In the late Pleistocene steppe ecosystems were dominating ecosystems on the planet. In Europe, northern Asia and northern America dominated Mammoth steppe-tundra. In more southern region mammoths were absent, but all steppe ecosystems were very similar. They all were characterized by high animal density, rich grass vegetation and high rates of bio cycle. Animal species could vary from ecosystem to ecosystem, but assemblage of animal “professions” stayed almost the same. For sustainable ecosystem was needed “elephants”, “cows”, “goats”, “wolves”, “tigers” etc. These ecosystems were extremely stable, they developed for hundred thousands of years and survived several deep glaciations and warm periods similar with Holocene. High animal density allowed grasses to be only dominating vegetation, since only grasses can sustain active grazing. Shrubs, moss and trees were either trampled or broken.
Analyses of bones collected in the North of Siberia allowed to calculate that animal biomass even in cold epochs in the north reached 10 ton/km2, and on average on each square kilometer of pastures roamed 1 mammoth, 5 bison, 6 horses and 10 reindeers. When climate warmed, pasture productivity increased and animal density rose, when cooled was vice versa. Number of animals in more southern and wet steppes was substantially higher then on the north.
Steppe ecosystems played an important role in driving the climate. During the glaciations steppe soils and especially soils of mammoth steppe-tundra were the main reservoir and sink for carbon on the planet, and during the deglaciations these soils released thousand of billions of tons of these carbon back into the atmosphere. High albedo of steppes allowed reflecting big portion of sun heat back to space. High productivity and transpiration of steppes allowed to keep soils dry and prevent wetland from appearing, therefore global emission, of such an important greenhouse gas as methane, was low.
12 thousands years ago happened last climate warming named Younger Dryas termination, but climate instead of gradual shift back to glacial conditions stayed stable. Steppe ecosystems throughout the planet vanished and were replaced by forests, wetlands or tundra. Happened global animal extinction comparable by scale only with dinosaur extinction. Why ecosystems which survived several glacial-interglacial cycles vanished right in Holocene? What is the difference between last deglaciation and previous ones?
In Pleistocene man played a minor role is steppe ecosystems. Man was the slowest and weakest animal, burdened with defenseless brood. Wild animals, and especially predators were deadly dangerous. Areal of man in late Pleistocene was limited to Africa, Australia, and south of Europe and Asia. But right after Bowling warming event 14650 man started active expansion to Siberia, Arctic and through Bering straight to America, where already after 200 years they reached southern part of South America. Simultaneously with rise in man population, increased their knowledge and skills. Climate warming provided wood and shelter in the places previously too severe for survival. Thanks to new technologies and possibilites man in steppe ecosystem from herbivore and carrion eater turned into predator. Appearance of a new predator is always a stress to the ecosystem as a whole and to herbivores in particular, and appearance of a predator which continuously develops its hunting abilities and behavior became a stress which ecosystem couldn’t sustain. Following the human migration wave followed extinction wave, which only increased in strength. If in Europe and Africa only small portion of animals became extinct, in South America 50 species with a body weight over 44kg became extinct. Animals which persisted through the migration wave, substantially decreased their numbers and had to hide in forests and mountains. As a result tens of millions of square kilometers of pastures and grasslands, were left without herbivores. Grass which grew in summer wasn’t eaten and was left as litter on the surface. Nutritions stayed in litter and were removed from the biocycle. New sprouts of shrubs and trees were not trampled. Decreased level of transpiration, increased soil moisture and run off. Nutritions were washed away. All this processes led to pasture degradation and in several hundred years richest steppe ecosystems weer replaced by low-productive modern ecosystems, and animal lost chance to recover their density – food base was gone.
Later with agriculture development big part of forests and wetlands were replaced artificial ecosystems of farms and domestic pastures. It is not possible to reconstruct rich steppes on these territories since they serve to maintain increasing human population. But on our planet exist vast territories which are not suitable for agriculture. This unused by man and biologically deserted regions can become home for oppressed wild animals and, what is not less important in light of climate warming, sink of carbon and mirror which will reflect bigger portion of heat from the sun.
Modern ecosystems are currently in the steady state. Same with steppe ecosystems in Pleistocene they have defending and compensating mechanisms which allow them to win competition with other ecosystems. For ecosystem to become sustainable dominating ecosystem this mechanisms just must start to work. Mammoth steppe-tundra in Pleistocene dominated in Alaska and in totally different climatic conditions in Iberian peninsula, same with modern tundra – it successfully develop as in Sweden so and in Siberian arctic. Man caused shift from steppes to forests and tundra simply decreasing number of animals and keeping this number low for period sufficient for pasture degradation. If we want to make reverse shift, we have to do opposite impacts – artificially increase number of animals on limited territory for period of time sufficient for pasture development. When shrubs, moss and trees will be vanished, nutritions in soil will increase, high productive grasses will dominate, and specie assemblage and “professions” will be sufficient, steppe ecosystem will become stable and self sustainable and will be ready for expansion.