“You may have horses…. But remember this: if you have horses everything will be changed for you forever.”
“The historian of the early military must first see how a society is fed before he can learn how it fights.”
“When civilized nations come into contact with barbarians the struggle is short, except where a deadly climate gives aid to the native race.”
It has is somewhat of a fashion among conservative thinkers to mark their defeat in America’s ‘culture wars’ by composing requiems for the classical education no longer taught at American universities.  When the lament turns to the study of history (as it invariably does), the eulogizer will raise two strains of melancholy, the first reporting the absence of even most basic historical knowledge among today’s university graduates and the second relating the sad eclipse of the field’s traditional focus on political, economic, and diplomatic history with the newer, less rigorous, and less relevant sub-fields like queer, gender, post-colonial, subaltern, and environmental history.
As an observer with conservative leanings I occasionally succumb to the temptation to don sack-cloth and wail in lamentation myself. The gist of the standard lament is true enough. As stated here I only regret the last sub-field’s inclusion in it. What environmental history shares with the other sub-fields listed is a popularity derived from progressive political concerns. The connection is unfortunate, for in most other ways these fields are—or at least, should be—completely different. The historian who sets out to study “Urban Culture and Transgender Sexualities in 18th Century Madrid,” for example, has committed him or herself to the intense examination of such a tiny and unrepresentative sliver of humanity as to make all of his or her conclusions useless for understanding the broader course of the human past. The same cannot be said for environmental history. There are few fields whose findings have such a clear and wide ranging impact on every other aspect of human civilization. The rise and fall of dynasties, the great deeds of armies and generals, the wealth and poverty of nations, and the daily life of men and women across human history were molded by the ecological setting in which they occurred.
A book I often recommend to those who doubt that environmental history is essential to making sense of human civilization is Alfred Crosby’s Ecological Imperialism: The Biological Expansion of Europe, 900-1900. Over the last three decades a torrent of books and articles have been written to explain why the West was able to rise above ‘the rest’ and establish global supremacy. While of the same vein as these works, the question that animates Ecological Imperialism is slightly different: why were Europeans so successful at reproducing European society (and completely displacing the previous inhabitants) of some locales but unable to accomplish this same feat in other locations? Why were white settlers in North America, New Zealand, Australia, Argentina, and South Africa fantastically more successful than their fellow colonists in Brazil, Mozambique, Panama, or New Guinea?
The answer, says Crosby, is ecology. European expansion was not just a movement of peoples, but of entire environments. For settlers to survive and thrive they must be able to secure food, construct buildings, and move about from place to place. European civilization allowed Europeans to do this on a scale new to human history, but the successes of Western civilization cannot be separated from the environment from which they sprang. Its great cities, armies, and ships were ultimately built upon a unique suite of European flora and fauna. When transplanted far from their homeland these alien organisms do better in some lands than in others. Places where climate and local disease prove hostile to European biota, like central Africa, are places where Western imperialists could only establish an ephemeral presence. Places like Brazil or Mexico, less deadly to European life but unsuitable for large-scale colonization without adopting indigenous crops and farming techniques, produced creole cultures that mixed European and Amerindian traditions. In Australia, New Zealand, North America, and the South African coastline the only environmental constraint European settlers faced was distance. The geography, climate, and ecology of these places were perfect for European biota, allowing them to displace native life without conscious effort.
The displacement was complete and utter. American readers may be surprised to find out how much of America’s ‘natural and wild’ wilderness is made of European aliens of recent import. Sparrows, starlings, house flies, honeybees, garden snails, earthworms, common rats, white clovers, dandelions, Kentucky blue-grass, stinging nettles, knot-grass, broadleaf plantains, Bermuda grass, periwinkles, mayweed, ground ivy, knapweed, milk thistles and almost every type of grass you can find east of the Mississippi originated in Europe and came to the United States in the two centuries after 1650. And that is just a small sample of the hundreds of plants and animals that came to America along with the Europeans. What started as a few alien weeds accidentally carried across the sea grew to dominate entire ecosystems. By 1940 an ecological survey in Southern California could report that “63% of herbaceous vegetation in the grassland types, 66% in the woodland, and 54% in the chaparral” were naturalized plants. 
The expansion of European biota across the land was not simply a consequence of European migration. In most cases it was an essential precursor to large-scale European immigration itself. European colonization of New Zealand’s South Island is a case in point. The first settlers in New Zealand did not believe that sheep could ever prosper there, for both islands, being carpeted by ferns or covered with dense forest, had no grass to speak of and nothing sheep could survive on. Initial attempts to remedy the situation by introducing flowering plants to New Zealand failed. The plants would grow but could not reproduce: New Zealand had no insect species adapted to pollinate them! It was not until settlers brought honeybees to the islands that the situation changed, leading to an explosion of European plants across both islands. The new grasses and clovers were perfect feed for English sheep, and it was not long before the sheep had reached such numbers that they became one of New Zealand’s chief exports and an economic pull for more migrants. 
While Professor Crosby does not use this term, it should be clear to today’s readers that the European settlers, crops, livestock, weeds and grasses were all part of one interconnected system upon which the settler communities were built. The colonization of the Americas, Africa, Polynesia, and Australia should be seen as the evolution of one ecological system into another.
This conception of environmental history is far more useful than many narratives historians have come to rely on when writing on this topic. Environmental histories are generally of two types. One divides the Earth into a world of nature and the world of man, the second advancing at the expense of the first. This is the tale of cleared forests, extinct species, polluted waters, and the ever growing ‘footprint’ of humanity.  The second narrative is one of exploitation and collapse, positing that societies decay and collapse when they overuse the ecological resources available to them.  Both present a moral tale easily used to condemn 21st century man’s ruinous relationship with nature.
These two narratives have some truth to them, even though their arrangement reveals more about current political concerns than actual historical dynamics. A much greater weakness with these narratives is that it is difficult to fit the type of transformations and systems Crosby discusses into their confines. In many ways the models used by ecologists to describe the role of individual species in their ecological systems and the transitions these systems make from one state to another are better suited to describe human interactions with the broader environment than any of the traditional narratives are. Humans are far from the only species that “engineer the environment to make it suitable for [their own population] growth”  and the basic models that describe the ecological dynamics of plankton, savanna grass, and rainforests should be studied just as closely by historians as documentary sources that reveal how past societies depicted or thought about their relationship with the environment.
This approach is not without its own difficulties. Perhaps the biggest problem is that ecologists themselves have developed few models that integrate the complexities of human energetics with their traditional field of study. As ecologists Erle Ellis and Navin Ramankutty note:
“More than 75% of Earth’s ice-free land showed evidence of alteration as a result of human residence and land use, with less than a quarter remaining as wildlands, supporting just 11% of terrestrial net primary production… [but] ecologists have long been known as the scientists who travel to uninhabited lands to do their work. As a result, our understanding of anthropogenic ecosystems remains poor when compared with the rich literature on “natural” ecosystems.” 
As time goes on we can only hope that more ecologists, geographers, environmental scientists, archaeologists, anthropologists, and historians work to provide the more nuanced understanding of human and environmental systems Ellis and Ramankutty advocate, one where we do not depict environmental history as the story of “natural systems, with humans disturbing them,” but one of “human systems, with natural ecosystems embedded within them.” 
 The nature of these changes was summarized briefly at T. Greer, “Do the Great Books Have a Place in the 21st Century?“, The Scholar’s Stage (27 May 2013).
 Alfred Crosby, Ecological Imperialism: The Biological Expansion of Europe, 900-1900 (Cambridge: Cambridge University Press, 1993), p. 154.
 ibid., pp. 217-269.
 The titles of the two most famous environmental histories of China, Mark Elvin’s Retreat of the Elephants: an Environmental History of China and Judith Shapiro’s Mao’s War Against Nature: Politics and the Environment in Revolutionary China, fit this narrative neatly into one sentence titles.
 For example, Jared Diamond’s Collapse: How Societies Choose to Fail or Suceed and Clive Pontig’s A New Green History of the World: the Environment and the Collapse of Civilizations (though this last title probably qualifies for both categories).
 Marten Scheffer, Critical Transitions in Nature and Society (Princeton: Princeton University Press, 2009), 16. The book contains several examples of these, but my favorite are the algae and fish in shallow water lakes covered in p. 111-141.
 Erle Ellis and Navin Ramakutty, “Putting People in the Map: Anthropogenic Biomes of the World,” Frontiers in Ecology and the Environment, vol 6 (2008), p. 439; 446.
 Ibid., 445.