This is a topic that is in need of further discussion, but these articles both document the loss of wildlife caused by human activities. Do we have the compunction to do anything about it?
Friday 28 October 2011
A potted history of life on Earth
In order for this blog to make sense, it needs to be placed into its historical context. This post is intended to provide some background to future discussion, hence enabling deeper analysis of mass extinction-related issues.
Recognisable life on earth was initiated around 700mya (million years ago) when single-celled organisms combined to create multicellular beings. 600mya witnessed the development of the skeleton, and between 500 and 400ma, the seas began to contain egg-laying fish. Mammalian life is thought to have begun around 260ma, at the end of the Palaeozoic era.
Fossil evidence suggests that there have been 5 mass extinction events to date, though these were likely to have been accompanied by smaller extinction events.
1) End-Ordovician (Ashgillian) 434 mya. This event took place over a period of several million years, and at a time of high global temperatures caused by greenhouse gases. Causes: sea level fluctuations, polar glaciations, changes in ocean temperatures, circulation and chemistry, also possibly due to extreme levels of CO2. It is thought that during this time, 90% of earth’s species vanished, and that the remaining 10% of species were severely affected by the ecological imbalance caused, so up to 99% of Palaeozoic species could have died out (Courtillot 2002).
2) Late Devonian (Frasnian-Framennian) 360 mya. Possible causes include bolide (meteor) collision, a fall in CO2 levels through increased uptake of plants, fluctuations in global sea level, and ocean anoxia. With regard to the exact causes of this event, McGhee (1988) notes that the most important question to answer is ‘what is the inhibiting factor that caused the cessation of new species originations?’
3) End-Permian 251 mya, also known as the ‘Great Dying’. It has been suggested by White (2002) and others that this was the worst loss of life the earth has ever witnessed. Perhaps up to 96% of marine species became extinct, and many land plant, reptiles, amphibians and insect species also vanished. Fossil evidence suggests that this event was incited by environmental disturbances. Oceans became stagnant and anoxic, with high levels of hydrogen sulphide, and large-scale methane released contributed to global warming. There is still much debate as to whether these instabilities came about due to changes within the earth system or because of a catastrophic event.
4) End-Triassic (Novian) 205mya. This event occurred between the Triassic and Jurassic Periods. 50% of genera were lost. It has been noted that this extinction occurred at the same time as the increase in volcanic activity caused by continental movements within the Pangaea earth mass (Deenen et al 2010). Though others argue that meteorite impact may have been responsible (Courtillot and Renne 2003). Extreme atmospheric CO2 levels, short-term sea level fluctuations, changes in ocean chemistry.
5) End-cretaceous (end-Maastrichtian) 65mya. This event was tends to be remembered because it marked the end of the dinosaur era, but also wiped out most other large land animals and plants. Other taxa, however, including freshwater fish, amphibians, turtles, crocodiles, snakes and lizards, and placental mammals were unaffected. On average, temperatures were between 6 and 14 degrees higher than at present, and up to 40 degrees higher at the poles. This extinction is thought to have been caused by the after effects of a bolide collision, evidence for which is visible in the Yucatan Peninsula, Southeast Mexico. This collision triggered tsunamis and volcanic eruptions, which released clouds of stratospheric volcanic dust and cooled the earth, creating a ‘nuclear winter’. Acid rain, methane release from continental slopes and intense greenhouse warming are also thought to have arisen. Over a period of hundreds of thousands of years, the combination of these effects led to large-scale species extinction.
The causes of mass extinction will be discussed in greater detail in later posts.
Tuesday 25 October 2011
Mass Extinction: An Introduction
This blog will discuss the causes and ramifications of mass extinction, and explore the debates surrounding the possibility of the onset of a sixth mass extinction. An understanding of the conditions preceding historical mass extinctions are important to our comprehension of the biological and evolutionary significance of our current climatic state on a potential forthcoming mass extinction.
As climatic conditions change, ecosystem variables shift, and environmental conditions alter, a certain number of species extinctions are likely to occur. Fossil evidence suggests that the majority of species have a lifespan of between 2 and 10 million years, this process of extinctions is considered to be ‘normal’. Although biodiversity loss occurs at a regional level, it influences the functioning of earth systems and leads to more wide reaching consequences. In contrast, mass extinctions can be defined as extinctions of a significant proportion of global biota in a geologically insignificant period of time (Sepkoski 1986).
Rockstrom et al (2009), amongst others, suggest that the current rate of extinction is between 0.2 and 0.5 per year per million species, somewhere between 100 and 1000 times greater than the ‘natural’ level. Discussion of the causes of mass extinctions, specifically whether they are due to climatic or anthropogenic influence will be tackled in later posts.
Mass extinction is a timely and relevant topic, and a rich source of academic debate. This blog will examine the complexities of mass extinction, and feels justified in doing so for the following reasons:
- A greater understanding of the causes of mass extinction will facilitate a more informed and committed approach to efforts to ensure the health of future ecosystems.
- Ocean feedbacks, and the development of their effects on life on earth, need to be understood in the context of mass extinctions.
- Levels of CO2 and other greenhouse gases, the rates and causes of fluctuations, need further examination, as several theories suggest that they are contributing factors to mass extinction events.
- The level at which humanity can responsibly continue to make long-term social and economic developments needs to be understood in the context of biodiversity loss (Rockstrom et al 2009).
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