Excellent study on Great Value of Wild Horses to Northern Ecosystems
Vegetation shift at the end of the Pleistocene, Europe, North America, Russia
Title of Example Vegetation shift at the end of the Pleistocene, Europe, North America, Russia
Certainty of Shift Proposed
Location Europe, North America, Beringia (eastern Russia, western Alaska)
System Type Ecological
Regime Shift Category 1a
Ecosystem Type Tundra
Type of Resource Use Other
Ecosystem Services Hunting; food and fibre
Resource Users Hunter-gatherers
Spatial Scale Sub-continental/Sub-regional
Number of Possible Regimes 2
Time Scale of Change Centuries
Reversibility of Shift Irreversible
Background At the end of the Pleistocene (10,000 – 12,000 years BP), the vegetation of Beringia saw a shift in dominance from grasslands to mosses, which coincided with the extinction of the megaherbivores. General circulation models suggest that the Pleistocene environment of Beringia had similar wind and precipitation to today, but was colder and had wetter soils. Simulation models based on plant competition for water and light, nutrient supply and plant sensitivity to grazers, predict that either a grass or moss dominated steppe could occur in Beringia, both now and during the Pleistocene.
Alternate Regimes 1. Semi-arid grassland with megafauna
2. Wet moss tundra with scattered trees, no megafauna
Fast or Dependent Variable(s) Plant species composition
Slower or Independant Variable(s) Soil temperature, depth of soil thaw
Disturbance or Threshold Trigger(s) Cessation of grazing by megaherbivores, climate change
External/Internal Trigger(s) External
Mechanism With the extinction of the megaherbivores, grazing pressure was relaxed. Ungrazed, unfertilised meadows are unstable in the absence of herbivores or continued disturbance. When dry uneaten litter accumulates, nutrients are sequestered in undecomposed organic matter and nutrient turnover declines. Dry grass litter is highly reflective and acts as a heat insulator, cooling the underlying soil and reducing the depth of soil thaw during the short summer. This promotes the growth of shallow rooted mosses. Mosses have a low rate of evapotranspiration, and low thermal conductance which promotes the development of water-logged soils. Eventually as the moss grows, thaw depth decreases to the point where only the moss layer itself thaws, excluding deeper rooted grasses altogether. Moss dominated ecosystems are avoided by mammalian herbivores due to their low tissue nutrients and high secondary metabolite concentrations making them poorly digestible and unpalatable.
In the model, when grazers were removed, dominance of mosses expanded rapidly, grasses declined and trees increased slightly. Neither climate change, nor the loss of grazers alone, was sufficient to cause the observed vegetation changes, but rather a combination of both. It is primarily at ecotones, where the climate is suitable to support more than one ecosystem type, that mammals will have their greatest impact.
The model was tested in the lowlands of Kolyma, 100km inland from the Arctic Ocean. In 1988, 25 yearling Yukatan horses were released and protected from poachers. Areas where the herd was concentrated did show an increase in the abundance of grasses and a decrease in the abundance of mosses, consistent with the model.
Reference(s) Zimov, S. A., V. I. Chuprynin, A. P. Oreshko, F. S. Chapin, J. F. Reynolds, and M. C. Chapin. 1995. Steppe-tundra transition: a herbivore-driven biome shift at the end of the pleistocene. The American Naturalist 146, no. 5: 765-94. (M)
Keywords Population Harvesting, model, Physical/Climate
Contact Name Jacqui Meyers
E-mail Address email@example.com
Institution and Address CSIRO Sustainable Ecosystems,
PO Box 284,
Canberra ACT 2601