Arguments that we are now entering a sixth mass extinction event have been discussed in the previous post, but are humans actually responsible for this dramatic loss of biodiversity?
Generally, the Holocene has been a time of regularity in terms of temperature and freshwater supply; hence current Earth systems are now very sensitive to small changes in climatic variables. Synergy hypotheses, those that link mass extinction events to the changes in climate, atmosphere and ecological conditions, can be used to explain or predict a potential move towards a sixth mass extinction. Investigations using DNA and phylochronology have demonstrated that modern interpretations of species richness and evenness are low relative to conditions considered to be normal a few thousand years ago.
Rokstrom et al (2009) and Barnosky et al (2011) are agreed that human behaviour is the main cause of global environmental change during the turbulent end-Holocene period, the Anthropocene. The species extinction rate has accelerated considerably during this time, and is currently estimated to be between 100 and 1,000 times greater than what is considered to be natural, suggesting that we are indeed in the midst of a sixth mass extinction.
The Sixth Mass Extinction is quite distinct from previous events, the primary difference being that it has been initiated by humans, a biotic factor, rather than by a physical cause. Humans are now a geophysical force, with a destructive impact capable of changing the atmosphere and climate as well as global flora and fauna. The global population has doubled in the past 60 years, oriented by selfish sexual and reproductive drives facilitated by the technical advances of agriculture, and has now exceeded the environment’s natural carrying capacity (Wilson 2005, Eldredge 2011).
The human population places huge demands on the environment. The ecosystems existing today began to evolve at the end of the last glacial period, at a time in which Homo sapiens had relatively little impact on the Earth. Human activity has since led to habitat fragmentation through changing land use, the introduction of non-native species and pathogens, removal of species, and changing global climate. These factors combine to cause regional level biodiversity loss, which influences the functioning of the earth system as a whole.
It is evident from comparisons with previous mass extinctions, such as that of the late-Pleistocene, that human influence has had a significant impact on loss of biodiversity. Whilst the late Triassic event is thought to have been caused by substantial disturbance to the global carbon cycle, sea level change and possibly bolide impact the late-Pleistocene, or K-T, event, can be explained using the Overkill and Infectious Diseases hypotheses (Tanner et al 2004). The Overkill Hypothesis is based on the observation that large-scale human hunting in newly discovered North America resulted in severe species losses.
Evidence for The Overkill Hypothesis
1) Climate change is not indicated in palaeoclimatological records. Extinctions were very sudden and appear to follow the spread of humans, and the main species targeted during this time were large mammals, which would suggest that hunting was to blame.
2) In Africa where humans and animals coevolved, fauna became adapted to survive human presence. Conversely, when Native American ancestors entered North America 14,000 years ago, the large creatures already living there had no inherent fear of humans, could be hunted with ease, and so became extinct very quickly, resulting in large-scale ecological disruption.
3) There is no evidence of competition from exotic species on a sufficient level to cause extinctions.
4) Arguments that fossil evidence is insufficient to indicate large-scale hunting have been countered by the suggestion that there was insufficient time to preserve all fossils during the time in which extinctions took place.
5) There is little evidence of small animals, which would not be of interest to humans, also dying during this time. Equally, occurrence of extinctions before human arrival in North America seems unlikely based on palaeo data.
6) The Overkill Hypothesis is supported by the Keystone Herbivore Hypothesis. This states that large animals are ecosystem engineers, and that without them, habitats would be sufficiently altered to result in the demise of other smaller creatures (American Museum of Natural History).
IPCC reports confirm that climate change is occurring, and albeit by small increments, this could lead to dramatic species disturbances. Amphibians have historically managed to escape extinction events relatively unscathed, but now, due to changing environmental conditions, up to a third of the 6,300 amphibian species are threatened with extinction. This trend is likely to accelerate because most amphibians live in small habitat ranges, aren’t capable of moving far or adapting quickly to habitat pressures imposed by humans, and their moist skin is vulnerable to changes in humidity and temperature (Pounds 2006, Wake and Vredenberg 2008).
These chilling statistics suggest that humanity needs to change its ways, and fast, but have we already passed the point of no return? Is there anything that can be done to reverse the trend of large-scale species loss? Await the next post for further discussion.
American Museum of Natural History, What is the Overkill Hypothesis?, [www] Available from http://www.amnh.org/science/biodiversity/extinction/Day1/overkill/Bit1.html, [Acessed 10 November 2011]
Barnosky A.D., Matzke N., Tomiya S., Wogan G.O.U., Swartz B., Quentel T.B., Marshall C., McGuire J.L., Lindsey E.L., Maguire K.C., Mersey B., Ferrer E.A. (2011), ‘Has the Earth’s sixth mass extinction already arrived?’, Nature, 471, 51-57
Eldredge, N. (2001) The Sixth Extinction. Retrieved [www] Available from http:/ /www.actionbioscience.
org/newfrontiers/eldredge2.html [Accessed 10 November 2011].
Pounds J.A., Bustamante M.R., Coloma L.A., Consuegra J.A., Fogden M.P.L., Foster P.N., La Marca E., Masters K.L., Merino-Viteri A., Puschendorf R., Ron S.R, Sa´nchez-Azofeifa G.A., Still C.J. and Young B.E. (2006) ‘Widespread amphibian extinctions from epidemic disease driven by global warming’, Nature, 439, 12, 161-167.
Rockstrom et al (2009) ‘A safe operating space for humanity’, Nature, 461, 472-475.
Tanner L.H., Lucas S.G. and Chapman M.G. (2004) 'Assessing the record and causes of Late Triassic extinctions', Earth Science Reviews, 65, 103-139.
Wake D.B. and Vredenberg V.T. (2008) ‘Are we in the midst of the sixth mass extinction? A view from the world of amphibians’.
Wilson E.O. (1993) 'Is humanity suicidal?', Biosystems, 31, 2-3, 235-242.