What was the main reason for the decline of native American populations after European contact?

In the 1500s, the ponderosa pine forests of Jemez province in New Mexico were home to between 5000 and 8000 people. But after Europeans arrived in the area, the native population plummeted by more than 80%, probably because of a series of devastating epidemics. A new study suggests the crash took place 100 years after the first contact with Europeans. It also suggests that the sudden drop in the local population had dramatic ecological effects, including an increase in forest fires.

The authors of the paper used a "terrific combination" of dendro-ecology—which uses the rings of trees to determine their ages and reconstruct past environments—fire ecology, and LiDAR, a remote sensing technique based on laser light, says Steve Lekson, a Southwestern archaeologist at the University of Colorado, Boulder, who wasn't involved in the study. "It's such an amazing approach," agrees Richard Nevle, an environmental scientist at Stanford University in Palo Alto, California, who has studied the ecological effects of the American Indian population crash. "No one has really pulled all these different pieces together so well before. It raises the bar."

American Indian populations plummeted after the arrival of Europeans in the New World, largely because of the spread of smallpox, typhus, measles, and other infectious diseases. But archaeologists and historians have debated the exact timing and severity of the decline. Did diseases race out ahead of colonial settlement, decimating communities that hadn't even met Europeans? Or did it take more sustained contact between the two populations to spark epidemics?

The new study suggests the latter, at least in the Jemez province in northern New Mexico. At one time, the region was full of villages and fieldhouses, says Matthew Liebmann, an archaeologist at Harvard University and lead author of the paper. "You can't walk around on top of these mesas for more than a couple hundred yards without coming across an archaeological site." Although few of these sites have been excavated, they have been well preserved because they lie within a national forest.

So Liebmann and his team just needed a way to map the ruins without the trees getting in the way. That's exactly what LiDAR allowed them to do: By sending pulses of laser light down from an airplane, the team soon had a map of the Jemez province, sans trees. Then, they converted the volume of rubble visible today into an approximate number of rooms that must have existed in each of the large villages. From this, they estimated how many people lived there just before the first Spanish explorers arrived in the area in 1541: between 5000 and 8000. That made the province "probably one of the more densely occupied areas of the American Southwest on the eve of European contact," Liebmann says.

To figure out when the Jemez villages were abandoned, Liebmann and his colleagues counted the rings of trees now growing on the landscape. When a tree is growing, say, inside what was once a house, "the age of that tree would give you at least a minimum time since the structure was last occupied," says Tom Swetnam, a dendrochronologist at the University of Arizona in Tucson who worked with Liebmann. Many trees now growing in and around the Jemez villages sprouted in the 1630s and 1640s, suggesting that the communities were abandoned around that time, the team reports today in the Proceedings of the National Academy of Sciences.

That's about 100 years after the first contact with Europeans, but it coincides with the establishment of Catholic missions, the first permanent European settlements in the region. That suggests the area's colonization triggered the depopulation, says Nevle, not something that happened before, such as occasional contact with Spanish explorers. The exact details remain unclear, but perhaps "it was something about bringing so many people together in the mission system that provided the critical mass for diseases to move across the population," Nevle says.

Colonial documents mention plagues sweeping through Jemez in the 17th century, which supports the idea that a large part of the depopulation was due to disease. The few survivors eventually retrenched in one village near the main mission, which may have also contributed to the abandonment of other settlements. Four decades later, by the early 1680s, there were fewer than 850 Pueblo people left in Jemez province, LiDAR data from the remaining village and Spanish colonial documents indicate. That's a loss of 87% of the original population. The native peoples' descendants, members of the Pueblo of Jemez, still live there today.

The tree samples also revealed scars from forest fires, which allowed Swetnam to see how the fire ecology of the Jemez province changed after the population crash. When the villages were still occupied, "people were using every stick of wood for firewood," Swetnam says, so forest fires didn't have much fuel to work with. Before 1620, extensive fires burned through Jemez about once every 17 years, he calculated; after 1620, that increased to every 11 years, presumably because people were no longer clearing the underbrush and chopping down trees for construction.

What's more, "there's no evidence that any of these villages that we're studying had any catastrophic fires go through them," Swetnam says. That is in stark contrast to today, as forest fires are becoming both more frequent and more intense in the Jemez region. "The role of ancient Pueblo people in culling or thinning understory provides a really intriguing model for how we might decrease the number of catastrophic fires," Lekson says. Liebmann says he is already working with the Pueblo of Jemez to help them apply his study's lessons about sustainably controlling fire.

The number of Native Americans quickly shrank by roughly half following European contact about 500 years ago, according to a new genetic study.

The finding supports historical accounts that Europeans triggered a wave of disease, warfare, and enslavement in the New World that had devastating effects for indigenous populations across the Americas.

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Using samples of ancient and modern mitochondrial DNA—which is passed down only from mothers to daughters—the researchers calculated a demographic history for American Indians. (Get an overview of human genetics.)

Based on the data, the team estimates that the Native American population was at an all-time high about 5,000 years ago.

The population then reached a low point about 500 years ago—only a few years after Christopher Columbus arrived in the New World and before extensive European colonization began.

Study co-author Brendan O'Fallon, a population geneticist who conducted the research while at the University of Washington in Seattle, speculates that many of the early casualties may have been due to disease, which "would likely have traveled much faster than the European settlers themselves."

For instance, the Franciscan friar Toribio de Benavente—one of the first Spanish missionaries to arrive in the New World in the early 1500s—wrote that Mexico was initially "extremely full of people, and when the smallpox began to attack the Indians, it became so great a pestilence among them ... that in most provinces more than half the population died."

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Some historians have questioned whether such effects were restricted to particular cities or regions, but the new study suggests mortality was widespread.

Pinpointing a Recent Native American Decline

The new analysis—published in this week's online early edition of the Proceedings of the National Academy of Sciences—found that modern Native Americans are more genetically similar to one another than those living before European contact. This suggests the population size was reduced sometime in the recent past.

Imagine "selecting two individuals from a very small village and asking them how many generations ago they first shared a common ancestor. It's likely to have not been that long," O'Fallon said.

"On the other hand, if the village is very large, you might have to go back a long way to find a common ancestor."

The results run counter to earlier genetic studies, which found no evidence of a recent population contraction among American Indians.

But those earlier studies did not include ancient DNA, which is crucial for establishing an accurate time line, O'Fallon said.

"One previous study did find a decline in population size among Native Americans but inferred the time of the decrease as around 1,000 to 2,000 years ago, [which is] hard to reconcile with what we know about Native American history," he said.

(Related: "American Indian Sailed to Europe With Vikings?")

Although the new study is based on DNA, the researchers caution that their use of statistical analysis means the findings aren't conclusive and can only suggest that a particular scenario most likely occurred.

"Our methods infer thousands of genealogies," O'Fallon said. "By looking at the bulk properties of all these genealogies we can begin to get a clearer picture of what likely happened."

In addition, the margin of error for the new study is rather large, O'Fallon said, so it's possible the decline happened more recently than 500 years ago.

"I don't think it would rule out European influence at all if the bottleneck happened a bit more recently than 500 years ago," he said.

Instead, a slightly more recent time frame might change "our interpretation [of the early cause of the decline] from disease to other causes such as war, societal disruption, loss of homelands, etc."

A Short-Lived Population Bottleneck?

Despite revealing a dramatic drop, the new study suggests that Native American populations eventually recovered to their predecline levels, likely aided by the development of resistance to European diseases.

Furthermore, the genetic health of the group did not appear to suffer long-term damage.

"Our study did not find a substantial reduction in genetic diversity," O'Fallon said. "The bottleneck was fairly short-lived and, while significant, didn't appear to eliminate many lineages that were present before Europeans arrived."

(Related: "Sixteen Indian Innovations: From Popcorn to Parkas.")

Overall, the new results "are some of the most detailed information scientists have about Native American ancestral population demography based on genetic data," said Quentin Atkinson, an evolutionary anthropologist at the University of Auckland in New Zealand, who was not involved in the study.

Commenting via email, Atkinson called the findings "intriguing and suggestive," but he said more work will be needed to reduce uncertainties in both the estimated magnitude and timing of the population reduction.