Chapter 2: A history of wildlife in North America
HUMANS AND WILDLIFE IN AMERICA
The following is a brief history of human-wildlife interactions in North America. The rather arbitrary "eras" used in this historical account follow Shaw (1985). Colonization of North America by Europeans began in the early 1600s when Europe was largely in a late agrarian stage of cultural development. The United States was founded during the early transition from a late agrarian to an early industrial economy. Much of the continent, especially the western regions, was settled during this transitional time period. In contrast, Native Americans were living in hunter-gatherer or horticultural societies at the time of European colonization and American expansion. The clashes between these two cultures had tragic consequences for the Native Americans and also resulted in dramatic declines in wildlife. The 20th century roughly corresponds with the transition of North America to a late industrial economy. Not surprisingly, a dramatic shift in the attitudes toward nature and the development of the conservation movement occurred during that time. This chapter documents these changes and illustrates them through extended examples of the history of bison and beaver.
Pre-European Era (11,500 B.C. to 1500 AD)
Humans invaded North America some time during the last ice age, roughly 13,500 years ago, when sea levels were lower and it was presumably possible to walk across the Bering Land Bridge connecting Siberia and Alaska (Flannery 2001). Although evidence is scanty, it seems likely that once the glaciers melted sufficiently to allow their passage out of Alaska, the colonizing humans spread across the continent and throughout South America in less than 1000 years. From the beginning, these people probably had a major impact on wildlife.
Before humans entered the picture, North America had an impressive assortment of large mammals and birds. The herbivores of this megafauna included 3 species of elephants (woolly mammoths, giant mammoths, and mastodons), horses, camels, giant bison, giant ground sloths, giant armadillos, tapirs, giant beaver, giant tortoises (roughly the size of Volkswagon bugs), and a peccary as large as the wild boars of Europe. An entire guild of now extinct mega-predators existed to feed on these large herbivores, including cheetahs, saber-toothed tigers, giant wolves, and two species of lion (one larger than the modern lions of Africa). There also existed a truly fearsome short-nosed bear, about twice the size of a modern grizzly bear, which ran its prey down like modern wolves do. Jaguars lived far north of their current tropical latitudes, into the boreal forests of Canada, as did many of the New World cats now restricted to Central and South America. There also existed a guild of large meat-eating birds, the largest of which were the teratorns, scavengers with wingspans up to five meters. The endangered California condor is the last remnant of these giant scavenger birds. There was even a giant vampire bat adapted to feeding off the blood of these enormous beasts.
The fate of all these species has been the topic of much scientific debate, but the majority of the evidence supports the hypothesis of "Pleistocene Overkill" (Martin and Wright 1967, Flannery 2001). This hypothesis suggests that as humans spread across the two continents, they preyed upon the large herbivores, such as mammoths, ground sloths, and horses, and wiped them out. Such large animals are more vulnerable to extinction than smaller ones because they cannot hide as easily, and because their lower reproductive rates cannot compensate for the losses due to hunting. They also may have had a fearlessness of humans, somewhat like the dodo bird, because these animals evolved with out human presence. When the large herbivores disappeared, their natural predators, such as saber-toothed tigers and short-nosed bears, became extinct as well. The large scavenger bird species, adapted to eating the remains of large animals, then followed into extinction. The California condor may have held on because it had access to the carcasses of marine mammals, which did not suffer high extinction rates at this time. The loss of the megafauna also impacted the diversity of smaller animals. Because large abundant animals (such as mammoths) alter plant communities by their intense grazing practices, their disappearance caused a major shift in the plant communities (e.g., from prairie to forest) resulting in the extinction of many smaller species that depended on the habitats maintained by the large grazers. In fact, there existed a grassland ecosystem in Alaska called the mammoth steppe that disappeared entirely once the woolly mammoth went extinct in that region, which is attributed the change in ecosystem processes that occurred when this keystone herbivore was lost (Flannery 2001).
The scenario of Pleistocene Overkill has been controversial. The principal alternative hypothesis to explain the rapid loss of this megafauna is the impact of climatic changes that occurred with the end of the ice age 13,000 years ago. There has also been a tendency to challenge the Pleistocene Overkill hypothesis by those who want to romanticize hunter-gatherers as living perfectly in balance with nature. But new data and discoveries by scientists increasingly confirm that the first Native Americans were indeed responsible for the extinction of these species. Evidence includes the finding of remains of mammoths and giant sloths butchered by humans and the general occurrence of widespread extinction coincident with the spread of humans. The Clovis people were the first humans to colonize North America. Their distinctive, beautifully made stone spearheads were well adapted to killing large herbivores, and have been found by archaeologists imbedded in the skeletons of large prey at many kill sites. This Clovis culture rapidly spread throughout North America, and then rather abruptly disappeared after about 300 years. The disappearance of the Clovis spearheads coincided almost exactly with extinction of the large game of North America. The physical conditions of mammoths at the time of the Clovis culture just before their extinction can be determined by looking at the growth rings in their tusks, which indicate that the animals were getting plenty of nutrition, reproducing frequently, and not experiencing the starvation stress that would accompany climate-driven extinction. In addition, many of the extinct species of megafauna had already survived several other glacial/interglacial climate cycles, and so presumably they could have survived one more. Furthermore, very similar patterns of extinction of megafauna occurred in Australia when humans first colonized that continent (Flannery 1994); this extinction event, however, did not coincide with a period of climate change. Over all it is estimated that the Pleistocene Overkill hypothesis illustrates a widely accepted fact: even hunter-gatherer humans were capable of having major effects on their environment.
One major, well-documented ecosystem alteration by Native Americans peoples was the burning of grasslands and forests, often deliberately, which kept them open and provided habitat for favored food animals such as bison and deer. In the absence of fire, many regions of prairie are invaded by trees and turn into forest. Even more drastic alterations of the landscape occurred when groups of Native Americans settled into permanent communities associated with the development of agriculture or fisheries. Such communities supported large numbers of people who cleared large tracts of land for agriculture. Where early agrarian civilizations developed, such as in the Ohio Valley (Mound Builders) or Central America (Mayan), forests in large regions were cleared or altered. In confined locations, such as the Hawaiian Islands, clearing of forests and hunting by the native peoples drove many species of animals to extinction; in continental areas it is likely that the populations of many species were greatly depleted, but few new extinctions occurred.
When the first Europeans arrived in North America and pushed their settlements into the interior, they were often impressed with the abundance of wildlife (Warren 2003). When Daniel Boone brought colonists over the Cumberland Gap to settle the Ohio Valley, he brought them into a wilderness of large trees, teeming with deer and bear. Two hundred years earlier, however, this same valley had been largely cleared for farms, tended by a dense population of Native Americans. The cause of the disappearance of so many Native Americans was disease. Smallpox, measles, and other diseases carried by the early explorers, from Columbus onward, apparently swept through the continent, decimating Native American populations, which had no resistance to them. Likewise, Cortez' conquest of Mexico was greatly assisted by the decimation of the Aztec population by a measles epidemic. Thus the first impact of Europeans on wildlife in the Americas was probably to increase wildlife populations through the tremendous and tragic reduction of the populations of indigenous peoples.
Disease is more likely to occur in dense populations of animals or humans, because transmission of the disease can occur more easily between individuals and there is a larger supply of susceptible hosts for the disease. Not surprisingly, the high population densities in the late agrarian and early industrial societies of Europe and Asia supported many highly pernicious communicable diseases, such as smallpox, measles, diphtheria, trachoma, whooping cough, chicken pox, bubonic plague (carried by fleas, which were carried by European rats), malaria, typhoid fever, cholera, yellow fever, dengue fever, scarlet fever, amoebic dysentery, influenza, and a number of worm infections (Figure 2.1). Most Europeans were relatively immune to diseases like measles and smallpox because they had been exposed in childhood, and because people from the Old World had experienced selection for more intrinsically defensive immune systems after living for centuries with epidemic diseases (Diamond 1999). When the Europeans came to America they brought these diseases with them. However, Native Americans had absolutely no antibodies to these diseases. To say that the effect of these illnesses on the population of the Americas was devastating would be an understatement. It has been estimated that by the end of the 17th century, between seventy and ninety percent of the population of the Native Americas had died of European-imported diseases.
Table of Contents
1. Roots of the modern environmental dilemma: A brief history of the relationship between humans and wildlife
2. A history of wildlife in North America
3. Climatic determinants of global patterns of biodiversity
5. Natural selection
6. Principles of ecology
7. Niche and habitat
8. Conservation biology
9. Conservation in the USA: legislative milestones
10. Alien invaders
11. Wildlife and Pollution
12. What you can do to save wildlife
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