Palaeontology is the science that studies the fossil record to investigate the evolution of life on Earth. From the fossil record it has been determined that there has been five previous major mass extinction events or periods in Earth's history, such as the most publicly known one resulting in the demise of the dinosaurs. It is the contention of many ecological and environmental scientists that the world is now undergoing a sixth mass extinction period (Purvis & Hector, 2000; Jenkins, Green & Madden, 2003; Thomas, Telfer, Roy, Preston, Greenwood, Asher et al., 2004; May, 2005).
It is estimated that there are from five to fifteen million species on the planet, but only around 1.6 million have so far been named and described (May, 2005), so how can scientists claim that a mass extinction event is occurring?
Lord May, the President of the Royal Society, in his address given to the Society on their anniversary meeting on the 30th of November 2005, devoted a portion of his speech to the issues of biological diversity. He both recognized the interpretation of research indicating current extinction levels and the responsibility human ecological effects in the world have on extinctions of primarily endemic species.
Although recognizing that habitat loss, over exploitation and the introduction of alien species, individually or in combination, and compounded by climate change, are the probable major causes of a significant proportion of current extinctions, May (2005) contends that an analysis of species life expectancy is the primary evidence that a sixth mass extinction period is now occurring. He states that certified extinctions in avian and mammal species during the 20th Century expressed forward gives an average species life expectancy of 10,000 years in comparison to the one to ten million years average determined from the fossil record as the average for the last 550 million years (May, 2005). It is this difference that results in the claims that current extinction levels are 100 to 1000 times greater than background levels, ten thousand being that proportion of one to ten million.
Habitat loss and fragmentation is generally considered the leading cause for extinctions. Purvis and Hector (2000) equate habitat loss to extinctions on the basis of species-area relationships, the loss of habitat results in local population extinctions which can lead to species extinctions and refer to tropical rainforest reductions of 0.8 to 2.0 percent. Jenkins, Green and Madden (2003) expand on this by giving examples of habitat loss in a broad range of ecosystems throughout the biosphere. Wilson's (1992) research on species-area relationships indicate approximately 27,000 species are becoming extinct per year.
Thomas et al. (2004) researched 40 years of records in the UK on butterflies, birds and plants and found that population and species extinctions during that period supports the species-area relationship concept.
Therefore, despite only partial data on the world's species, by calculating species life expectancies and species-area relationships in habitat types that are being significantly reduced by human activities, many scientists consider sufficient evidence to have been compiled to justify concluding Earth's sixth wave of mass extinctions is underway.
Jenkins, M., Green, R. & Madden, J. (2003) The challenge of measuring global change in wild nature: are things getting better or worse?. Conservation Biology 17, 2023.
May, R. (2005) Threats to tomorrow's world. Notes & Records of the Royal Society (2006) 60: 109-130.
Purvis, A. & Hector, H. (2000) Getting the measure of biodiversity. Nature 405, 212219.
Thomas, J.A., Telfer, M.G., Roy, D.B., Preston, C.D., Greenwood, J.J.D., Asher, J., Fox, R., Clarke, R.T. & Lawton, J.H. (2004) Comparative losses of British butterflies, birds and plants and the global extinction crisis. Science 303, 18791881.