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With the steep decline in populations of many animal species, from frogs and fish to tigers, some scientists have warned that Earth is on the brink of a mass extinction like those that occurred only five times before during the past 540 million years.
Each of these 'Big Five' saw three-quarters or more of all animal species go extinct.
In a study to be published in the March 3 issue of the journal Nature, University of California, Berkeley, paleobiologists assess where mammals and other species stand today in terms of possible extinction, compared with the past 540 million years, and they find cause for hope as well as alarm.
"If you look only at the critically endangered mammals -- those where the risk of extinction is at least 50 percent within three of their generations -- and assume that their time will run out, and they will be extinct in 1,000 years, that puts us clearly outside any range of normal, and tells us that we are moving into the mass extinction realm," said principal author Anthony D. Barnosky, UC Berkeley professor of integrative biology, a curator in the Museum of Paleontology and a research paleontologist in the Museum of Vertebrate Zoology
"It looks like modern extinction rates resemble mass extinction rates, even after setting a high bar for defining 'mass extinction,'" Barnosky said.
Dead zones are hypoxic (low-oxygen) areas in the world's oceans,
In March 2004, when the recently established UN Environment Programme published its first Global Environment Outlook Year Book (GEO Year Book 2003) it reported 146 dead zones in the world's oceans where marine life could not be supported due to depleted oxygen levels. Some of these were as small as a square kilometre (0.4 mi²), but the largest dead zone covered 70,000 square kilometres (27,000 mi²). A 2008 study counted 405 dead zones worldwide
Aquatic and marine dead zones can be caused by an increase in chemical nutrients (particularly nitrogen and phosphorus) in the water, known as eutrophication.
is the addition of artificial or natural substances, such as nitrates and phosphates, through fertilizers or sewage, to an aquatic system. In other terms, it is the "bloom" or great increase of phytoplankton in a water body. Negative environmental effects include hypoxia, the depletion of oxygen in the water, which induces reductions in specific fish and other animal populations.
Although anoxic events have not happened for millions of years,the geological record shows that they happened many times in the past. Anoxic events may have caused mass extinctions.
The Cenomanian-Turonian boundary event, also known as the Cenomanian-Turonian extinction event, the Cenomanian-Turonian anoxic event, and referred to in Europe as the Bonarelli Event, was the latter of two anoxic extinction events early in the Late Cretaceous period. The event occurred approximately 91.5 ± 8.6 million years ago, and brought about the extinction of the ichthyosaurs and pliosaurs. Although the cause is still uncertain,the result starved the Earth's oceans of oxygen for nearly half a million years, causing the extinction of approximately 27 percent of marine invertebrates. This global environmental disturbance increased atmospheric and oceanic temperatures.
The authors admitted to weaknesses in the study. They acknowledged that the fossil record is far from complete, that mammals provide an imperfect benchmark of Earth's biodiversity and further work is needed to confirm their suspicions.