Grass, Ruminants and Climate Change

The planet continually experiences Climate Change, but this post will describe the effects of climate change on an ecosystem since the beginning of the Oligocene. 

Climate and Tectonics (from http://en.wikipedia.org/wiki/Cenozoic)

After the tectonic creation of Drake Passage, when South America fully detached from Antarctica during the Oligocene, the climate cooled significantly due to the advent of the Antarctic Circumpolar Current which brought cool deep Antarctic water to the surface. The cooling trend continued in the Miocene, with relatively short warmer periods. When South America became attached to North America creating the Isthmus of Panama, the Arctic region cooled due to the strengthening of the Humboldt and Gulf Stream currents,^[7] eventually leading to the glaciations of the Pleistocene ice age, the current interglacial of which is the Holocene Epoch.

I aim to show that this cooling of the planet for the last 34 Myrs was the driving force behind evolution of the biosphere, and lead to the creation of an ecosystem which co-evolved using Mutualism  i.e. a probable service-service relationship.

http://en.wikipedia.org/wiki/Mutualism_(biology)

Ecology of Grasses (from http://en.wikipedia.org/wiki/Grasses )

Biomes dominated by grasses are called grasslands. If only large contiguous areas of grasslands are counted, these biomes cover 31% of the planet’s land.^[5] Grasslands go by various names depending on location, including pampas, plains, steppes, or prairie.

In addition to their use as forage worldwide by many grazing mammals, such as cattle and other livestock, deer, and elephants, grasses are used as food plants by many species of butterflies and moths; see List of Lepidoptera that feed on grasses.

The evolution of large grazing animals in the Cenozoic has contributed to the spread of grasses. Without large grazers, a clearcut of fire-destroyed area would soon be colonized by grasses and, if there is enough rain, tree seedlings. The tree seedlings would eventually produce shade, which kills most grasses. Large animals, however, trample the seedlings, killing the trees. Grasses persist because their lack of woody stems helps them to resist the damage of trampling.

Evolution of Grasslands (from http://en.wikipedia.org/wiki/Grassland)

The appearance of mountains in the western United States during the Miocene and Pliocene epochs, a period of some 25 million years, created a continental climate favorable to the evolution of grasslands. Existing forest biomes declined, and grasslands became much more widespread. Following the Pleistocene Ice Ages, grasslands expanded in range in the hotter, drier climates, and began to become the dominant land feature worldwide.

Human impact and economic importance

Grasslands are of vital importance for raising livestock for human consumption and for milk and other dairy products.

Grassland vegetation remains dominant in a particular area usually due to grazing, cutting, or natural or manmade fires, all discouraging colonisation by and survival of tree and shrub seedlings. Some of the world’s largest expanses of grassland are found in African savanna, and these are maintained by wild herbivores as well as by nomadic pastoralists and their cattle, sheep or goats.

Grasslands may occur naturally or as the result of human activity. Grasslands created and maintained by human activity are called anthropogenic grasslands. Hunting peoples around the world often set regular fires to maintain and extend grasslands, and prevent fire-intolerant trees and shrubs from taking hold. The tallgrass prairies in the US Midwest may have been extended eastward into Illinois, Indiana, and Ohio by human agency. Much grassland in northwest Europe developed after the Neolithic Period, when people gradually cleared the forest to create areas for raising their livestock.

Ruminants Abundance, distribution, and domestication (from http://en.wikipedia.org/wiki/Ruminant)

Wild ruminants number at least 75 million and are native to all continents except Australia and Antarctica. Nearly 90% of all species are found in Eurasia and Africa alone. Species inhabit a wide range of climates (from tropic to arctic) and habitats (from open plains to forests).^[10]

The population of domestic ruminants is greater than 3.5 billion, with cattle, sheep, and goats accounting for about 95% of the total population. Goats were domesticated in the Near East at approximately 8,000 B.C. Most other species were domesticated by 2,500 B.C., either in the Near East or southern Asia.^[11]

Evolution of Bovidae (from http://en.wikipedia.org/wiki/Bovidae)

The bovid family is known through fossils from the early Miocene, around 20 million years ago. The earliest bovids, such as Eotragus, were small animals, somewhat similar to modern gazelles, and probably lived in woodland environments. The bovids rapidly diversified, and by the late Miocene, the number of bovid species had greatly expanded. This late Miocene radiation was partly due to the fact that many bovids became adapted to more open, grassland habitat.^[8] There are 78 genera known from the Miocene (compared to 50 today).

Early in their evolutionary history, the bovids split into two main clades: Boodontia and Aegodontia. This early split between Boodontia (of Eurasian origin) and Aegodontia (of African origin) has been attributed to the continental divide between these landmasses. When these continents were later rejoined, this barrier was removed, and both groups expanded into each other’s territory.^[9]

The largest number of modern bovids is found in Africa, while substantial but less diverse populations are in Asia and North America. Some scientists^[who?] has suggested that many bovid species that evolved in Asia could not survive predation by humans arriving from Africa in the late Pleistocene^{[citation needed]}. By contrast, African species had many thousands or a few million years to adapt to the gradual development of human hunting skills. Yet many of the commonly domesticated bovid species (goats, sheep, water buffalo and yak) originated in Asia. This may be because Asian bovids had less fear of humans and were more docile.

The small number of modern American bovids are relatively recent arrivals over the Bering land bridge, but they long predate human arrival.

Miocene (from http://en.wikipedia.org/wiki/Miocene)

The earth went from the Oligocene Epoch through the Miocene and into the Pliocene as it cooled into a series of Ice Ages. The Miocene boundaries are not marked by a single distinct global event but consist rather of regional boundaries between the warmer Oligocene and the cooler Pliocene.

Miocene Life (from http://en.wikipedia.org/wiki/Miocene)

Flora

Grasslands underwent a major expansion; forests fell victim to a generally cooler and drier climate overall. Grasses also diversified greatly, co-evolving with large herbivores and grazers, including ruminants. Between 7 and 6 million years ago, there occurred a sudden expansion of grasses which were able to assimilate carbon dioxide more efficiently but were also richer in silica, causing a worldwide extinction of large herbivores.^[4] The expansion of grasslands and radiations among terrestrial herbivores such as horses can be linked to fluctuations in CO₂.^[5] Cycads between 11.5 and 5 m.y.a. began to rediversify after previous declines in variety due to climatic changes, and thus modern cycads are not a good model for a “living fossil”.

Fauna

Approximately 100 species of apes lived during this time. They ranged over much of the Old World and varied widely in size, diet, and anatomy. Due to scanty fossil evidence it is unclear which ape or apes contributed to the modern hominid clade, but molecular evidence indicates this ape lived from between 15 to 12 million years ago.

extract from Miocene Paleogeography (http://en.wikipedia.org/wiki/Miocene)

The global trend was towards increasing aridity caused primarily by global cooling reducing the ability of the atmosphere to absorb moisture. Uplift of East Africa in the Late Miocene was partly responsible for the shrinking of tropical rain forests in that region, and Australia got drier as it entered a zone of low rainfall in the Late Miocene.

Grass, Ruminants and Climate Change

The hypothesis I will now give is based on clues contained in the above Wikipedia extracts.

As the earth cooled due to continental rearrangement and mountain building the grassland biomes came to dominate the earth. This domination was aided by the co-evolution of herbivores in the Miocene; see extract below

(There is evidence for grassland being much the product of animal behaviour and movement;^[6] some examples include migratory herds of antelope trampling vegetation and African Bush Elephants eating Acacia saplings before the plant has a chance to grow into a mature tree.) from Wikipedia

The Ruminants have co-evolved with grasses during the Miocene, this co-evolution can be seen as a mutualistic as the grazing and trampling and fertilizing activities contribute to the maintenance and spread of grasslands. Both groups benefit in a service-service relationship, and they came to dominate the Miocene, Pliocene and Pleistocene terrestrial landscape

tbc