Severe Drought, Other Changes Can Cause Permanent Ecosystem Disruption

This giant waterbug, once the top insect predator in a stream in Arizona's French Joe Canyon, has now disappeared in some places due to severe drought. (Credit: Photo by Michael Bogan)

Science Daily  — An eight-year study has concluded that increasingly frequent and severe drought, dropping water tables and dried-up springs have pushed some aquatic desert ecosystems into "catastrophic regime change," from which many species will not recover.

"Populations that have persisted for hundreds or thousands of years are now dying out," said David Lytle, an associate professor of zoology at Oregon State University. "Springs that used to be permanent are drying up. Streams that used to be perennial are now intermittent. And species that used to rise and fall in their populations are now disappearing."

The findings, just published in the journal Freshwater Biology, raise concerns that climate change, over-pumping of aquifers for urban water use, and land management may permanently affect which species can survive.

The research, done by Lytle and doctoral candidate Michael Bogan, examined the effect of complete water loss and its subsequent impact on aquatic insect communities in a formerly perennial desert stream in Arizona's French Joe Canyon, before and after severe droughts in the early 2000s.

The stream completely dried up for a period in 2005, and again in 2008 and 2009, leading to what researchers called a rapid "regime shift" in which some species went locally extinct and others took their place. The ecosystem dynamics are now different and show no sign of returning to their former state. Six species were eliminated when the stream dried up, and 40 others became more abundant. Large-bodied "top predators" like the giant waterbug disappeared and were replaced by smaller "mesopredators" such as aquatic beetles.

"Before 2004, this area was like a beautiful oasis, with lots of vegetation, birds and rare species," Lytle said. "The spring has lost a number of key insect species, has a lot less water, and now has very different characteristics."

The phenomena, the researchers say, does not so much indicate the disappearance of life -- there is about as much abundance as before. It's just not the same.

"Our study focused on a single stream in isolation, but this process of drying and local extinction is happening across the desert Southwest," Bogan said. "Eventually this could lead to the loss of species from the entire region, or the complete extinction of species that rely on these desert oases."

Small streams such as this are of particular interest because they can be more easily observed and studied than larger rivers and streams, and may represent a microcosm of similar effects that are taking place across much of the American West, the researchers said. The speed and suddenness of some changes give species inadequate time to adapt.

"It's like comparing old-growth forests to second-growth forests," Lytle said. "There are still trees, but it's not the same ecosystem it used to be. These desert streams can be a window to help us see forces that are at work all around us, whether it's due to climate change, land management or other factors."

The researchers noted in their report that the last 30 years have been marked by a significant increase in drought severity in the Southwest. The drought that helped dry up French Joe Canyon in 2005 resulted in the lowest flow in Arizona streams in 60 years, and in many cases the lowest on record. At French Joe Canyon, the stream channel was completely dry to bedrock, leaving many aquatic invertebrates dead in the sediments.

That was probably "an unprecedented disturbance," the researchers said in their report. Community composition shifted dramatically, with longer-lived insects dying out and smaller, shorter-lived ones taking their places.

Conceptually similar events have taken place in the past in plant communities in the Florida Everglades, floodplains in Australia, and boreal forests following fire disturbance, other researchers have found. In the Southwest, climate change models predict longer, more frequent and more intense droughts in the coming century, the scientists noted in their study.

The research was supported by the National Science Foundation.

100,000-Year-Old Ochre Toolkit and Workshop Discovered in South Africa

Science Daily — An ochre-rich mixture, possibly used for decoration, painting, and skin protection 100,000 years ago, and stored in two abalone shells, was discovered at Blombos Cave in Cape Town, South Africa.

The findings were published in the journal Science on Oct. 14, 2011.

"Ochre may have been applied with symbolic intent as decoration on bodies and clothing during the Middle Stone Age," says Professor Christopher Henshilwood from the Institute for Human Evolution at the University of the Witwatersrand, Johannesburg, who, together with his international team discovered a processing workshop in 2008 where a liquefied ochre-rich mixture was produced.

The two coeval, spatially associated toolkits were discovered in situ (not having been moved from its original place of deposition), and the kits included ochre, bone, charcoal, grindstones, and hammerstones. The grinding and scraping of ochre to produce a powder for use as a pigment was common practice in Africa and the Near East only after about 100,000 years ago.

"This discovery represents an important benchmark in the evolution of complex human cognition (mental processes) in that it shows that humans had the conceptual ability to source, combine and store substances that were then possibly used to enhance their social practices," explains Henshilwood.

"We believe that the manufacturing process involved the rubbing of pieces of ochre on quartzite slabs to produce a fine red powder. Ochre chips were crushed with quartz, quartzite and silcrete hammerstones/grinders and combined with heated crushed, mammal-bone, charcoal, stone chips and a liquid, which was then introduced to the abalone shells and gently stirred. A bone was probably used to stir the mixture and to transfer some of the mixture out of the shell."

The quartz sediments in which the ochre containers were buried were dated to about 100,000 years using Optically Stimulated Luminescence (OSL) dating. This is consistent with the thermoluminescence dating of burnt lithics and the dating of calcium carbonate concretions using uranium-series dating methods.

"The recovery of these toolkits adds evidence for early technological and behavioural developments associated with humans and documents their deliberate planning, production and curation of pigmented compound and the use of containers. It also demonstrates that humans had an elementary knowledge of chemistry and the ability for long-term planning 100,000 years ago," concludes Henshilwood.

The two specimens will be on display at the Iziko Museum in Cape Town from Friday, 14 October 2011.

*Ochre is the colloquial term used by archaeologists to describe an earth or rock containing red or yellow oxides or hydroxides of iron

*Note: The Blombos Cave is situated on the southern Cape Coast, 300km east of Cape Town, South Africa.

Future Forests May Soak Up More Carbon Dioxide Than Previously Believed

An aerial view of the 38-acre experimental forest in Wisconsin where U-M researchers and their colleagues continuously exposed birch, aspen and maple trees to elevated levels of carbon dioxide and ozone gas from 1997 through 2008. (Credit: David Karnosky, Michigan Technological University)

Science Daily — North American forests appear to have a greater capacity to soak up heat-trapping carbon dioxide gas than researchers had previously anticipated.

The results of a 12-year study at an experimental forest in northeastern Wisconsin challenge several long-held assumptions about how future forests will respond to the rising levels of atmospheric carbon dioxide blamed for human-caused climate change, said University of Michigan microbial ecologist Donald Zak, lead author of a paper published online this week in Ecology Letters.As a result, they could help slow the pace of human-caused climate warming more than most scientists had thought, a U-M ecologist and his colleagues have concluded.

"Some of the initial assumptions about ecosystem response are not correct and will have to be revised," said Zak, a professor at the U-M School of Natural Resources and Environment and the Department of Ecology and Evolutionary Biology in the College of Literature, Science, and the Arts.

To simulate atmospheric conditions expected in the latter half of this century, Zak and his colleagues continuously pumped extra carbon dioxide into the canopies of trembling aspen, paper birch and sugar maple trees at a 38-acre experimental forest in Rhinelander, Wis., from 1997 to 2008.

Some of the trees were also bathed in elevated levels of ground-level ozone, the primary constituent in smog, to simulate the increasingly polluted air of the future. Both parts of the federally funded experiment -- the carbon dioxide and the ozone treatments -- produced unexpected results.

In addition to trapping heat, carbon dioxide is known to have a fertilizing effect on trees and other plants, making them grow faster than they normally would. Climate researchers and ecosystem modelers assume that in coming decades, carbon dioxide's fertilizing effect will temporarily boost the growth rate of northern temperate forests.

Previous studies have concluded that this growth spurt would be short-lived, grinding to a halt when the trees can no longer extract the essential nutrient nitrogen from the soil.

But in the Rhinelander study, the trees bathed in elevated carbon dioxide continued to grow at an accelerated rate throughout the 12-year experiment. In the final three years of the study, the CO₂-soaked trees grew 26 percent more than those exposed to normal levels of carbon dioxide.

It appears that the extra carbon dioxide allowed trees to grow more small roots and "forage" more successfully for nitrogen in the soil, Zak said. At the same time, the rate at which microorganisms released nitrogen back to the soil, as fallen leaves and branches decayed, increased.

"The greater growth has been sustained by an acceleration, rather than a slowing down, of soil nitrogen cycling," Zak said. "Under elevated carbon dioxide, the trees did a better job of getting nitrogen out of the soil, and there was more of it for plants to use."

Zak stressed that growth-enhancing effects of CO₂ in forests will eventually "hit the wall" and come to a halt. The trees' roots will eventually "fully exploit" the soil's nitrogen resources. No one knows how long it will take to reach that limit, he said.

The ozone portion of the 12-year experiment also held surprises.

Ground-level ozone is known to damage plant tissues and interfere with photosynthesis. Conventional wisdom has held that in the future, increasing levels of ozone would constrain the degree to which rising levels of carbon dioxide would promote tree growth, canceling out some of a forest's ability to buffer projected climate warming.

In the first few years of the Rhinelander experiment, that's exactly what was observed. Trees exposed to elevated levels of ozone did not grow as fast as other trees. But by the end of study, ozone had no effect at all on forest productivity.

"What happened is that ozone-tolerant species and genotypes in our experiment more or less took up the slack left behind by those who were negatively affected, and that's called compensatory growth," Zak said. The same thing happened with growth under elevated carbon dioxide, under which some genotypes and species fared better than others.

"The interesting take home point with this is that aspects of biological diversity -- like genetic diversity and plant species compositions -- are important components of an ecosystem's response to climate change," he said. "Biodiversity matters, in this regard."

Co-authors of the Ecology Letters paper were Kurt Pregitzer of the University of Idaho, Mark Kubiske of the U.S. Forest Service and Andrew Burton of Michigan Technological University. The work was funded by grants from the U.S. Department of Energy and the U.S. Forest Service.