Humans changed the ecosystems of Central Africa more than 2,600 years ago

Fields, streets and cities, but also forests planted in rank and file, and dead straight rivers: humans shape nature to better suit their purposes, and not only since the onset of industrialization. Such influences are well documented in the Amazonian rainforest. On the other hand, the influence of humans was debated in Central Africa where major interventions seem to have occurred there 2,600 years ago: Potsdam geoscientist Yannick Garcin and his team have published a report on their findings in the Proceedings of the National Academy of Sciences. The research team examined lake sediments in southern Cameroon to solve the riddle of the "rainforest crisis." They found that the drastic transformation of the rainforest ecosystem at this time wasn't a result of climatic change, it was humankind.

Farming activities in North-West Region of Cameroon [Credit: Better World Cameroon]

More than 20 years ago, the analysis of lake sediments from Lake Barombi in southern Cameroon showed that older sediment layers mainly contained tree pollen reflecting a dense forested environment. In contrast, the newer sediments contained a significant proportion of savannah pollen: the dense primitive forest quickly transformed into savannahs around 2,600 years ago, followed by an equally abrupt recovery of the forest approximately 600 years later. For a long time, the most probable cause of this sudden change, dubbed the "rainforest crisis," was thought to be climate change brought about by a decrease in precipitation amount and increase in precipitation seasonality. Despite some controversy, the origin of the rainforest crisis was thought to be settled.

High volumes of precipitation in the region (over 3,000 mm annually) have ensured that the lake has not dried out over
the millenia. This heavy rainfall has created large volumes of sediment, which are then washed into the lake.
These circumstances make it possible to perform sediment analyses with the utmost precision
[Credit: B. Brademann/GFZ]

Yet Garcin, a postdoctoral researcher at the University of Potsdam, and his international team of scientists from UP, CEREGE, IRD, ENS Lyon, GFZ, MARUM, AMU, AWI, and from Cameroon suspected that other causes could have led to the ecosystem's transformation. By reconstructing both vegetation and climate change independently -- through stable isotope analysis of plant waxes, molecular fossils preserved in the sediment -- the team confirmed that there was a large change in vegetation during the rainforest crisis, but indicated that this was not accompanied by a change in precipitation.

"The rainforest crisis is proven, but it cannot be explained by a climate change," says Garcin. "In fact, in over 460 archaeological finds in the region, we have found indications that humans triggered these changes in the ecosystem." Archaeological remains older than 3,000 years are rare in Central Africa. Around 2,600 years ago, coincident with the rainforest crisis, the number of sites increased significantly, suggesting a rapid human population growth -- probably related to the expansion of the Bantu-speaking peoples in Central Africa. This period also saw the emergence of pearl millet cultivation, oil palm use, and iron metallurgy in the region.

This floating platform can be completely taken apart and transported overseas. The platform enabled the collection of
sediment samples in the approximately 100-meter-deep Lake Barombi, which were then analyzed in the laboratory
[Credit: Y. Garcin/University of Potsdam]

"The combination of regional archaeological data and our results from the sediments of Lake Barombi shows convincingly that humans strongly impacted the tropical forests of Central Africa thousands of years ago, and left detectable anthropogenic footprints in geological archives," says Dirk Sachse at the Helmholtz Center Potsdam -- Research Center for Geosciences (GFZ). Sachse was one of the major contributors to the development of the method for analyzing plant wax molecular fossils (termed biomarkers).

"We are therefore convinced that it was not climate change that caused the rainforest crisis 2,600 years ago, but it was the growing populations that settled in the region and needed to clear the forest for exploiting arable land," says Garcin. "We are currently observing a similar process underway in many parts of Africa, South America, and Asia." But the work of Garcin and his team also shows that nature has powerful regenerative abilities. When anthropogenic pressure decreased 2,000 years ago forest ecosystems reconstituted, but not necessarily as before: as in the Amazonian rainforest, field studies show that the presence of certain species is very often related to past human activity.

Source: GFZ GeoForschungsZentrum Potsdam, Helmholtz Centre [February 26, 2018]

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Action plan released to conserve one of Africa's richest sites for biodiversity

A team of scientists led by WCS (Wildlife Conservation Society) has developed a conservation blueprint to protect one of the most biodiverse regions in Africa: the Albertine Rift, home to mountain and Grauer's gorillas, golden monkeys, chimpanzees, elephants, and 162 vertebrate, and 350 plant species unique to this region.

Grauer's gorilla. A team of scientists led by WCS (Wildlife Conservation Society) has developed a conservation blueprint
 to protect one of the most biodiverse regions in Africa: the Albertine Rift, home to mountain and Grauer's gorillas,
golden monkeys, chimpanzees, elephants, and 162 vertebrate, and 350 plant species unique to this region
[Credit: A.J. Plumptre]

Based on work by WCS and participants from five countries in the region, the "Conservation Action Plan for the Albertine Rift" summarizes the results of 16 years of research and commitment to the conservation of six key landscapes within the Albertine Rift, which runs through five countries (Uganda, Democratic Republic of Congo, Rwanda, Burundi, and Tanzania) and stretches from the southern tip of Lake Tanganyika to the northern tip of Lake Albert

Building on an initial framework plan developed in 2004, the new plan highlights the importance of the region for global biodiversity and goes further to outline the main steps required for the conservation of each landscape. The plan assesses where within each landscape is most important for the conservation of the many unique and threatened species, both now and under projected climate change, and identifies which species remain unprotected.

"The Albertine Rift is the most important site for vertebrate conservation in Africa, with more endemic and globally threatened vertebrates than any other region of the continent," said Dr. Andy Plumptre, Senior Scientist for WCS's Africa program. "We know of 163 terrestrial vertebrates that are unique to this region and we keep discovering new species. We also know the lakes in this region have incredible fish diversity and that at least 350 species of plant are unique to the region."

WCS has conducted surveys of the biodiversity of the Albertine Rift over decades, supporting surveys of some species and specific sites as early as 1959 in the case of eastern gorillas (one of the endemic species). A more comprehensive program started by WCS in 2000 compiled region-wide data on mammals, birds, reptiles, amphibians and plants. WCS worked with other NGO partners and the environmental protection authorities of Burundi, the Democratic Republic of Congo, Rwanda, Tanzania, and Uganda to identify six key landscapes and to establish cooperative protection at ground-level in each.

Based on work by WCS and participants from five countries in the region, the 'Conservation Action Plan for
the Albertine Rift' summarizes the results of 16 years of research and commitment to the conservation of six key
 landscapes within the Albertine Rift, which runs through five countries (Uganda, Democratic Republic of Congo,
Rwanda, Burundi, and Tanzania) and stretches from the southern tip of Lake Tanganyika
to the northern tip of Lake Albert [Credit: WCS]

Threats to the landscapes are substantial because this part of Africa also contains some of the highest human population densities on the continent. Habitat loss is the most critical threat for most of the species. Modeling work described in the report showed that the endemic and threatened species have already lost on average 40 percent of suitable habitat to agriculture. Climate change is likely to drastically reduce the remaining suitable habitat.

"We predict that by the end of this century, endemic species will further decline in response to climate change as many of these species will need to move to higher elevations as the climate warms. These up-slope movements will result in a dramatic 75 percent reduction in suitable habitat," said Sam Ayebare, a conservationist for WCS Uganda.

Many of the areas currently under protection are essential for the conservation of these species. Three additional areas, totaling over 10,000 square kilometers, that were gazetted in 2016, Itombwe, Ngandja and Kabobo Reserves, were critical for protecting many additional endemic species, both now and under future climate change. The report assesses the optimum ways to conserve these endemic and globally threatened species, and identifies which areas not currently under protection remain important for the conservation of some of the species.

"These critical sites outside of the existing protected areas mostly occur in DR Congo," said Deo Kujirakwinja, Technical Advisor for WCS in DR Congo. "We need to focus our attention on these sites before they, and the unique species they contain, are lost."

Supporting the conservation and management of the six landscapes within the Albertine Rift will require a dedicated effort from governments and from the conservation community. However, investment in conservation in this region yields tremendous value because of its incredible species richness.

Available at: www.albertinerift.org

Source: Wildlife Conservation Society [February 15, 2018]

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Tiny fossils, huge landslides: Are diatoms the key to Earth's biggest slides?

The biggest landslides on Earth aren't on land, but on the seafloor. These mega-slides can move thousands of cubic kilometers of material, and sometimes trigger tsunamis. Yet, remarkably, they occur on nearly flat slopes of less than three degrees.

Seismic reflection data [Credit: Morelia Urlaub and colleagues, and Geology]

Morelia Urlaub, a marine geoscientist at the Geomar Helmholtz Center for Ocean Research in Kiel, Germany, voices the obvious question: "How can you fail on a slope that is so flat?" Now, Urlaub and colleagues may have discovered the answer. The smoking -- or in this case, oozing -- gun is a layer of siliceous microfossils called diatoms.

The study, published online ahead of print for the Geological Society of America's journal Geology, is the first to identify the weak layer responsible for a submarine mega-slide. Although the nature of these critical weak layers has been highly debated, studying them has been nearly impossible because they are typically destroyed along with the slides.

Urlaub was compiling ocean drilling data from 1980 when she realized that the core sampled the seafloor just outside the Cap Blanc slide, a 149,000 year-old mega-slide off the coast of northwest Africa. She correlated that data with high resolution seismic reflection data recorded in the same area in 2009. Together, these data revealed diatom-rich layers, up to ten meters thick, that traced directly from the core to the base of slide layers within the mega-slide complex.

What's more, each diatom layer was topped by a layer of clay-rich sediment. That clay is apparently key. "Diatom layers are very compressible and water rich," Urlaub says. As pressure builds, she explains, water would be squeezed from the diatom layer into the clay. Ultimately the clay or the interface between the clay and diatoms fails, sending the sediments above sliding.

At the Cap Blanc slide, the seafloor slopes at just 2.8 degrees. Yet when it broke loose, the slide transported over 30 cubic kilometers of material, and extended at least 35 kilometers. Another submarine mega-slide 8,500 years ago off Norway moved a staggering 3,000 cubic kilometers, causing a damaging tsunami. And some scientists speculate that the 2011 Tohoku tsunami in Japan may have been amplified by a submarine mega-slide.

Although such slides don't occur very often, says Urlaub, their size makes them quite significant. "One-fifth of all tsunamis may be caused by undersea mega-slides," she says. If diatom layers are a major factor, then understanding where paleoclimate conditions may have favored diatom growth might help reveal potential mega-slide sites.

Source: Geological Society of America [February 12, 2018]

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