CORVALLIS, Ore. - Scientists have long struggled to explain how tropical forests can maintain their staggering diversity of trees without having a handful of species take over - or having many other species die out.
The answer, researchers say, lies in the soil found near individual trees, where natural "enemies" of tree species reside. These enemies, including fungi and arthropods, attack and kill many of the seeds and seedlings near the host tree, preventing local recruitment of trees of that same species.
Archaeological data allow us to consider human actions over extended periods of time in a way that few other sources can. This is particularly true when it comes to studying human resilience in the face of environmental disasters. From approximately A.D. 450-1400, a Native American group known today as the Hohokam overcame a harsh desert environment along with periodic droughts and floods to settle and farm much of modern Arizona. They managed this feat by collectively maintaining an extensive infrastructure of canals with collaborative labor.
The new excavations, however, were able to employ optically stimulated luminescence dating methods that reveal how long-ago quartz sand particles were heated by the fiery desert sun. With this new dating technique, the researchers were able to identify three distinct damaging floods that occurred between A.D. 1000 and 1400.
After each flood the Native American communities that relied upon the canal system to irrigate their fields banded together to repair the canal intakes, clear the channels of accumulated sediments, and repair canal walls and berms. Responding to disasters, however, strains social systems, even in the best of times.
Dr. Scott Johnson, author of Why Did Ancient Civilizations Fail, notes “Throughout human history, from the Egyptians and Romans to the Maya, the more that people modify their surroundings, the more they become dependent on those alterations.” By A.D. 1300, Hohokam populations throughout the Southwest were rising, resulting in increasing strains on natural resources and human social organization. The third flood identified by Desert Archaeology, Inc. brought more drastic consequences for the Native American communities living along the Salt River, with shrinking populations and only minimal repairs to the canals.
Johnson adds that “As the environment changes over time, for both natural and anthropogenic reasons, the more difficult it becomes to maintain those modifications. We see it in the Hohokam canals, Mesopotamian flood agriculture, Maya wetland farming, and our own society's dependence on fossil fuels. We ignore the examples of the failure to adapt throughout the ancient world at our peril.”