The Mammoth Steppe

The term "Mammoth Steppe" describes a vast area that spanned northern Eurasia and parts of northern America, primarily within the Arctic Circle, during the late Pleistocene epoch. This environment persisted from approximately 129,000 years ago until the onset of the Holocene epoch around 11,700 years ago (Cohen et al., 2013).

Evidence from pollen, ancient DNA, and plant macrofossils indicates that the Mammoth Steppe was predominantly grassland interspersed with some trees, resembling a savanna. This diverse habitat supported large populations of bison, horses, elk, reindeer, and mammoths (Zimov et al., 2012).

As the climate shifted, marking the end of the ice age, and with the arrival of human hunters, large herbivore populations dwindled, leading to the extinction of species such as the mammoth. The ongoing debate revolves around whether climate change (Guthrie, 2006) or human hunting (Macias-Fauria et al., 2020; Zimov et al., 2012) was the primary driver of these extinctions.

Regardless of the cause, it is evident that megaherbivores played a crucial role in suppressing woody vegetation during the late Pleistocene. Trials that excluded modern large herbivores demonstrated an increase in woody plants, emphasizing the importance of these animals in maintaining grassland ecosystems.

Grazing Behavior and Ecosystem Maintenance

Modern examples, such as elephants in the African savanna, illustrate how large herbivores maintain grasslands by uprooting shrubs and toppling trees (Bakker et al., 2016). Numerous grazing animals contribute to the upkeep of grasslands by disturbing mosses and shrubs. The location of grass growing buds, which are situated at or below ground level, enables grasses to recover better from grazing.

A decline in grazing animals would allow leaf litter to accumulate, insulating the soil, lowering summer temperatures, and ultimately reducing grass productivity and nutrient availability. This shift would favor the growth of mosses and shrubs over grasses, diminishing the habitat for grazing animals. The reintroduction of these herbivores could potentially rejuvenate the grassland ecosystem (Zimov et al., 2012).

The benefits of restoring grassland ecosystems include:

• Grassland plants exhibit a higher albedo compared to tundra shrubs and larch trees.
• The exposed snow in grasslands has a higher albedo, reflecting more sunlight.
• Trampling by large herbivores reduces snow depth, allowing deeper permafrost freezing.
• Fewer shrubs and trees lead to less snow trapping and buildup.
• Increased transpiration from grasses results in lower soil moisture and reduced waterlogging.
• Herbivores enhance nutrient cycling through digestion, contrasting with the slower decomposition by microorganisms in cold soils.
• Grasses and forbs possess deeper root systems than shallow-rooted shrubs and trees, enabling better soil carbon storage (Macias-Fauria et al., 2020).

Reintroducing Large Herbivores

To stabilize permafrost and counteract the climate warming feedback loop linked to carbon and methane emissions, a sufficient density of large herbivores must be reintroduced to the Arctic. Suggested species include bison, cattle, and horses. Bison would consume tree and shrub bark, while cattle would graze on twigs and grass. Horses, exclusively grass grazers, would trample snow, exposing grass beneath and reducing the insulating layer (Cromsigt et al., 2018). Reindeer, or caribou, are already present in the tundra, so they are excluded from this proposal (Macias-Fauria et al., 2020).

Estimates of large herbivore densities during the Pleistocene suggest 1 mammoth, 5 bison, 7.5 horses, and 15 reindeer per square kilometer (Zimov et al., 2012). While mammoths would have significantly reduced woody vegetation, they are now extinct. Although cloning mammoths is not feasible as a live specimen is required (Shapiro, 2015), efforts are underway to genetically modify elephants for cold adaptations.

The Wyss Institute at Harvard University is currently working on introducing DNA with CRISPR technology into the Asian elephant genome, the mammoth's closest living relative. This technology aims to modify genes influencing cold adaptation traits such as blood hemoglobin levels, ear size, body fat, and hair density (Shapiro, 2015). The introduction of cold-adapted, mammoth-like genetically engineered elephants could help restore grasslands by controlling tree and shrub growth.

However, mammoth-like elephants are not yet available, and the mammoth's role in controlling tree growth is unnecessary in northern Arctic regions, which have minimal tree cover. In contrast, larger mammals in lower latitudes could manage woody vegetation effectively (Macias-Fauria et al., 2020).

As global climatic conditions grow harsher, society may become more receptive to natural climate solutions, including rewilding and megafaunal ecological engineering, alongside efforts to reduce greenhouse gas emissions.

Challenges and Opportunities in Rewilding

Rewilding the Arctic Circle to restore Pleistocene grasslands is a complex endeavor, primarily due to the vast number of animals required. However, it could commence with the establishment of a trans-Arctic network of experimental reserves (Macias-Fauria et al., 2020).

The vision of reviving the great cold plains of grasslands teeming with large herbivore herds, continuously sequestering carbon, necessitates collective human cooperation and mutual assistance.

The video titled "What is Pleistocene Rewilding?" explores the concept of bringing back the ecosystems from the Pleistocene era and the role of large herbivores in maintaining these environments.