What Was the Mammoth Steppe? The Largest Biome That No Longer Exists

The evidence indicates a cold, dry, productive grassland rather than the wet mossy tundra that occupies much of the same latitude today. Pollen records, plant remains preserved in permafrost, and the gut contents and dung of frozen animals all point to a landscape dominated by grasses, sedges and herbs, with a high proportion of nutritious forbs, and with trees largely absent because of aridity and wind. Low precipitation combined with thin snow cover meant vegetation stayed accessible through winter, which is the crucial difference from modern tundra where deep snow and waterlogged ground restrict grazing. The soil was mineral-rich rather than peaty, since cold dry conditions and constant grazing prevented the accumulation of organic matter that produces today's acidic boggy ground. The apparent paradox, that a cold dry environment supported more large animal biomass than the same region does now, is why the system is sometimes described as productive beyond what modern analogues would predict.

The animal community was dominated by large grazers and their predators:

• •Woolly mammoth, the most numerous large grazer over much of the range and the animal the biome is named for
• •Woolly rhinoceros, steppe bison, horses and reindeer in very large numbers, with horses particularly abundant in many assemblages
• •Saiga antelope, musk ox and giant deer, each with a different feeding niche
• •Cave lions, cave hyenas, wolves and bears as the predator and scavenger community
• •Humans, present across much of the range through the later period, hunting the same animals and leaving substantial archaeological records
• •A soil and plant community sustained by the grazers, since their trampling, dunging and grazing maintained the grassland against encroachment by moss and shrubs

The biome collapsed at the end of the last glacial period and the cause is one of the long-running debates in Quaternary science. The climate explanation holds that warming and increased precipitation converted the dry grassland into wet tundra and boreal forest, deep snow cover restricted winter grazing, waterlogging produced peat, and the animals lost their habitat. The hunting explanation holds that human predation removed the large grazers, and that without their maintenance the vegetation shifted, so the habitat change followed the extinction rather than causing it. The evidence supports contributions from both, with regional variation, and the timing differs between continents and species in ways that complicate any single account. What is agreed is that the two processes interact, since a population reduced by climate stress is more vulnerable to hunting, and a grazing system that maintains its own habitat can collapse abruptly once grazer density falls below a threshold.

Knowing what a vanished ecosystem looked like requires evidence from several independent sources, and this one is unusually well supplied:

• •Pollen cores from lake sediments and peat, which record the proportions of grasses, herbs and trees through time at a given location
• •Plant macrofossils, meaning seeds, leaves and stems preserved in permafrost, which identify species where pollen can only reach the genus
• •Frozen carcasses recovered from permafrost, whose stomach contents and dung record exactly what an individual animal ate in its final days
• •Stable isotopes in bone and tooth enamel, which indicate diet and the conditions in which the plants grew
• •Ancient environmental DNA extracted directly from sediment and permafrost, which has revealed plants and animals leaving no other trace and has revised the picture towards a higher proportion of herbs
• •Radiocarbon dating of the remains themselves, which builds the chronology of when each species disappeared from each region, and which is what shows the extinctions were staggered rather than simultaneous

The argument that grazers maintained the grassland has produced practical experiments. A long-running project in northeastern Siberia has introduced horses, bison, musk ox and other large herbivores to a fenced area with the stated aim of restoring the ecosystem, and has reported changes in vegetation and in snow compaction consistent with the hypothesis. The additional claim attached to it concerns permafrost, since trampling snow in winter removes its insulating effect and allows cold to penetrate the ground, which the project argues could slow thawing and the release of the very large quantities of carbon that northern permafrost holds. That proposal has attracted serious scientific attention and substantial scepticism about whether it could operate at the scale required. A related strand is the effort to produce mammoth-like elephants by genetic modification, which has drawn funding and criticism in roughly equal measure, with the criticisms concerning feasibility, welfare, and whether restoration of an ecosystem requires the specific animal or simply large grazers of any kind.

A dry cold grassland ran continuously from Spain to Canada and supported more large animal biomass than the same latitudes do now, because thin snow and mineral soils kept vegetation accessible through winter. Warming, wetter conditions and human hunting both contributed to its collapse, and the two interact. Grazers maintained the habitat themselves, which is the premise of ongoing attempts to recreate it in Siberia.