HISTORY OF THE PLEISTOCENE
At the end of the Pleistocene, steppe ecosystems were the dominant ecosystems on the planet. In Europe, Northern Asia and Northern America the Mammoth steppe ecosystem dominated. In more southern regions, mammoths were absent, but all steppe ecosystems were very similar, characterized by a high density of animals, rich herbaceous vegetation, and high rates of biogeochemical cycling. Animal species varied from ecosystem to ecosystem, but the roles played by these different types of animals remained relatively constant. For an ecosystem to be sustainable it must have large heavy grazers, such elephants, ruminants such as cows and goats, predators such as wolves and tigers etc. Steppe ecosystems were extremely stable, since they developed over hundreds of thousands of years and survived several deep glaciations and warm periods similar to the Holocene. High animal density allowed only grasses to be the dominating vegetation, since only grasses can sustain active grazing. Shrubs, moss and trees were either trampled or broken.
An analysis of bones collected in northern Siberia has allowed scientists to calculate that the animal biomass, even in the coldest periods in the north, reached 10 ton/km2, and that the average per square kilometer of pasture included one mammoth, five bison, six horses, and 10 reindeer. The number of animals in the southern steppes or wet steppes was significantly higher than in the north.
When the climate warmed, pasture productivity increased, and animal density rose, while the opposite occurred during cooling periods. Steppe ecosystems played an important role in driving the climate of the earth. During glaciations steppe soil, especially the soil of the mammoth steppe-tundra, was a main reservoir, or “sink”, for carbon on the planet. While during climate warming (deglaciation) periods the steppe soil released thousands of billions of tonnes of carbon back into the atmosphere. Steppe vegetation has a high albedo, thus significant portion of heat from the sun was reflected back into space. The high productivity and transpiration rate of steppes meant that the soil in the steppes remained, therefore wetlands were rare, and emission of methane, an important greenhouse gas, was low. The mammoth steppe tundras had higher albedo and were drier than moss and shrub tundra ecosystems found in the north today.
Roughly 14,650 years ago the Bowling warming event occurred. This warming wasn’t more pronounced than warming events which took place in previous glacial cycles, but it led to unprecedented changes. A global extinction of animals took place that was comparable in scale only to dinosaur extinction. The steppe ecosystems across the planet disappeared and have been replaced by forests, wetlands, or tundra. The glacial-interglacial cycle was broken - instead of gradually cooling to glacial conditions, the climate stayed warm and stable throughout the entire Holocene. Why did the Mammoth-steppe ecosystems, which survived several glacial-interglacial cycles, vanish in the Holocene? What is the difference between this most recent deglaciation and the previous ones?
In the Pleistocene, man, played a minor role in the steppe ecosystems. Man was the slowest and weakest animal, burdened with defenseless offspring. easily preyed upon by wild animals. In the late Pleistocene man occupied only Africa, Australia, and the south of Europe and Asia. Right after the Bowling warming event in 14650 BP man started an active expansion into Siberia, the Arctic and through the Bering straight to America, reaching the southern part of South American within 200 years. Along with the increase in human population, came an increase in knowledge and skills. Climate warming provided wood and shelter in the places previously too severe for survival. Thanks to new technologies and possibilities man changed from being a herbivore and carrion eater into a predator within the steppe ecosystem. The appearance of a new predator is always a stress to the ecosystem as a whole and to herbivores in particular, and the appearance of a predator which continuously developed its hunting abilities and behavior became a stress which the ecosystem couldn’t sustain.
The wave of human migration was followed by a wave of extinction, which only increased in strength as it went. In Europe and Africa only a small portion of animals became extinct, while in South America 50 species with a body weight over 44kg became extinct. The populations of animals which persisted through the migration wave substantially decreased and they had to hide in forests and mountains. As a result, tens of millions of square kilometers of pastures and grasslands, were left without herbivores. Summer grass was left uneaten as litter on the ground surface. Nutrients were retained in the litter and thus removed from the biological cycle. New sprouts of shrubs and trees were not trampled. Levels of transpiration decreased, which caused an increase in soil moisture and run off. Nutrients were washed away. These processes led to pasture degradation and in several hundreds of years the richest steppe ecosystems were replaced by modern ecosystems of low productivity, and animal populations could not recover their density:the food base was gone In the centuries since, with the development of agriculture, most of the forests and wetlands were replaced with artificial ecosystem management and domestic pastures.
A NEW PLEISTOCENE ERA
It is not possible to recover the rich steppes of the past in many areas, since they are used to support the development of the human population. However, vast territories unsuitable for agriculture do still exist, providing an opportunity to re-establish the steppe ecosystem. Reconstructing these ecosystems would allow Man to correct what he has done in the past, to provide a refuge for populations of wild animals and to increase the overall productivity of the planet. This would be desirable also because steppe ecosystems governed the glacial- interglacial cycles. They provided sinks for atmospheric carbon, storing it in the soil or permafrost. Their light surface (high albedo) served as a mirror to reflect much of the sun’s heat back to the atmosphere. If enough area is covered by steppe it would diminish the effects of global warming caused by increased anthropogenic greenhouse gas emissions.
Modern ecosystems are currently in a stable condition, as was the steppe ecosystem during the Pleistocene. Ecosystems have protective and compensatory mechanisms that allow them to out-compete other ecosystems. The Mammoth steppe-tundra in the Pleistocene was dominant in Alaska, as well as under different climatic conditions on the Iberian Peninsula. The same is true for the modern tundra - it has successfully developed both in Sweden and in the Siberian Arctic. Man has caused the transition from the steppes to the forest and tundra simply by reducing the number of animals and keeping this number low for a period sufficient for the degradation of pastures.
If we want to effect a reverse ecosystem shift, we need to artificially increase the number of animals in a limited area for a period of time sufficient for pasture development. Animals would trample all vegetation including shrubs trees and moss. By fertilizing the soil they would increase the rate of biogeochemical cycling. More nutrients would accumulate in the soil, allowing higher grass productivity. Higher transpiration would keep soils dry. Under continuous pasture pressure and fertilization, grasses would once again become the dominant vegetation, and in combination with various steppe animal species they would form a modern steppe ecosystem, which would be sustainable and ready to expand.
Header Photo By Mauricio Antón