Thor Hanson

Environmental Geology

March 22, 1999

Topic: Pleistocene Park: Restoring the Mastodon Steppe

Annotated Bibliography

Hopkins, D. M. 1967. The Cenozoic history of Beringia--a synthesis.

Pages 451-484 in D. M. Hopkins, ed. The Bering Land Bridge. Stanford

University Press, Stanford, CA.

Beginning with land bridges in the early Tertiary, this chapter summarizes

the geologic and biological history of eastern Siberia and western Alaska,

a region known collectively as Beringia. The author draws from multiple

disciplines and studies to comment on flora, fauna, climate and

landscape-level changes.

The Quaternary Period was marked by great climate fluctuations that

continuously redistributed the regions three major biomes: tundra, steppe

grasslands, and taiga forest. Taiga typically separated the tundra from

the steppe, but that forest belt diminished during dry periods surrounding

the Illinoian and Wisconsin glaciations. Artemisia pollen, and fossil

evidence of great herbivore herds indicate that grassland steppes

dominated much of this period, favored by a drier climate, loess

deposition, and trampling of grazers.

Hopkins subscribes to the theory that climate change and overkill together

caused the extinction of herbivores. Climate change reduced the available

grassland habitat, and efficient hunters eliminated remnant populations.

He states that most herbivores couldnt survive the shift to tundra, and

notes an interesting modern analog:

Bison have been introduced and pack horses have been left to fend for

themselves through the winter in small areas in central and southern

Alaska where wild grass is abundant on the braided outwash plains of

modern glacial rivers and on adjoining areas of active sand movement or

active deposition of windblown silt. The hoses survive and the bison

thrive, but neither animal has been able to expand its feral range beyond

the highly specialized and isolate environment in which it has been

placed.

McNaughton, S. J. Grazing as an optimization process: grass-ungulate

relationships in the Serengeti. American Naturalist 113: 691-703.

Herbivory causes a number of documented positive impacts on plant

productivity:

increased photosynthetic activity in residual tissues.

reallocation of nutrients from elsewhere in the plant.

removal of older, less-productive tissues.

increased light to productive, younger, underlying tissues.

slower leaf senescence--longer photosynthetic term per leaf.

release of hormones that stimulate growth.

conservation of soil moisture by decreased transpiration.

increased nutrient cycling from urine and dung.

increased growth from hormones in ruminant saliva.

The author predicts that an optimum level of grazing should exist between

herbivores and their forage, bestowing positive impacts on both.

Exclosures in the Serengeti exposed treatments to varying levels of

grazing by wildebeest. The mean level of natural grazing was beyond the

level of optimum growth for grasses, perhaps because over-stimulation

caused regrowth with higher nutrient concentrations--more benefit per unit

of effort for the grazers. Most notable to the Siberian study, long-term

exclosures analogous to herbivore extinction showed a dramatic shift in

plant species composition, indicating that certain grasses are coevolved

with grazers.

Owen Smith, N. 1987. Pleistocene extinctions: the pivotal role of

megaherbivores. Paleobiology 13: 351-362.

This paper proposed the herbivore keystone hypothesis as a mechanism for

massive extinctions at the end of the Pleistocene. The author dismisses

three major extinction theories as insufficient. Climate alone fails to

explain the lack of extinctions associated with earlier post-glacial

environments, while contributing everything to human hunting, or overkill,

doesnt explain why so many non-game species also disappeared. Even a

combination of these factors doesnt clarify the asynchronous pattern of

extinctions in different areas.

In Africa, elephant, rhinoceros, hippopotamus and other megaherbivores

provide modern analogs for some of the extinct Pleistocene species. These

animals are resistant to drought and other weather stresses and largely

immune to non-human predation. Their populations tend, if undisturbed, to

exert a dramatic influence over their environment, maintaining forest

openings, or even converting woodlands into grasslands. Elimination of

megaherbivores from parts of Africa has led to habitat changes similar to

many of those recorded following the extinctions of Pleistocene species

including mastodon, gomphotheres and certain bison.

The author concludes that while climate and overkill contributed to

Pleistocene extinctions, the habitat changes following a reduction in

megaherbivores accelerated extinction rates and impacted a wider range of

species dependant on megaherbivore-maintained habitats. Zimov et al

referenced this paper for its strong statements about habitat change in

the absence of herbivores.

Pastor, J. and W. M. Post. 1988. Response of northern forests to

CO2-induced climate change. Nature 334: 55-58.

Climate change is expected to alter the composition and productivity of

northern forest communities. Forest responses, however, are dependant on

soil moisture and nitrogen availability, the limiting factors for tree

growth. The authors combined models for climate change with those for

forest productivity and soil processes, manipulating variables including

recruitment and mortality of trees, and the decomposition of their litter.

At doubled levels of CO2, they predict the greatest rate of change at the

boreal/northern hardwood boundary. But those changes were strongly

influenced by soil type, and by a positive feedback between CO2 and

nitrogen cycles. On moist soils, boreal spruce/fir changed to more

productive hardwoods, while droughty soils saw a decrease in productivity

from spruce/fir to stunted pines and oak.

Zimov et al referenced this paper to illustrate how climate change alone

cant account for all vegetation shifts.

Pitelka, Louis F. 1997. Plant migration and climate change. American

Scientist 85: 464-473.

This paper from our week #5 discussion gives a good summary of plant

dispersal and migration in response to climate change. Re-reading it with

Zimov in mind, Im surprised that the authors largely dismiss the role of

animals as dispersers or habitat modifiers. They stress human changes to

the landscape and how that may impact plant migration, but fail to mention

the theory that animals can also be keystone species.

Stone, Richard. 1998. A bold plan to re-create a long-lost Siberian

ecosystem. Science 282: 31-34.

This News Focus piece summarizes Zimovs 1995 paper and highlights his

ongoing efforts to test the keystone herbivore theory. With help from a

team of American, Canadian and Russian scientists, Zimov hopes to create

Pleistocene Park, a 160-square-kilometer reserve where herds of grazers

will transform the tundra into a lush grassland. He has brought in horses

and applied to Ted Turner for a grant to introduce forest bison from

Canada. Theoretically, their trampling hooves will disturb the moss and

sedge-dominated tundra, allowing grasses to become established. The

higher transpiration rates of grasses will decrease soil moisture, a

positive feedback leading to a self-perpetuating steppe-savanna ecosystem.

The project faces scientific criticism from proponents of the climatic

biome-shift theory, who maintain that grasslands wont survive without

drier Pleistocene weather conditions. Zimov is also challenged by the

deteriorating economic conditions in Russia, but remains hopeful that hell

have a Siberian Serengeti in twenty years.

Zimov, S. A., V. I. Chuprynin, A. P. Oreshko, F. S. Chapin III, J. F.

Reynolds, and M. C. Chapin. 1995. Steppe-tundra transition: a

herbivore-driven biome shift at the end of the Pleistocene. American

Naturalist 146: 765-794.

This is the seminal paper in herbivore keystone theory for Siberia (from

which all those countless other Siberian herbivore keystone papers were

undoubtedly derived). Zimov et al present their case thoroughly, drawing

on dozens of previous studies as they review the failure of climatic

theory alone to explain the shift from steppe to tundra. They suggest

that the Pleistocene overkill removed herbivores necessary for the

maintenance of a grassland ecosystem.

Evidence suggests that while the Pleistocene climate was colder, it may

not have been significantly drier than current conditions. Recent weather

data indicate that Siberia receives enough net energy input to

evapotranspire one to three times its annual precipitation. The high

water retention of mosses may better explain the tundras saturated

condition. Laboratory experiments show that steppe grasses transpire far

more water than mosses, and could account lead to arid soils and the

associated plant community of the Mastodon steppe. Continued trampling by

herbivores also favor grasses over moss. Finally, a simulation model

incorporating climate change, loess deposition and the loss of grazers

pointed to grazing as the key element in the shift from steppe to tundra.