‘The Sixth Extinction’
Alarm Bells Ringing
Astudy released in June
2015 has re-emphasised that the
extinction of various species has escalated rapidly in recent times.
In a widely discussed paper titled "Accelerated modern human-induced species losses: Entering the sixth mass extinction" Gerardo Ceballos, Paul R Ehrlich and others have stated, "The oft-repeated claim that Earth's biota is entering a sixth "mass extinction" depends on clearly demonstrating that current extinction rates are far above the "background" rates prevailing between the five previous mass extinctions. Earlier estimates of extinction rates have been criticized for using assumptions that might overestimate the severity of the extinction crisis. We assess, using extremely conservative assumptions, whether human activities are causing a mass extinction. First, we use a recent estimate of a background rate of 2 mammal extinctions per 10,000 species per 100 years, which is twice as high as widely used previous estimates. We then compare this rate with the current rate of mammal and vertebrate extinctions. The latter is conservatively low because listing a species as extinct requires meeting stringent criteria. Even under our assumptions, which would tend to minimize evidence of an incipient mass extinction, the average rate of vertebrate species loss over the last century is up to 100 times higher than the background rate."
Further this study asserts, "Under the background rate, the number of species that have gone extinct in the last century would have taken, depending on the vertebrate taxon, between 800 and 10,000 years to disappear. These estimates reveal an exceptionally rapid loss of biodiversity over the last few centuries, indicating that a sixth mass extinction is already under way. Averting a dramatic decay of biodiversity and the subsequent loss of ecosystem services is still possible through intensified conservation efforts, but that window of opportunity is rapidly closing."
This study emphasizes that there is no exaggeration, actually the reverse may be true. To quote "We emphasize that our calculations very likely underestimate the severity of the extinction crisis because our aim was to place a realistic "lower bound" on humanity's impact on biodiversity. Therefore, although biologists cannot say precisely how many species there are, or exactly how many have gone extinct in any time interval, we can confidently conclude that modern extinction rates are exceptionally high, that they are increasing, and that they suggest a mass extinction under way-the sixth of its kind in Earth's 4.5 billion years of history...The evidence is incontrovertible that recent extinction rates are unprecedented in human history and highly unusual in Earth's history. Our analysis emphasizes that our global society has started to destroy species of other organisms at an accelerating rate, initiating a mass extinction episode unparalleled for 65 million years. If the currently elevated extinction pace is allowed to continue, humans will soon (in as little as three human lifetimes) be deprived of many biodiversity benefits".
In a chain reaction extinction of one species can lead to further losses. This extinction has been linked to climate change, deforestation and pollution, but a complexity of many factors is at work. The number of species on the verge of extinction or threatened with extinction is of course much higher. According to the International Union for Conservation of Nature, about 41 percent of all amphibian species and 26 percent of all mammals are threatened with extinction. This extinction caused by human-made factors poses many threats to human beings as well, apart from providing a warning to human beings about the impact of human-made environmental threats. As Prof Paul R Ehlrich, a co-author of this study says, "We are sawing off the limb we are sitting on".
Elizabeth Kolbert, author of the "The Sixth Extinction" said recently in an interview that a third of all reef-building corals, a third of sharks and rays, a quarter of all mammals, a fifth of all reptiles and a sixth of all birds are now vulnerable to extinction.
This view is supported by another widely discussed research paper titled "Planetary Boundries- Exploring the Safe Operating Space for Humanity" by Johan Rockstrom and 26 other scientists. This paper says, "...Currently about 25% of species in well-studied taxonomic groups are threatened with extinction (ranging from 12% for birds to 52% for cycads). Until recently, most extinctions (since 1500) occurred on oceanic islands. In the last 20 years, however, about half of the recorded extinctions have occurred on continents, primarily due to land-use change, species introductions, and increasingly climate change, indicating that biodiversity is now broadly at risk throughout the planet.
This paper also indicates how biodiversity loss and extinction can lead to other catastrophic changes, "Like land-system change, local and regional biodiversity changes can have pervasive effects on Earth System functioning and interact with several other planetary boundaries. For example, loss of biodiversity can increase the vulnerability of terrestrial and aquatic ecosystems to changes in climate and ocean acidity, thus reducing the safe boundary levels for these processes... Accelerated biodiversity loss during the Anthropocene is particularly serious, given growing evidence of the importance of biodiversity for sustaining ecosystem functioning and services and for preventing ecosystems from tipping into undesired states. A diversity of functional response mechanisms to environmental variation among species in an ecosystem maintains resilience to disturbances. Consequently, ecosystems (both managed and unmanaged) with low levels of response diversity within functional groups are particularly vulnerable to disturbances (such as disease) and have a greater risk of undergoing catastrophic regime shifts".
The loss of species is also linked to other critical areas like climate change and global fresh water. Regarding the impact of climate change this paper says "Significant risks of deleterious climate impacts for society and the environment have to be faced even if the 2°C line (of temperature rise) can be held ...Despite uncertainties related to the degree of hysteresis in the relationship between ice growth and ice creation in response to temperature change, raising CO2 concentration above 350 ppm may lead to crossing a threshold that results in the eventual disappearance of some of the large polar ice sheets, with a higher risk of crossing the threshold as the CO2 concentration approaches the upper end of the range...The contemporary climate is thus moving out of the envelope of Holocene variability, sharply increasing the risk of dangerous climate change. Observations of a climate transition include a rapid retreat of summer sea ice in the Arctic Ocean, retreat of mountain glaciers around the world, loss of mass from the Greenland and West Antarctic ice sheets, an increased rate of sea-level rise in the last 10-15 years, a 4° latitude pole-ward shift of subtropical regions, increased bleaching and mortality in coral reefs, a rise in the number of large floods, and the activation of slow feedback processes like the weakening of the oceanic carbon sink."
Clearly this has serious consequences for bio-diversity and species loss. As for changes in global fresh water use, this study says, "The global freshwater cycle has entered the Anthropocene because humans are now the dominant driving force altering global-scale river flow and the spatial patterns and seasonal timing of vapour flows. An estimated 25% of the world's river basins run dry before reaching the oceans due to use of freshwater resources in the basins. Global manipulations of the freshwater cycle affect biodiversity, food, and health security and ecological functioning, such as provision of habitats for fish recruitment, carbon sequestration, and climate regulation, undermining the resilience of terrestrial and aquatic ecosystems".
Vol. 48, No. 7, Aug 23 - 29, 2015