009), there may be little reason to control against the two types crossing or to remove populations of the plains type, for example the Delta Junction herd.
It is important to think about the pressures that may have led to the disappearance of these species in the past. The extinctions of megafauna worldwide are widely and consistently attributable to modern humans (Araujo et al., 2017; Bartlett et al., 2016; Lemoine et al., 2023; Sandom et al., 2014), but it makes sense that subarctic areas with lower human densities might have offered more opportunities for refugia (Chaput et al., 2015). As an example, muskoxen (Ovibos moschatus) were notably present further south in the boreal region of the Yukon until about 3500 years ago (Monteath et al., 2021), and their current range may be a due to lower human density rather than habitat requirements, also evidenced by their recent migration into boreal areas in the Northwest Territories (Adamczewski et al., 2021). The arrival and expansion of new human cultures into Alaska and Canada from Siberia starting ~5-6000 years ago, which displaced or integrated with existing populations in the subarctic (Briere & Gajewski, 2020; Stone, 2019), may have added additional pressure to dwindling populations of mammoth and horse. There are accounts in stories by people from Inuit and Dene cultures of animals very reminiscent of woolly mammoths (Johnson, 2019). While later extinctions of wapiti and bison are near-concurrent with European colonization, there was little influx of Europeans themselves into the interior of Alaska and the Yukon before the 1840s or later (Coates, 1980), and there are no European accounts of either species in these regions from this time (that I could find), making their disappearance somewhat of a mystery. There were trade routes between Russians and indigenous peoples on the Pacific coast earlier in the 18th century, potentially leading to technological influx, notably of rifles, which might have improved hunting efficacy, as they did amongst hunters in other colonized parts of the world (Braga Pereira et al., 2020; Grinëv, 2017). As a comparison, the earlier development of other technologies such as dog sleds may have put muskoxen at greater risk in tundra regions due to the ability of dogs to transport carcasses long-distance (Lent, 1998). The Little Ice Age, a period of widespread cooling that occurred prior to the 20th century, would also have been concurrent (Anderson et al., 2011), potentially leading to further stress on populations of more continental species like bison and elk. Diseases introduced by Europeans may also have played a role by severely reducing local human populations (Periferakis, 2019), which may actually have had a negative effect on large herbivores, since prescribed burning would become less common, leading to greater dominance of spruce (Picea), which does not provide usable forage (Chapin et al., 2008).
Although all of this is speculative, that last point is probably the most relevant for rewilding herbivores in the Yukon and Alaska today. Of all the large herbivores that exist in the boreal region now, only one is actually adapted to live in the dominant habitat type, which is relatively closed spruce forest. Woodland caribou (Rangifer tarandus) survive off lichen and moss that can grow in the shady, damp, and acidic conditions in these old-growth coniferous forests (Brown et al., 2007), which also protect them from predators that their tundra relatives instead avoid through migration (Seip, 1991). All other Yukon herbivores rely on early-succession plant types to meet their caloric needs, whether it be grasses, herbs, or woody broadleaves (Jung et al., 2015). Fast-growing, palatable trees and shrubs like alder (Alnus), birch (Betula), aspen/poplar (Populus), and willow (Salix) all provide food for moose (Alces alces), beaver (Castor canadensis), and other herbivores, while tannin-heavy conifers provide very little. The trade-off is that conifers generally require far more time to recover after a fire, though notably pine (Pinus) and larch (Larix) require less time to do so than spruce or fir (Abies). The early establishment of larch in Siberia after the last glacial is credited with its current dominance in Eurasian taiga, in contrast to the dominance of spruce in Canada and Alaska, as fires burn less intensely but more often in larch forests and this would prevent spruce from growing to a reproductive stage (Schulte et al., 2022). While closed-canopy spruce forests were certainly present in the past (Schweger et al., 2011; Schweger & Matthews Jr, 1991), alternative stable states maintained by disturbances like fire, large animals, and later people would hypothetically have created a much more heterogeneous landscape.
One point that is worth exploring is just how the presence of mammoths would have encouraged open conditions. As mammoths were closer to Asian elephants (Elephas maximus) than to African elephants (Loxodonta) taxonomically, we can probably deduce much about their ecology from similarities in the living species (Olivier, 1982). The degree to which living megaherbivores (>1000 kg) create habitat openness seems to be somewhat dependant on productivity, with the least productive areas being naturally open already. Elephants can seemingly be decisive in converting closed habitats to open ones in areas of intermediate productivity, whereas in high-productivity habitats elephants do not change the overall forested nature of the habitat, but can still create and expand open patches and passageways. The boreal region has low productivity (Cramer et al., 1999), suggesting that elephants should have an effect, but it is difficult to compare with current elephant habitats because low-palatability trees, i.e. conifers, are dominant. However, while probably not preferred, mammoths did consume conifers, with twigs of larch, pine, and spruce all occurring in preserved intestines and faeces (van Geel et al., 2008). With the exception of larch (probably due to availability), these species are often given to captive elephants as browse, though as with the mammoth (Olivier, 1982), deciduous hardwoods like alder, birch, aspen/poplar, and willow seem to be preferred (personal observation and various elephant husbandry guides). It is also true that elephants do not only damage trees that they intend to eat (Midgley et al., 2005). Elephants are intelligent animals that will break things for the simple enjoyment of it or, in the case of males that do much of the tree damage, to vent their frustration (Midgley et al., 2005). Trampling of conifer saplings in heavily used open areas could potentially also potentially suppress their growth, especially since open areas might also experience more frequent, less intense fires. As such, the impact that mammoths might have on coniferous habitat is somewhat unclear, though any impact at all would potentially promote heterogeneity, especially in combination with other large grazers and browsers.
Fig 3. Woolly mammoths were potentially ecosystem engineers in Yukon, their browsing and behaviour likely increased mortality amongst trees and may have promoted openness
Terms of use: This image is licensed under a Attribution 2.0 Generic. It is attributed to FunkMonk, and the original can be found here. The image is unedited
The specific impact of other herbivores is worth considering. Bison can also damage adult conifers by rubbing on them or stripping their bark, for example (Bamforth, 2011; Nieszała et al., 2022; Safronov et al., 2012). Moose and wapiti can also have strong effects on both adult and seedling conifers, though this may affect non-spruce species like pine and fir (Abies) more so (Brandner et al., 1990; Lyly et al., 2014; Nichols & Spong, 2014). A big limitation on the impacts of the living species is numbers and biomass. While these species may have survived later than we originally thought, it was likely at much lower densities and confined to less accessible and less optimal habitats. Bison in northern North America today often exploit burned areas or sedge-dominant wetland habitats, for example, where the alkaline and waterlogged conditions favour openness even in the absence of herbivory (Jensen et al., 2003). Bison and other living herbivores can go a long way towards maintaining open conditions in high densities, but we cannot expect low densities to create open habitats from closed ones if they cannot use forest habitats in the first place. Something would first have to break the monoculture, whether that be fire or the activity of mammoths. Inevitably, forests would form in areas with rough and inaccessible topography or following periodic herbivore die-offs, but greater disturbance by fire and herbivores (as well as logging and insect outbreaks) would create and maintain alternative stable states of parkland and wood-pasture around and within these forests (Mack et al., 2021; Oldén, 2016; Payette & Delwaide, 2003). If we consider widespread reintroduction of the living species, the nature of these feedback loops and the absence of mammoths are things that we need to consider.
Another consideration is carrying capacity and predation. Alaska and the Yukon already have relatively large populations of coyotes (Canis latrans), wolves (Canis lupus), wolverines (Gulo gulo), lynx (Lynx canadensis), and bears (Ursus americanus, arctos). Wolves in Alaska and the Yukon today sustain themselves primarily on moose, caribou, and sheep (Ovis dalli), with different populations relying on these species in different proportions. Up until roughly 7500 years ago, however, a different wolf was present, with a different prey preference – the Beringian wolf (Leonard et al., 2007). Although it has no taxonomic distinction (Canis lupus cf. spelaeus might be close), we know that these wolves were genetically and morphologically distinct from wolves living in the area today. Although about the same size as wolves living in Alaska and the Yukon today, Beringian wolves were more robust, with shorter, stronger mandibles and larger teeth (Leonard et al., 2007). This would have allowed them to take down larger prey and to more easily crack open bones. Although moose, caribou, and sheep were all present in the late glacial and early-middle Holocene, none of these seems to have been their preferred prey. Instead, as much of half their diet seems to be have been horse (Landry et al., 2021), the reduction and eventual disappearance of which was likely related to the robust varieties’ disappearance in favour of the more gracile living form.
Wolves, alongside bears, would likely still play an important role in predating on horses and other reintroduced ungulates, but it is worth noting that if we look at other megafaunal systems, it is productivity rather than predation that determines herbivore biomass (Hopcraft et al., 2010). Predation can play an important role in determining how that biomass is structured, typically favouring the larger species, and in altering the behaviours of some species (Boyce, 2018), but “natural” herbivore numbers are generally much higher than modern management systems are typically ready to accommodate, at least outside of Africa. Even in low productivity areas, carrying capacities of over 10,000 kg per square kilometre are common in African systems (Fløjgaard et al., 2022). The fossil record supports similar or higher figures for temperate and arctic areas as well, though in the absence of mammoth 6-7500 kg might be more accurate, similar to recorded densities in Aspen parklands like in Elk Island NP (Telfer & Scotter, 1975). Those species less affected by predation would instead be food-regulated, through starvation and affected reproduction, or through periodic disease associated with overcrowding (Hopcraft et al., 2010). In a rewilded boreal landscape of herbivore parklands, humans would also still be significant predators.
Herbivore and fire-mediated open habitats in Alaska and the Yukon could potentially support very high biodiversity. Many or even most birds in this area breed in open and/or mixed deciduous habitats , along with butterflies, rare grasshoppers, and relict populations of grassland plant species otherwise associated with open habitats further south (Schwarz et al., 1986; Schwarz & Wein, 2011; Strong, 2015). Favourite shrubs of elk and bison in the boreal-prairie transition, such as saskatoonberry (Amelanchier), dogwood (Cornus), and mountain ash (Sorbus), have populations in Alaska and the Yukon as well and could provide additional fodder ((FEIS); Telfer & Scotter, 1975). Better-defended shrubs like juniper (Juniperus) and salmonberry (Rubus) would potentially provide protection for the more saplings of more palatable species (Hegland et al., 2021; Vera, 2000). These open habitats would be grazed and browsed into mosaics of shrubs, individual or clusters of trees, grasses of different heights, bare ground, and diverse collections of herbs and forbs that are otherwise rare or only found in less competitive environments (Bråthen et al., 2021). Many species would also take advantage of microhabitats created by herbivores, such as wallows, sandbaths, and temporary pools of water (Galetti et al., 2018). Birds would benefit from animal hair in their nests and an abundance of insects produced by greater heterogeneity, as well as huge amounts of dung and carrion, the latter of which would benefit scavengers as well (Galetti et al., 2018).
More open, herbivore-dominated landscapes with fewer conifers and less overall plant biomass would also be more resistant to wildfire, and would potentially act as better or at least more secure carbon sinks, as I discussed in my LinkedIn article titled “How Can Rewilding Help Mitigate the Boreal Wildfire Problem?”. Since climate-induced wildfires are already forcing a shift towards more birch- and aspen-dominated ecosystems in many areas, due to the vulnerability and long recovery time of conifers (Mack et al., 2021), the opportunity to turn this much more edible biomass into belowground carbon stocks by reintroducing herbivory exists now (Kristensen et al., 2022). There would also be significant economic opportunities. Sustainable harvesting of meat from a greater diversity and number of ungulates, as well as gamebirds that benefit from open and deciduous conditions (e.g. Bonasa, Tympanuchus), and greater space for edible plants that compete poorly with conifers would improve food security. Large animal-based ecotourism could also provide additional revenue and job opportunities in areas where both have typically been low. I am sure all of this would require far more investigation and organization. Nevertheless, global changes are forcing us to consider drastic changes to how we view landscapes and management, and the return of the missing and more dynamic ecosystem elements could go a long way towards finding a new balance.
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