Saturday, December 25, 2010

EARLY SETTLERS RAPIDLY TRANSFORMED NEW ZEALAND FORESTS WITH FIRE

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New research indicates that the speed of early forest clearance following human colonisation of the South Island of New Zealand was much faster and more intense than previously thought.

Charcoal recovered from lake-bed sediment cores show that just a few large fires within 200 years of initial colonization destroyed much of the South Island's lowland forest. Grasslands and shrubland replaced the burnt forest and smaller fires prevented forests from returning.

The findings - by an international team led by Dave McWethy and Cathy Whitlock from Montana State University- have just been published in the prestigious journal Proceedings of the National Academy of Sciences in the United States and will be explored further under new grants from the National Science Foundation Geography and Spatial Science (GSS) and Partnerships in International Research and Education (PIRE) programs (www.wildfirepire.org).

Previous studies by co-authors Matt McGlone and Janet Wilmshurst at Landcare Research in New Zealand showed that closed forests covered 85-90% of New Zealand prior to the arrival of Polynesians (Māori) 700-800 years ago, but by the time Europeans settled in the mid 19th century, grass and shrubs had replaced over 40% of the South Island's forests. Despite this information, questions over the timing, rapidity, and cause of the extensive forest clearance have remained.

The international team of scientists reconstructed the environmental history of 16 small lakes in the South Island, New Zealand. They used pollen records to reconstruct past vegetation, charcoal fragments to document fires, and algae and midge remains to quantify changes in lake chemistry and soil erosion.

The cores showed several high-severity fire events occurred within two centuries of known Māori arrival in the 13th Century.

"The impacts of burning were more pronounced in drier eastern forests where fires were severe enough to clear vast tracts of forest and cause significant erosion of soils and nutrients. Because the initial Māori populations were small, we can only conclude that forests were highly vulnerable to burning," McWethy said.

Wilmshurst said archaeological evidence suggests that successful cultivation of introduced food crops, such as kumara and taro, was only possible in warmer northern coastal areas and the starch-rich rhizomes of bracken fern, which replaced the burnt forests, provided an essential part of Māori diets in colder regions.

"In their efforts to increase the productivity of lowland forests for food, Māori encouraged a more heterogeneous and economically useful fern-shrubland at the same time as making travel easier to search for food and stone resources for making tools," Wilmshurst said.

Newly derived records of past climate enabled the team to disprove the hypothesis that unusual climate conditions encouraged fire at around the time of Māori settlement.

"Our evidence suggests that human activity was the main cause of the fires, and that these fires were not related to any unusually dry or warm conditions at the time," McGlone said.

Before human arrival in New Zealand, fire was naturally rare in most forests, with lightning-started fires occurring perhaps only once every 1-2 thousand years.

"What is remarkable is that small mostly subsistence-based groups of people were able to burn large tracts of forests throughout the relatively large South Island (151,215 km2) in only a few decades," McWethy said.

Whitlock said "Changes in the fossils and chemistry of the lake sediments showed that soil erosion followed initial forest clearance. In some regions, this degradation was exacerbated by intensive clearance in the 19th Century by European pastoralists who developed the land for grazing sheep and farming."

This study shows the extent to which a small number of settlers can transform a vast and topographically complex landscape through land-use change alone, and highlights how exceptionally vulnerable New Zealand forests were to fire in the past. The authors suggest that understanding the history of people and fire in New Zealand will help researchers and managers develop informed forest fire management and conservation strategies.

Montana State University

HUMANS HELPED VULTURES COLONIZE THE CANARY ISLANDS

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The Egyptian vulture population of the Canary Islands was established following the arrival of the first human settlers who brought livestock to the islands. A genetic comparison of Iberian and Canarian birds, published in the open access journal BMC Evolutionary Biology, found that the Egyptian vulture population in the Canary Islands was likely established around 2500 years ago – around the same time as humans began to colonise the islands.

Rosa Agudo worked with a team of researchers from the Doñana Biological Station, Seville, Spain, to investigate genetic and morphological changes between 143 Iberian birds and 242 from Fuerteventura, one of the Canary Islands. She said, "We found that the island vultures are significantly heavier and larger than those from Iberia. The establishment of this insular population took place some 2500 years ago, matching the date of human colonization. Our results suggest that human activity can trigger divergent evolution and that this process may take place on a relatively brief time scale".

The authors suggest that before the arrival of humans, the Canary Islands would not have been able to support vultures, as food resources would have been scarce, consisting only of the remains of seabirds and sea mammals, or of rodents. They say, "The introduction of new and abundant food sources by humans could have allowed not only colonization by vultures, but also their demographic expansion and their putative adaptation to the new island environment". For once, human activity has actually assisted in the diversification and adaptation of the Egyptian vulture, now globally threatened and classified as 'Endangered' on the IUCN Red List.

BMC

DEMISE OF LARGE SATELLITE MAY HAVE LED TO THE FORMATION OF SATURN'S RINGS AND INNER MOONS

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Simulations performed at Southwest Research Institute may explain how Saturn's majestic rings and icy inner moons formed following the collision of a Titan-sized satellite with the planet, according to a paper published in Nature magazine's Dec. 12 Advance Online Publication.

Saturn's rings are at present 90 to 95 percent water ice. Because dust and debris from rocky meteoroids have polluted the rings, the rings are believed to have consisted of pure ice when they formed. This composition is unusual compared to the approximately half-ice and half-rock mixture expected for materials in the outer Solar System. Similarly, the low densities of Saturn's inner moons show that they too are, as a group, unusually rich in ice.

The previous leading ring origin theory suggests the rings formed when a small satellite was disrupted by an impacting comet. "This scenario would have likely resulted in rings that were a mixture of rock and ice, rather than the ice-rich rings we see today," says the paper's author, Dr. Robin M. Canup, associate vice president of the SwRI Planetary Science Directorate in Boulder.

The new theory links the formation of the rings to the formation of Saturn's satellites. While Jupiter has four large satellites, Saturn has only one, Titan. Previous work suggests that multiple Titan-sized satellites originally formed at Saturn, but that those orbiting interior to Titan were lost as their orbits spiraled into the planet.

As the final lost satellite neared Saturn, heating caused by the flexing of its shape by the planet's gravity would cause its ice to melt and its rock to sink to its center. Canup uses numerical simulations to show that as such a satellite crosses the region of the current B ring, planetary tidal forces strip material from its outer icy layers, while its rocky core remains intact and eventually collides with the planet. This produces an initial ice ring that is much more massive than Saturn's current rings.

Over time, collisions in the ring cause it to spread radially and decrease in mass. Inwardly spreading ring material is lost, while material spreading past the ring's outer edge accumulates into icy moons with estimated masses consistent with the inner moons seen today.

"The new model proposes that the rings are primordial, formed from the same events that left Titan as Saturn's sole large satellite," says Canup. "The implication is that the rings and the Saturnian moons interior to and including Tethys share a coupled origin, and are the last remnants of a lost companion satellite to Titan."

During its extended mission, the Cassini spacecraft will measure the rings' current mass and will indirectly measure the pollution rate of the rings. This should provide an improved estimate of the rings' age and a test of the new ring origin model.

(Photo: Southwest Research Institute)

Southwest Research Institute

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