1. Rise of the Grassland Biome, Central North-America
Axelrod, D. (1985). Botanical Review, 51(2), 163-201
The author uses paleoecological evidence to explain the rise of the grassland biome in North America. Evidence includes paleo-pollen analysis, plant and animal fossils, current ranges for a variety of plant species, and disjunctive plant populations. Evidence indicates glasslands first emerged in the western portion of the North American continent, along the rain shadow created by the Cordillera uplift (Rocky Mountains) 40 million years ago (mya). Grasslands expand eastward into the Midwest during an arid period in the late Miocene early Pliocene 10 mya. Grasslands rapidly expand eastward, to the Atlantic states with the advent of humans and fire at the end of the Pleistocene. The author concludes the modern grassland biome is the product of feedbacks between an increasingly arid environment, vegetation, herbivorous animals, and humans using fire.
2. A Postulated Natural Origin for the Open Landscape of Upland Scotland
Fenton, J. H. C. (2008). Plant Ecology & Diversity, 1(1), 115-127
This paper challenges the dominant ecological perception that the treeless Scottish uplands are a product of humans grazing livestock, tree cutting, and burning, and that without these disturbances, the Scottish highlands would have been forest. The paper investigates historical feedbacks between vegetation, soils, herbivores, fires and tree-felling to identify plausible factors to account for the open landscape without inferring human influence as the primary driver. The author proposes the current open state is equivalent to previous interglacial periods where woodland expansion is followed by woodland regression. If this analysis is correct, upland Scottish moorlands could be one of the most natural landscapes in Europe. This concept has major implications for Scottish Highland conservation planning, where to date forest ‘restoration’ is dominant narrative.
3. The Ecology of the Earth’s Grazing Ecosystems
Frank, D. A., McNaughton, S. J., & Tracy, B. F. (1998). Bioscience, 48(7), 513-521
This paper summarizes the relationship between grassland vegetation and grazing animals. The authors conclude that ungulates are an important component of energy and nutrient flows in grazing ecosystems. When ungulates are removed from grasslands, the functional character of the system changes, edible plants are soon replaced by inedible plants, and the food web shifts from herbivore-carnivore to decomposers. The result is a less functional ecosystem.
4. The Origin of Grass-dominated Ecosystems
Jacobs, B., Kingston, J., & Jacobs, L. (1999). Annals of the Missouri Botanical Garden, 86(2), 590-643
Approximately half of the Earth’s vegetative cover comprises savannas, grasslands, and other graminoid-dominated ecosystems. Grasses are known from all continents except Antarctica. Fossil evidence and stable isotope analysis suggest five stages in the origin of grass-dominated ecosystems: (1) the late Paleocene origin of Poaceae 55 mya, (2) the transition of Paleocene-Eocene forests into open woodland in the early Tertiary 45 mya, (3) an increase in C(3) grasses 35 mya, (4) the origin of C(4) grasses in the middle Miocene 15 mya, and (5) the spread of C(4) grass-dominated ecosystems in the late Miocene 7 mya. Herbivore morphology conducive to grass-dominated ecosystems first appears in South America at the Eocene-Oligocene boundary 35 mya, and later in northern continents in the early Miocene 20 mya. By 7 mya, savanna grassland grazing ecosystems are the most advanced and ecologically functional terrestrial ecosystems ever to have existed on the earth.
5. The Origins and Evolution of the North American Grassland Biome: The Story from the Hoofed Mammals
Janis, C.M., Damuth, J., Theodora. 2002. Palaeogeography, Palaeoclimatology, Palaeoecology, Vol. 177, Issues 1–2
The authors suggest the North American grassland biome emerged 18 million years ago (mya) where the fossil record indicates a shift in mammal tooth formula from a browsing diet change to a grazing diet. This suggests the South American and North America grasslands have coevolved with grazing animals for 45 to 18 mya respectively.
6. Grazing Lawns – Animals in Herds, Plant Form, and Coevolution
McNaughton, S.J. 1984. American Naturalist 124 (6), Pages: 863-886
The concept of the “Gazing Lawn”as a plant community is introduced to the scientific community in this classic paper. Grazing lawns occur when herbivores frequent particular landscapes in a predictable time space pattern. For example, herbivores frequent shade created by open-grown trees during sunny hot summer days. The shade pools are subsequently subjected to periods of intense grazing. Over time, plant and animal communities associated with grazing lawns evolve particular traits that reinforce each other through a series of complex mutualistic feedbacks. The author concludes grazing lawns represent highly evolved biotic-communities, where plants and animals are dependent upon one another, in fact, plant species associated with grazing lawns can be considered “graze obligates”. Interestingly, the grazing lawn concept has not caught on in North America, where plant and animal relationships are viewed instead under the context of predator prey relationships.
7. Coevolution of Grasses and Herbivores
Stebbins, G.L. 1981. Annals of the Missouri Botanical Garden, Vol. 68, No. 1, 1981
The author conducts a literature review to determine when biomes on different continents transitioned from forest communities with browsing animals to savanna grasslands with grazing animals. Evidence indicates grasses and herbivores mutualistically coevolved starting first in South America during the Eocene 55-45 million years ago (mya) and in North America 25 mya. The paper provides an excellent summary for the emergence of keystone grass and animal species.