Monday 9 January 2012

The End?

The purpose of this blog has been to investigate and explain arguments surrounding the mechanisms and causes of mass extinction. Two main debates arise as to the driver of species loss: the influence of climate, and of humans.

Climate as the cause?

Historic mass extinctions, the Late Permian (251mya), and End Triassic (205mya), for instance, demonstrate that climate change is certainly capable of initiating large-scale species loss. This recent paper agrees that climatically driven habitat shifts could conceivably have resulted in the loss of many species of megafauna, but it also highlights the uncertainty of this topic. We cannot pinpoint a single cause of extinctions, rather, it seems that a number of factors had a part to play.
In a modern context, our understanding of the complex dynamics of nature is still limited. We don’t fully comprehend short term versus long-term environmental stochasticity, or population processes in relation to community dynamics and stability. Thus it is hard to reach conclusions about the potential behaviour of communities and ecosystems from studies of individuals and populations.

There are consistent large-scale environmental responses to low average rates of climate change, suggesting that the modern landscape is inflexible to ecosystem change in response to climate alterations, hence the widespread loss and fragmentation of habitats. With further warming may come dire ecological and socio-economic consequences (Walther et al. 2002).

Source: http://www.skepticalscience.com/empirical-evidence-for-global-warming.htm

Humans as agents of change?

While the reasons behind historic events remain unclear, evidence points fairly convincingly towards humans as the cause for the current mass extinction. Anthropogenic pollution of the atmosphere which results in untimely feedbacks and planetary warming, and destructive, insensitive expansion of man-made environments, must surely lead to changes of such magnitude that habitats are unable to support the species that evolved to live there. Estimations (Novacek and Cleland 2001, Rockstrom et al. 2009) of future extinction rates based on comparison with background rates suggest that 30% of species currently present on Earth could vanish by the middle of the 21st century. Even our closest genetic relatives, Chimpanzees, are threatened because of human activity. A combination of habitat loss, disease, hunting, and human population increase means that the most endangered Chimpanzee species, Pan troglodytes vellerosus, could go extinct in just 20 years.

Rainforest destruction, intensive agriculture, and degradation and overexploitation of marine ecosystems present similarly large threats to species diversity. Alteration of global biogeochemical cycles, and changes to feedbacks between the hydrosphere, atmosphere and lithosphere could well accelerate rate of species loss. In spite of Rachel Carson’s emphatic 1960s discourse warning of the deleterious impact of DDT and other chemicals, human use of pesticides has tripled in the past 40 years to a current level of 2.5 m tones/year, and human activity has doubled the amount of Nitrogen in global cycles. 

Novacek andCleland (2001) state that shifting land use is the most intensive driver of terrestrial environmental change. By the year 2030, there may be 8.2bn people to feed, which will require the grain harvest to be increased by 2%/year. If rates of topsoil removal seen in the past 20 years continue, there may be no suitable platform on which to do this.


Lessons from the past?

Fossil data doesn’t provide an effective illustration of the exact cause of previous mass extinction events, but gives us a powerful indication of the reality of extinction nonetheless.

The difference now, though, is that while previous mass extinction events took place over long timescales, and the situation for recovery was similar, we do not have the luxury of time to allow ecosystems to rebound (Novacek and Cleland 2001).

What to do?

·       The current trend could reverse itself, but this would take long time, and according to Malthusian theory, would require fewer humans to exist on Earth.
·       Recovery could occur if a considerable protection policy were to be implemented. This would include large-scale ecosystem management and mitigation of current disruption of biogeochemical cycles (Novacek and Cleland 2001).

All is not lost?

Primatologist Jane Goodall argues that in spite of everything, there are reasons to be optimistic about the future. The powerful human brain is capable of complex problem solving, and companies have begun to become more attentive to greening operations. In addition, we should have faith in the resilience of nature. If species can re-colonise areas destroyed by atomic activity, then resistance to change should be possible elsewhere.

Finally, some snippets from performances in ‘Saving Species. Sustaining Life’ by A.L. Kennedy and Miles Chambers respectively, remind us of our place within nature, and our responsibility to care for, and protect it, lest it vanish forever.

‘Do we remember we are animals as well as people, and tend ourselves with mercy? Do we remember we are people as well as animals, allow ourselves joys within moderation, a place within nature, a place in the balance of the world that can be beautiful, but has no mercy? If we kill it, it will kill us back’.

‘This place is all we’ve got. This is our home, and our children’s, children’s, children’s home. Don’t tell me you forgot’.


References

Lorenzen E.D., Nogues-Bravo D., Orlando L, Weinstock J., Binladen J. Marske K.A., Ugan A., Boregaard M.K., Gilbert M.T.P., Nielsen R., Ho S.Y.W., Goebel T, Graf K.E., Byers D., Stenderup J.T., Rasmussen M., Campos P.F., Leonart J.A., Koepfli K.-P., Froese D., Zazula G., Stafford T.W., Aaris-Sorensen K., Batra P., and Haywood A.M., Singarayer J.S., Valdes P.J., Boeskorov G., Burns J.A., Davydov S.P., Haile J., Jenkins D.L., Kosintev P., Kuznetsova T., Lai X., Martin L.D., McDonald H.G., Mol D., Meldgaard M., Munch K., Stephan E., Sablin M., Sommer R.S., Sipko T., Scott E., Suchard M.A., Tikhonov A., Willerslev R., Wayne R.K., Cooper A., Hofreiter M., Sher A., Shapiro B., Rahbek C. and Willerslev E. (2011) ‘Species-specific responses of Late Quaternary megafauna to climate and humans’, Nature, 479, 359-364.

Novacek M.J. and Cleland E.E. (2001) ‘The current biodiversity extinction event: Scenarios for mitigation and recovery’, Proceedings of the National Academy of Sciences of the United States of America, 98, 10, 5466-5470.
Rokstrom J., Steffen W., Noone K., Persson A., Chapin F.S., Lambin E.F., Lenton T.M., Scheffer M., Folke C., Schellnhuber H.J., Nykvist B., de Wit C.A., Hughes T., van der Leeuw S., Rodhe H., Sorlin S., Snyder P.K., Constanza R., Svedin U., Falkenmark M., Karlberg M., Karlberg L., Corell R.W., Fabry V., Hansen J., Walker B., Liverman D., Richardson K., Crutsen P. and Foley J.A. (2001) ‘A safe operating space for humanity’, Nature, 461, 472-475.
Walther G.-R., Post E., Convey P., Menzel A., Parmesan C., Beebee T.J.C., Fromentin J.-M., Hoegh-Guldberg O., and Bairlein F. (2002) ‘Ecological responses to recent climate change’, Nature, 416, 389-395.


Saturday 7 January 2012

Case Study: Hedgehogs

During the past century, hedgehogs have suffered from dramatic declines in population numbers. Current estimations of hedgehog population numbers from the wildlife conservation unit at Oxford University stand at 1.5 million, a figure that was around 30 million in the 1950s. 


REASONS FOR DECLINE

RURAL POPULATIONS
Millward (2011) and O’Connell (2007) list the ways in which the rural hedgehog
population has been hampered by life in the modern anthropocene:

  • Changes to farming methods and increases in relative proportion of arable land are reducing the areas available for hedgehogs to live.
  • Extensive use of pesticides has resulted in removal of many components of the hedgehog diet.
  • Although the exact influence on hedgehogs is unclear, but University of Bristol researchers speculate rodenticides may invoke subtle changes in reproductivity or resilience during fights because of limited blood clotting ability.
  • Expansion of urban areas, and thus, reductions in natural habitat area.
  • Hedgehogs, and predatory badgers being forced to share habitats.
  • 50,000 hedgehogs are run over annually by motorists.
  •  Hedgehog migration into urban areas, where they are threatened by strimming, mowing, and being caught in netting.


HEDGEHOGS IN URBAN AREAS
Surprisingly, urban hedgehog populations are thriving more than those in rural areas. Tough even in towns and cities, the picture is bleak, and populations have declined by a third in the past 15 years.

Changing human habits are contributing to the demise of the species. Townspeople no longer leave food out for these nocturnal creatures, and the wild, overgrown gardens that were once a hedgehog haven are now small and neatly kempt.

HIBERNATION
Climatic changes and less seasonal predictability are confusing for hedgehogs. Research has shown that they are often hibernating in January, rather than November. This means that energetic expenditure is higher, and hedgehogs are not in an optimal state at the onset of hibernation. This problem is likely to affect other hibernating creatures, for which, hot summers and cold winters are ideal. Due to climate change, seasons have become unclear, and winters tend to be warmer.

TRANSLOCATION
There is a small glimmer of hope, however. Although it seems unlikely that hedgehog numbers in mainland Britain will recover, the island of Uist in the Hebrides is overrun with them. The generalist nature of hedgehogs, and ability to find new territory with relative ease, makes them ideal for translocation, and this may go some way towards maintaining species numbers.


 References

Millward D. (2011) ‘Hedgehogs may become extinct within 15 years’, [www], available from: http://www.telegraph.co.uk/news/uknews/8696170/Hedgehogs-may-become-extinct-within-15-years.html (2nd January 2012).
O’Connell S. (2007) ‘Hedgehogs: Over the hedge’, [www] available from: http://www.independent.co.uk/environment/nature/hedgehogs-over-the-hedge-399493.html, (2nd January 2012).
Taylor C. (2011) ‘Help save the hedgehogs this autumn’, [www] available from http://www.guardian.co.uk/commentisfree/2011/sep/27/hedgehogs-autumn-population (2nd January 2012). 

A piece of history; the role of climate in extinction of megafauna and the Woolly Mammoth.

INTRODUCTION

The Woolly Mammoth is an example of a typical megafaunal creature, a large mammal weighing over 1000kg. Over the past 100,000 years, many megafaunal species have been removed from Earth in waves of extinction, Eurasia and North America, for instance, suffered losses of between 36% and 72% 500ka (Lorenz et al. 2011).

The causes of these extinctions are a hotly debated topic, and there is much uncertainty, and many problems associated with reaching a conclusion as to the main driver. While it is key to bear in mind that suggestions of the cause of extinctions are all speculative, the two dominant paradigms of this episode of mass extinction are climate and vegetation change, and human hunting and disturbance, disease may also have been instrumental (Wroe et al. 2004, Lorenz et al. 2011, Stuart 2005, Nicholls 2011, Faith 2011).

Figure 1. Ranges of megafaunal species. Source: Lorenz et al. (2011).

CLIMATE

There is little doubt that climate has played a large part in species population change during the past 50,000 years. Fossil and DNA evidence suggests that populations would have shrunk as environmental conditions changed, because of losses of genetic variation and adaptive flexibility (Lister and Stuart 2008, Lorenz et al. 2011).
Building on this idea of changing climate causing environmental shifts, Faith (2011) proposes that the extinction of North American megafauna was caused by an ecological mechanism.
In modern ecosystems, the interaction between plant nutrient content, nitrogen cycling and herbivore-plant relationships produces feedback mechanisms. These vary between modes of nutrient acceleration and deceleration, and are influenced by atmospheric CO2 concentration, temperature and precipitation. Lateglacial climate change, Faith suggests, may have caused increases in atmospheric CO2, and is likely to have shifted ecosystem dynamics away from nutrient acceleration and towards a deceleration mode, causing megafaunal populations to shrink.


HUMANS

Fossil remains indicate that human populations expanded across northern Eurasia 40ka, bringing with them a wave of disturbance.

The Blitzkrieg hypothesis offers an explanation of the variation in rates of megafaunal extinction between global landmasses during the late Quaternary, based on five major observations:

·       Rates of megafauna extinction were considerably higher in areas outside of Africa where humans and mammals coevolved.
·       Remote island species that had no previous exposure to humans were particularly vulnerable.
·       Hunter-gatherers preferentially selected large prey.
·       Late Quaternary extinctions had a greater impact on large animals than small ones.
·       Climatic fluctuations during the late Quaternary were not reflected in extinctions of megafauna (Nogues-Bravo et al. 2008, Lister and Stuart 2008).


Wroe et al. (2004) and Stuart (2005) dispute this line of thinking, noting that the simplicity of the Blitzkrieg model is appealing, but the exact causes of extinction are still unclear. They argue that:
·       Human-related extinctions of both large and small taxa, rather than just large, may have occurred on remote islands.
·       Translocated species could have contributed to extinctions.
·       The role of humans in megafaunal extinction remains unclear. Perhaps modern hunters, with good technology and organisation did contribute to species loss. But equally, cultural variations such as specialised technologies, diet, and hunting behaviours, suggest that not all human societies would have been to blame for the rapid demise of large mammals.

MULTIPLE FACTORS

Many suggestions point to a multifactoral cause of megafaunal extinctions (Lorenz et al. 2011, Wroeet al. 2004, Stuart 2005, Nicholls 2011, Faith 2011).

Lorenz et al. (2011) acknowledge that it is difficult to fully explain the link between climate change, population size and species extinctions. Changes to climatic conditions are thought to have reduced the carrying capacity of landscapes, making populations vulnerable to extinctions caused by environmental or anthropogenic processes (Faith 2011)

Evidence suggests that individual species responded differently to climatic shifts, environmental changes, and human encroachment. Expansion into different areas in order to increase chances of survival may have been dependent on population density, and the characteristics of the new area, such as predation by other creatures and by humans. It has also been suggested that vulnerability to extinction may have been caused by low fecundity, rather than simply large body size (Lister and Stuart 2008).


MAMMOTHS
The Woolly Mammoth, Mammuthus primigenius was an herbivorous mammal that lived in cool, dry open steppe-tundra in Northern Hemisphere from the late Middle Pleistocene 300ka BP. These creatures vanished from Eurasia and North America during the Holocene, 3.6 ky BP.

CAUSES OF MAMMOTH EXTINCTION

Testing hypotheses of climatic and anthropogenic causes of these extinctions is a challenge, due to complications with gaining quantitative estimates of the relationship between contraction of the geographic range of the mammoth and these two hypotheses.

CLIMATE
DNA research into mammoth behaviour is aided by modern proxy evidence. Modern elephants produce 50 litres of urine each day, so it seems reasonable that mammoths would have done the same. This means that vast land areas will be covered in mammoth DNA. Genetic studies have shown that the range of wooly mammoths had been in decline for several thousand years before their disappearance. 126ky BP, Earth’s climate became progressively colder and drier, until the Last Glacial Maximum, LGM, when conditions became warmer and wetter. These climatic oscillations are thought to be responsible for changes to vegetation and reduced and fragmented geographic range of mammoth habitats (Nicholls 2011, Nogues-Bravo et al. 2008).
Mammoth habitat range contracted again 12kya, and at this time, mainland populations rapidly moved north. Strangely, this disappearance was not correlated with any pattern of warming and spread of shrub-grassland vegetation over much of Europe. During the cold Younger Dryas phase, on the other hand, when steppe-tundra was allowed to re-establish, the Mammoth moved back into northeast Europe (Stuart 2005).
Woolly Mammoth remains have been unearthed on Wrangel Island, Siberia. C14 data suggest that Mammoths inhabited this area for as long as 6000 years after it vanished from mainland Siberia. It is thought that the species was forced to migrate to Wrangel by the movement of the forest/tundra line, even though the area was perhaps not ideally suitable. (Vartanyan 1995, Nogues-Bravo et al. 2008).
CLIMATE AND HUMANS
It is likely that the collapse of the climatic niche of the mammoth resulted in a drop in their population size, and increase in vulnerability to human hunting pressure.

The hunting intensity model suggests that mammoth extinction was caused by a synergy between collapse of suitable climatic conditions and northward increase in human population densities. This is based on the idea that regardless of cull rate; the percentage of mammoth population that must be killed to drive the species to extinction, hunting intensity has varied through time, so hunting is unlikely to be the only reason for extinctions (Nogues-Bravo et al. 2008).

Like the mammoth, the Straight-Tusked Elephant, Eurasian Musk Ox and Woolly Rhinoceros were forced to retreat because of climatic changes, loss of habitat, and vegetation shifts. While there are arguments suggesting that the extinction of these creatures can be explained by climate alone, anthropogenic effects are thought to be responsible for the loss of the wild horse and steppe bison.

RELEVANCE

These studies demonstrate the complex causes of mass extinction. A greater understanding (Lister and Stuart 2008) of the role of climate in the process of mass extinction may help us to predict the ways in which current species may adapt to future ecosystem shifts.
With regard to the current extinction, arguments that climate is the cause may well be rooted in truth, but, as demonstrated, a variety of other factors also have a part to play. The unprecedented expansion of humans and the resulting climatic warming and destructive activity must surely be responsible for a considerable portion of species loss?

References
Faith J. T. (2011) ‘Late Pleistocene climate change, nutrient cycling, and the megafaunal extinctions in North America’, Quaternary Science Reviews, 30, 1675-1680.

Lister A.M. and Stuart A.J. (2008) ‘The impact of climate change on large mammal distribution and extinction: Evidence from the last glacial/interglacial transition’, Comptes Rendus Geoscience, 340, 615-620.
Lorenzen E.D., Nogues-Bravo D., ORlando L, Weinstock J., Binladen J. Marske K.A., Ugan A., Boregaard M.K., Gilbert M.T.P., Nielsen R., Ho S.Y.W., Goebel T, Graf K.E., Byers D., Stenderup J.T., Rasmussen M., Campos P.F., Leonart J.A., Koepfli K.-P., Froese D., Zazula G., Stafford T.W., Aaris-Sorensen K., Batra P., and Haywood A.M., Singarayer J.S., Valdes P.J., Boeskorov G., Burns J.A., Davydov S.P., Haile J., Jenkins D.L., Kosintev P., Kuznetsova T., Lai X., Martin L.D., McDonald H.G., Mol D., Meldgaard M., Munch K., Stephan E., Sablin M., Sommer R.S., Sipko T., Scott E., Suchard M.A., Tikhonov A., Willerslev R., Wayne R.K., Cooper A., Hofreiter M., Sher A., Shapiro B., Rahbek C. and Willerslev E. (2011) ‘Species-specific responses of Late Quaternary megafauna to climate and humans’, Nature, 479, 359-364.
Nicholls H. (2011) ‘Last days of the mammoth’, New Scientist, 209, 2805, 54-57.
Nogues-Bravo D., Rodriguez J., Hortal J., Batra P., Araujo M.B. (2008) ‘Climate change, humans, and the extinction of the woolly mammoth’, Public Library of Science Biology, 6, 4, 685-692.
Stuart A.J. (2005) ‘The extinction of woolly mammoth (Mammuthus primigenius) and straight-tusked elephant (Palaeoloxodon antiquis) in Europe’, Quaternary International, 126-128, 171-177.
Vartanyan S.L. (1995) ‘Radiocarbon dating evidence for mammoths on Wrangel Island, Arctic Ocean, until 2000 BC’, Radiocarbon, 37, 1, 1-6.
Wroe S., Field J., Fullagar R., and Jermin L.S. (2004) ‘Megafaunal extinction in the late Quaternary and the global overkill hypothesis’, Alcheringa: An Australasian Journal of Palaeontology, 28, 1, 291-331.