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 21^st 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.

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. 

Source: http://www.telegraph.co.uk/news/uknews/8696170/Hedgehogs-may-become-extinct-within-15-years.html

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, (2^nd 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.

Saving Species, Sustaining Life

Highly topical and timely, this week’s Saving Species program on Radio 4 focuses on the impact of humans on the planet. The panel; Jacqueline McGlade from the European Environment Agency, Aubrey Manning - University of Edinburgh, Jon Bridle - University of Bristol, and celebrated environmentalist Vandana Shiva, discuss some of the areas in which human influence has caused significant ecological damage.

Fishing

Demand for fish has increased considerably of late; directly, for human protein, and indirectly, as food for other fish. Vandana Shiva observes that where fish was once a luxury item, it is now considered acceptable for the rich to consume it on a regular basis. Large-scale fishing leads to the disruption of ecosystems. The removal of sharks and large predators is a prime example of this. Without top predators, a large ecosystem cannot be established, and the intricate ocean food webs cannot be maintained.

Jon Bridle explains that the root of the problem is governance. Decision makers and the small communities that manipulate the oceans have very limited understanding of nature. Hence, fisheries are a classic case of the ‘commons’ (Hardin 1968), where decisions are made at the expense of what is out of site, and therefore, out of mind. Further to this, subsidies distort ecosystem value, and the economy needs to be changed in order to use the planet more sustainably to support a given number of people.

Predators

Where once humans were both prey and predator, we are now very much at the core of habitat destruction. The Amazon rainforest, ‘the lungs of the world’, is vital to production of oxygen, carbon dioxide neutralisation, and is rich in highly valuable medicinal species. This unique resource is in jeopardy because of the unrealistic demands of the West.  Vast areas of forest have been cleared to make way for cheap meat production, and soya and palm oil plantations, a topic which is explained in detail here by a fellow blogger.

Too many people

Vandana Shiva describes the current inefficient resource intensive system in fishing, forestry and food production as ‘unsustainable and unjust’. She notes that the human species must recognize that it is our own abundance that is causing such damage to biodiversity. Shiva calls for an increase in ecological justice, restoration of love and appreciation for the planet, and a healing of the deep rift between humans and nature that was caused by the Cartesian revolution in science. There is talk of the need to banish the illusion that we are separate from nature, and recognize that we are part of the community of the earth. The program ends with the thought that the more we save species, the more we save ourselves.

Listen here

Case Study: Amphibians

INTRODUCTION

There is little doubt that human activity is causing loss of species through considerable damage to natural environments, and amphibians have suffered the most. Indeed Sodhi et al (2008) observe that due to their major declines in population, susceptibility to disease, and morphological deformities, amphibians epitomise the modern biodiversity crisis.

Amphibians are cold-blooded vertebrate animals, which metamorphose from water-breathing juveniles to air-breathing adults (Stuart et al. 2004).The current extent of amphibian extinction is perplexing, because during their 350 million year period of evolution, they have managed to overcome mass extinctions similar to that of the present. The exact characteristics enabling these creatures to survive extreme conditions have been the subject of much research (Wake and Vredenberg 2008).

Deep-sea sediment cores suggest that over the past 65 million years, global climate change has been almost continuous, but the recent rapid warming is of particular concern (Carey and Alexander 2003). By comparing the current amphibian extinction rate with background fossil rate, we can improve our understanding of the magnitude of the current biodiversity crisis and the extent to which humans are responsible (McCallum 2007).

Figure 1. Relative percentages of species loss from different altitudes. Source: Pounds et al. (2006).

STATISTICS

Recent research has shown that the current extinction rate exceeds that of both the 1500 and 1980 level, rates of amphibian declines are catastrophic, and projected losses for the future are even more intense.

The 1989 Congress Of Herpetology marked the start of scientific concerns about amphibian population declines, and the IUCN GAA (Global Amphibian Assessment) was conducted in order to gauge the severity of these declines. Results from this indicate that 43.2% of amphibian species are declining, 122 are possibly extinct, and up to a third are at risk of extinction (Stuart et al.2004, Pounds et al. 2006, Wake and Vredenberg 2008, McCallum 2007).

CAUSES

Colins and Storfer (2003) note that there are six underlying hypotheses explaining amphibian declines. Three of these; the invasion of alien species, overexploitation and excessive harvesting of amphibian populations, and land use change, are relatively well understood. By removing, introducing, or changing constants, it is likely that amphibians will suffer. The remaining hypotheses involve complex interactions between global change, infectious diseases and trends amongst amphibian populations.

·      Global Warming

Humans influence on the earth’s climate, large scale warming, and aerosol formation intensify the hydrological cycle and shift the balance between ecological conditions, in turn leading to threats to species survival (Walther et al. 2002, Pounds et al. 2006).

In the Cascade area of Western North America, shallow lakes and ponds provide an ideal location for Western Toads, Bufo boreas, to lay their eggs. Recently, though, the embryos of these toads have begun to die before becoming properly developed. Climate change induced reductions in pond level are thought to be to blame. Shallower water overlying eggs means that protection against exposure to UV-B is reduced, for instance, where water depth is less than 20cm, 80% of toad embryos are killed by Saprolegnia, whereas this figure is only 12% in water depth of 50cm (Pounds2001).

The indirect effects of global climate change include changes to the phenology of breeding, in the case of amphibians, the seasonal variation in timings of egg laying. Evidence suggests that breeding seasons mirror climatic conditions; this puts those hatching early at a disadvantageous risk of mortality due to cold temperatures.

Figure 2. Relationship between climate change, amphibian declines, and extinctions. Source: Pounds (2001).

·      Characteristics of amphibians

Amphibians are directly influenced by temperature and moisture. Their cellular and physiological processes are controlled by heat exchange with air, water, and solar heat. Hence, severe daily temperature fluctuations are dangerous for bodily function.

Most amphibians are found in specific tropical geographic ranges, and aren’t capable of adapting well to different environmental conditions. It is to be expected, therefore, that further habitat changes will accelerate species loss, particularly of those dwelling in ecologically pristine areas. The disappearance of the Monteverde toad and Harlequin frog during unusually warm years illustrate this (Daszak et al. 1999, Wake and Vredenberg 2008, Pounds et al. 2006).

In reproductive terms, water is vital for amphibian existence. Eggs and larvae are deposited in standing water, and so annual variation in rainfall will influence both the number of eggs laid and hatched (Carey and Alexander 2003).

The permeable skin and hormonally regulated development of amphibians makes them highly vulnerable to endocrine disruption. For instance, the herbicide Atrazine, a common contaminant of ground and surface water where amphibians breed, is highly active at low concentrations. This compound chemically castrates and feminizes male amphibian larvae, retards development and growth, and is the cause of unusual behaviour and immunosuppression (Hayes et al. 2006). Although now banned in the EU, Atrazine is still widely used in the USA.

·      Disease

Chytridiomycosis is a panzootic fungal amphibian disease, caused by Batrachochytrium. It develops and spreads in moist aquatic habitats, particularly during the winter. The disease is persistent at low densities, and attacks the moist skin of amphibians by degrading cellulose, chitin and keratin, and producing zoospores. The discovery of a new form of chytrid fungus, Batrachochytrium dendrobatidis, coincided with the observation that amphibian declines were taking place, and sporangia of this fungus were found within mouthparts of tadpoles, particularly from montane habitats (Daszak et al. 1999, Carey and Alexander 2003, Pounds et al. 2006, Pounds 2001). Similarly, the Saprolegnia ferax fungus has caused mortality of amphibian eggs in the Pacific North West, and loss of the Western toad Bufo boreas. Ranaviruses are another concern, and spread of these has increased due to humans (Collins and Storfer 2003, Carey andAlexander 2003).

In addition to these problems, limited knowledge of the true numbers of creatures no longer existing on earth presents further challenges to ensuring the survival of those remaining (McCallum 2007, Stuart et al. 2004).

References

Carey C. and Alexander M.A. (2003) ‘Climate change and amphibian declines: is there a link?’, Diversity and Distributions, 9, 111-121.
Collins J.P. and Storfer A. (2003) ‘Global amphibian declines: sorting the hypotheses’, Diversity and Distributions, 9, 89-98.

Daszak P., Berger L., Cunningham A.A., Hyatt A.D., Green D.E. and Speare R. (1999) ‘Emerging infectious diseases and amphibian population declines’, Emerging infectious diseases, 5, 6, 735-748.

Hayes T.B., Case P., Chui S., Chung D., Haeffele C., Haston K., Lee M., Mai V.-P., Marjuoa Y., Parker J., and Tsui M. (2006) ‘Pesticide Mixtures, Endocrine Disruption, and Amphibian Declines: Are we underestimating the impact?’, Environmental Health Perspectives, 114, 1, 40-50.

McCallum M.L. (2007) ‘Amphibian Decline or Extinction? Current Declines Dwarf Background Extinction Rate’, Journal of Herpetology, 41, 3, 483-491.

Pounds J.A. (2001) ‘Climate and amphibian declines’, Nature, 410, 369-340.

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., Sanchez-Azofeifa G.S., Still C.J. and Young B.E. (2006) ‘Widespread amphibian extinctions from epidemic disease driven by global warming’, Nature, 439, 161-167.

Sodhi N.J., Bickford D., Diesmos T.M.L., Lian P.K., Brook B.W., Sekercioglu C.H. and Bradshaw C.J.A. (2008) ‘Measuring the meltdown: drivers of global amphibian extinction and decline’, Public Library of Science, 3, 2, 1-8.

Stuart S.N., Chanson J.S., Cox N.A., Young B.E., Rodrigues A.S.L., Fischman D.L. and Waller R.L. (2004) ‘Status and trends of amphibian declines and extinctions worldwide’, ScienceExpress, [www] available from: http://people.nnu.edu/jocossel/Stuart%20et%20al%202004.pdf, [19/12/2011].

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’, 105, 1, 11466-11476.

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.

Ecosystem Services

The Biodiversity Crisis

As discussed previously, humans have a notable impact on the Earth. An estimated 83% of the global terrestrial biosphere is under human influence, and perhaps as much as 36% of the bioproductive surface of the Earth is controlled exclusively by man (Harbel and Krausmann 2010). Species diversity represents a dynamic equilibrium between extinction and speciation. Since human colonization, however, this delicate balance has been upset. Evidence from marine ecosystems demonstrates the impact of humans over the past century. During this time 15% of Pacific Island birds have gone extinct, 20 of 297 mussel and clam species and 40 of 950 fishes have perished in North America, amounting to 1 extinction every 20 minutes. The current level of species loss has been compared to that of the late Cretaceous extinction 65mya, in which the dinosaurs and two thirds of species on earth were killed off, possibly due to asteroid impact (Karieva and Marvier 2003).

Anthropogenic activity has a marked influence on trophic skew. By removing species through hunting, fishing down of food webs, elimination of prey, and altering biophysical conditions, dramatic shifts in vegetation composition may occur, causing alterations to trophic levels (Novacek and Cleland 2001).

Another leading cause of biodiversity loss is habitat fragmentation. This is due to both climate change and population expansion and the resulting resource exploitation and alteration of land use patterns. Fragmentation increases local rates of extinction by reducing species population sizes and colonization from similar habitats, eliminating keystone predators or mutualists, enhancing genetic bottlenecks, promoting edge effects, and interrupting landscape-scale processes (Singh 2002). In the future habitat fragmentation is likely to reduce opportunities for speciation and restrict gene flow between species groups. This may be particularly acute amongst larger species such as primates, which are already prone to high rates of speciation and extinction (Levin and Levin 2002).

Severe habitat destruction, overexploitation of populations, freak meteorological events, or the emergence of new disease often results in direct and abrupt species loss, with small populations more likely to go extinct due to these freak events. The final descent into extinction, however, is often driven by synergistic processes that are disconnected from the original cause of species decline (Karieva and Marvier 2003). Habitat degradation and species extinction taking place over short timescales are likely to reset the future evolution of earth’s biota. Evidence from the fossil record suggests that the recovery of global ecosystems takes place over tens of millions of years (Novacek and Cleland 2001).

Ecosystem services

Ecosystem services are benefits to humans from resources and processes that are supplied by natural ecosystems. This definition was formalized in 2004 following the Millennium Ecosystem Assessment. Society is highly dependent on ecosystem products and services for food, shelter and healthcare. As human populations grow, resource demands imposed on ecosystems increase, and the environmental impacts of human ecosystem exploitation; overfishing, deforestation, industrialisation, and landscape degradation become more evident. The relationship between species and the services they provide needs to be understood in order to assess the implications of population change on humanity’s life support systems (Kareiva and Marvier 2003).

Biodiversity is of immense value to human health as ecosystem function and stability are reliant on it. Healthy functioning ecosystems provide humankind with a multitude of economic benefits including timber and fibre whilst being  essential for human survival. Constanza et al.(1997) have estimated the value of ecological services to be between $16 and 54 trillion per year. On a global scale, biodiversity represents a balance between rates of speciation and extinction, with greater biodiversity resulting in access to more resources (Singh 2002). If only a few individuals of an endangered species remain, however, they are unlikely to be able to make any meaningful contribution to ecosystem function (Balmford et al. 2003) and will be of considerably lower value.

How to measure ecosystem services

Prediction of extinction risk is dependent upon environmental and biological setting. Diversity is not uniformly distributed on earth, and typically increases from poles to equator (Brook 2008, Singh 2002). Complications arise in estimating the value of ecosystems when unidentified species become extinct. Under such conditions, ecosystem value may be reduced if the loss of these unknown species has a detrimental impact on ecosystem function (Duffy 2003).

Biodiversity hotspots

Tropical ecosystems are home to more unique species than any other habitat, and are hence considered to be biodiversity hotspots - areas containing high concentrations of endemic species. Since the coining of the term ‘hotspots’ by Norman Myers in 1988, research and conservation funding has been focused on these areas, neglecting other species-poor regions such as the Arctic. This is not necessarily an optimal approach, however, as the biological value of ecosystem services should also be considered when deciding which areas are deserving of more attention. Species that are of high value to humans are not found solely in biodiversity hotspot areas, so conservation efforts should perhaps be focused on ensuring that no major ecosystems suffer anything greater than a given percentage of biodiversity loss. It may be equally important to save higher taxonomic groups under threat than areas rich in endemic species, as by only conserving the species in a small area, evolutionary patterns may be altered (Kareiva and Marvier 2003).

Figure 2. Biodiversity Hotspots. Source: Myers et al. (2002). http://se-server.ethz.ch/staff/af/Fi159/M/My042.pdf

Measurement of biodiversity

In measurement of biodiversity, there are 4 key indicators.

·   Population richness. The number of populations of a species in a given area.

·  Population size. Number of individuals per population, which indicates the frequency distribution of population sizes. It is necessary to understand the contribution of each population to functioning ecosystems.

·   Population distribution. The spread of populations relative to their maximum possible extent within an area.

·   Genetic differentiation. More genetic variation within populations may provide better resilience to environmental change.

In order to fully understand population change and biodiversity decline, all four indicators must be considered. Biodiversity loss needs to include both species and population-based approaches (Luck et al. 2003).

Human Appropriation of Net Primary Productivity – HANPP, is an indicator that is used to estimate the relative scale of human activities and natural processes. Net primary production – NPP, is the net biomass produced by plants on an annual basis. It can be lost due to human-induced changes in ecosystem productivity, and provides a good indication of trophic energy flows within ecosystems. HANPP represents the extent to which land conversion and biomass harvest change the availability of NPP in ecosystems. Exact definitions of HANPP vary, but Harbel and Krausmann (2010) regard it as being the difference between the amount of NPP available in an ecosystem in the absence of human activities and the amount of NPP remaining in the ecosystem.

HANPP is a significant and useful indicator of human impact for several reasons. It provides a good measure of the physical size of the economy relative to that of the ecosystem. It gives an estimate of what proportion of the potential trophic energy, that could be used for wild animals and other heterotrophs, is still available and indicates human domination of ecosystems. Further to this, utilizing NPP as a basis for ecosystem functioning, human-induced changes of NPP and the way in which they affect patterns, processes and functions of ecosystems can be understood.

Figure 1. Map of Global HANPP. Source: Harbel and Krausmann (2010). http://www.eoearth.org/article/Global_human_appropriation_of_net_primary_production_(HANPP)

Balmford etal. (2003) suggest that the impact of humanity on nature can be gauged using estimations of extinction rates. Habitat loss data is combined with model predictions of changes in species number according to habitat area. Fossil records are incomplete and biased towards abundant and widely distributed species, this limits the applicability of this approach. Further to this, extinctions are difficult to document as only a small fraction of living systems are being fully monitored, there can also be a large time lag between habitat loss and species disappearance and statistical difficulties arise when combining datasets from different studies.

References

Balmford A., Green R.E. and Jenkins M. (2003) ‘Measuring the changing state of nature’, Trends in Ecology and Evolution, 18, 7, 326-330.

Brook B.W., Sodhi N.S. and Bradshaw J.A. (2008) ‘Synergies among extinction drivers under global change’, Trends in Ecology and Evolution, 23, 8, 453-460.

Constanza R. (1997) ‘The value of the world’s ecosystem services and natural capital’, Nature, 387, 6630, 253-260.

Duffy J.E. (2003) ‘Biodiversity loss, trophic skew and ecosystem functioning’, Ecology, 6, 680-687.

Harbel H., Erb K.-H., Krausmann F. (2010) ‘Global human appropriation of net primary production (HANPP), [www], available from http://www.eoearth.org/article/Global_human_appropriation_of_net_primary_production_(HANPP)

Kareiva P. and Marvier M. (2003) ‘Conserving biodiversity Coldspots’, American Scientist, 91, 4, 344-351.

Levin P. and Levin D. (2002) ‘The real biodiversity crisis’, American Scientist, 90, 1, 6.

Luck G.W., Daily G.C. and Ehrlich P.R. (2003) ‘Population diversity and ecosystem services’, Trends in Ecology and Evolution, 18, 7, 331-336.

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.

Singh J.S. (2002) ‘The biodiversity crisis: A multifaceted review’, Current Science, 82,6, 638-647.

A Haven For Endangered Species?

The Norfolk Broads is a unique managed environment, providing a shelter for a number of endangered British species, including Cetti’s Warbler and the Norfolk Hawker Dragonfly.

A recent press release highlights the value of such protected regions; however, the Norfolk Broads is by no means safe. 

Climate change is accelerating rates of sea level rise, placing the Broads at risk of flooding. Seawater intrusion is likely to result in salinisation, and subsequent ecological damage (Broads Authority).

Research has found that beyond a critical threshold, habitat fragmentation, a likely effect of sea level rise in the Norfolk Broads area, results in loss of genetic diversity, population decline and extinction. Shallow lakes are prone to dramatic state shifts following excessive nutrient loading, leading to eutrophication and species loss (Scheffer et al 2001). Worryingly, Thomas et al (2004) and Walther et al (2002) also note that shifts in timing of seasonal activities of plants and animals due to climate change, combined with habitat fragmentation, can result in large scale species loss.

Although undoubtedly valuable, protected areas such as the Norfolk Broads are still susceptible to the repercussions of climate change. With this in mind, is there any hope of preventing a mass extinction?

References

Opdam P. and Wascher D. (2004) ‘Climate change meets habitat fragmentation: linking landscape and biogeographical scale levels in research and conservation’, Biological Conservation, 117, 285-297.

Scheffer M., Carpenter S., Foley J.A., Folke C. and Walker B. (2001) ‘Catastrophic shifts in ecosystems’, Nature, 413, 591-596.

Thomas C.D., Cameron A., Green R.E., Bakkenes M., Beaumont L.J., Collingham Y.C., Erasmus B.F.N., Ferreira de Siqueira M., Grainger A., Hannah L., Hughes L., Huntley B., van Jaarsveld A.S., Midgley G.F., Miles L., Ortega-Huerta M.A., Townsend Peterson A., Phillips O.L. and Williams S.E. (2004) ‘Exctinction risk from climate change’, Nature, 427, 145-148.

Walther G.R., Post E., Convery 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, 389–95