Evidence for the 'Sixth Mass Extinction'.

Species are currently disappearing at a faster rate than ever before with the exception of cataclysmic events such as asteroid impacts or unusual volcanic activity (IUCN 2000). This time it appears that humans may be the cause.

Actual species extinction rates are extremely difficult to quantify with different scientists having different opinions. Most conservative estimates place the current rate at 1000 times the background rate as derived from the fossil record, but some estimates place it as high as 10,000 times greater (Call of Life 2009).

(Graph showing species extinction rates since 1800 from: http://www.whole-systems.org/extinctions.html)

This graph is based on a mathematical model linking species to habitat and was developed by a group of scientists led by Edward Wilson. The graph assumes that there is a total number of 10 million species on the Earth today. Background extinction rate from the fossil record is one extinction per million species per year, suggesting that the natural background extinction rate is 10 species per year.

The modern day extinction rate estimates that this model takes into consideration are (from: http://www.whole-systems.org/extinctions.html):

• Edward Wilson - 27,000 species lost per year.
• Niles Eldridge - 30,000 species lost per year.
• Georgina Mace - 14-22% of loss of species and subspecies over the next 100 years.
• Paul Erlich - Based on total energy use estimates extinction rates at 7,000-13,000 times the natural rate - 70,000 to 130,000 species lost per year.

This graph clearly shows that since 1920 there has been a dramatic rise in the number of extinctions. Of course as with all models there will be inaccuracies, but the increase is so dramatic that one cannot deny that the losses are abnormal compared to the natural extinction rate.

Historically man-made extinctions were primarily due to hunting such as the predation of slow-moving species e.g the wooly mammoth and the moa. Another factor may have also been the human introduction of new diseases by man or domesticated animals. The loss of these species led to the extinction of other dependent species such as giant vultures.

The cascade of current extinctions is mostly related to the destruction of habitat and displacement by introduced species. In the last 500 years human activity has forced 816 species into extinction, with 103 bird species extinctions since 1800 (IUCN 2000).

In September 2000 the World Conservation Union released a 'Threatened Species Survey' also known as the Red List, with shocking results. 11,046 species of plants and animals are considered threatened. This means that they are considered vulnerable, endangered or critically endangered. These species are considered to face high risk of extinction in the near future, in almost all cases this is seen as a result of human activities. This accounts for 24% of mammals and 12% of birds (IUCN 2000).

Since 2000 the Red List has been updated numerous times, with the number of threatened species increasing each time (IUCN 2009).
 Here is the IUCN Red List website which may be of interest: http://www.iucnredlist.org/

Rates of species loss are not globally consistent, here are some of the worst affected places (IUCN 2000):

• Madagascar - Which is home to more critically endangered primates than anywhere in the world. The island is a biodiversity hotspot with high levels of endemism (home to species that cannot be found anywhere else in the world). The island has already lost over 90% of its original vegetation. 
• Philippines - Another biodiversity hotspot which has lost 97% of its original vegetation and is home to more critically endangered birds than any other country. 
• Indonesia - Home to 135 threatened mammals, more than anywhere else in the world. 

These shocking figures show that species extinction rates do appear to be occurring at a rate far above the natural baseline rate. It also appears that a large proportion of these losses are due to human influence. Something which I will continue to investigate throughout this blog.

The next post will focus upon the feature documentary "Call of life: Facing the mass extinction", which investigates the growing threat to Earths life support systems from the unprecedented loss of biodiversity (Call of Life 2009).

References:

Call of Life: Facing the mass extinction. Monte Thompson (2009).

IUCN. (2000). Compiled by C. Hilton-Taylor. 2000 IUCN Red List of threatened species. Thanet Press Ltd.: Margate.

IUCN. (2009). The IUCN Red List of threatened species 2009 update.

Wilson, E. O. (1992). The diversity of Life, The Belknap Press of Harvard university press: Cambridge.

www.whole-systems.org/extinctions.html.