Pack Rat Middens

Another proxy method that scientists use to reconstruct past periods of climate change is through analysis of plant remains from fossil pack rat middens. The U.S. Geological Survey (USGS) is currently involved in research along these lines in the southwestern United States. Fossil pack rat (also known as wood rat) middens (crystallized urine) often con -tain abundant fossilized remains of leaves, twigs, fruits, seeds, bones, shells, and other dateable materials. These findings help reconstruct the past environment, illustrating what the climate conditions were at the time. Information about past atmospheric conditions is also contained in the middens. Scientists derive this data through analysis of the ratios of stable isotopes of oxygen, carbon, and deuterium. According to the USGS, radiocarbon dating has identified some middens that are more than 50,000 years old.

Because these samples are useful so far back in time, their analysis has become important for reconstructing biotic communities and envi ronmental conditions. To date, the USGS has been involved in pack rat midden and climate change studies in Arizona, California, Colorado, and Utah.

In a cave in Colorado, other researchers—Tony Barnosky, a paleo -biologist from the University of California, Berkeley, and Chris J. Bell,

Pack rats are roughly the size of laboratory rats. They gather plant materials at close range and accumulate them in dry caves and crevices. The plant remains and other debris become cemented into large masses of crystallized urine that can survive for tens of thousands of years and be used as climate proxies. (NOAA Paleoclimatology Paleo Slide Set; Ken Cole, USGS, photographer)

Utah Pack Rats
Paul Martin of the University of Arizona is examining a pack rat midden. It was not until the 1960s that paleoecologists realized that middens could be used as a tool to reconstruct past climatic conditions. (NOAA Paleoclimatology Paleo Slide Set; W.G. Spaulding, photographer)

an associate professor of geology at the University of Texas at Austin— discovered a pack rat collection of teeth and bones that dates back to between 600,000 and 1 million years ago. Due to the arid cave's controlled temperature and humidity, the specimens were well preserved. According to Tony Barnosky, "Everything in the cave has been nicely preserved at a controlled temperature and humidity, like putting the stuff in a refrigerator for 750,000 years. This is the first study where we've actually taken a living species and looked back almost a million years at the population level to see how it changes through time. Our study suggests that species adapt to handle routine climate change, and only something out of the ordinary initiates significant evolutionary change. It takes a long time for a species to change, and even the major climatic change 800,000 years ago wasn't dramatic enough to cause the origin of a new species."

Based on his studies, Barnosky concludes, "It's likely that specia-tion takes place over a longer time interval than extinction. So, climate changes like the global warming we are seeing today are probably happening too fast to cause anything but extinction."

In their studies, they have analyzed environmental conditions of the area over a large span of time and have identified evidence of climate changes, such as glacial and interglacial events. A major climate change that took place 800,000 years ago was identified. One of the interesting conclusions they made is that of species evolution and adaptation. Through past glacial and interglacial episodes, they were able to detect adaptations in certain species, such as voles.

One thing they caution is that these natural adaptations took place due to the climate over long periods of time. Today, however, with global warming, climate change is happening much faster than it has in the past; some species may not be able to adapt quickly enough and may face extinction. Pack rat midden studies in North American deserts, especially in caves and rock shelters of the arid interior, are currently under investigation by several universities as valuable proxies by which to study climate change and global warming.

Fossilized insect remains are also used as paleoclimatic proxies. A study in British Columbia, Canada, conducted by S. Bruce Archibald and Rolf W. Mathewes in 2000 determined that in the early Eocene, several different species of insects existed: bees, ants, March flies, dip-lopterine cockroaches, dinidorid bugs, and seed weevils. Some of these insects were larger than their cousins today. Archibald and Mathewes concluded that because of their presence in the fossil record, the Eocene in British Columbia was much warmer than it is today.

According to the National Oceanic and Atmospheric Administration (NOAA), the beetle (classified in the order coleoptera) is commonly found in both freshwater and land sediments. There are several different species of the beetle, each preferring a specific type of climate. Because scientists know what climatic conditions the corresponding present-day beetle species like, as well as dating the sediments the beetles were buried in, scientists can infer what the climate was once like using this proxy evidence.

For more than 1,000 years, scientists in China have monitored the abundance of locust populations. They have long been aware that locust outbreaks are most common when the climate is cold and wet. They have linked locust records with temperature and precipitation from 957 to 1956. Their results match the climate record of both floods (high locust populations) and droughts (low locust populations) in the lower Yangtze River.

According to Marianne S. V. Douglas, Roberto Quinian, and John P. Smol, in the Canadian high Arctic sedimentary records in three ponds on Ellesmere Island show major shifts in pond communities over the past 200 years. The results indicate that aquatic insect populations greatly increased in size and diversity in the early 1800s. They believe these changes happened because of shifts in algal (diatom) populations, which indicate an enhanced food chain. The increase in food supply was most likely due to climate warming, which reduced the ice cover in ponds. They predict that future warming in the Arctic may cause even more drastic ecological changes. According to the authors, the multitude of shallow ponds that have existed in the region for millennia are now completely drying out during the polar summers. Through the comparison of measurements of pond water to those made in the 1980s, the disappearance of the ponds is attributed to increased evaporation-precipitation ratios. They believe this is due to global warming. They state that the "final ecological threshold for these aquatic ecosystems has now been crossed—that of complete desiccation."

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