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

Case study: The Honey Bee

Bees are an integral component of both agricultural and wild species pollination, widely acknowledged as being economically indispensable. It has been estimated (vanEngelsdorp et al. 2008) that up to a third of the food we consume is pollinated by bees.

Increasingly, however, honeybee populations have been declining, leading to a pollination crisis, in which reductions in pollinators results in the loss of plant species (Ghazoul 2005), Klein et al. 2007).

Honey bees require a diverse community of flowering plants that bloom throughout the spring and summer. Climate change induced shifts in floral density and distribution can lead to alterations in species interactions. In particular, dry regions are predicted to become drier, which will deprive honeybees of necessary moisture, rendering them unable to cope. Yields of pollinator dependent crops have declined, perhaps due to expansion and simplification of agricultural areas on such a scale that numbers of bees simply cannot meet the demand placed upon them. Despite these negative anthropogenic effects, new research suggests that the introduction of new plant species, whose pollen is rich in protein, can be beneficial to bees.

Honeybee mortalities cannot be attributed purely to common diseases and pests, such as the mite Varroa destructor (Sammataro et al. 2000). A recent paper points out that Apis mellifera, the Western honeybee, is under increasing threat from anthropogenically induced habitat fragmentation. In addition to this, the introduction of non-native species, such as the Italian Bee Apis mellifera carnica, and the aggressive African honeybee Apis mellifera scutellata, has resulted in reductions in A. mellifera populations (Ghazoul 2005).

CCD, Colony Collapse Disorder, has been blamed for widespread reductions in honeybee numbers. The disorder is characterised by fewer adult bees within hives, accompanied often by disease pathogens (Olroyd 2007). This phenomenon is not yet fully understood, but it is thought to be multifactoral, a combination of attack from viruses and fungi, depleted immunity, and the narrow genetic base of colonies (Ghazoul 2005). Colony loss is so severe that in the USA, colony numbers have fallen from 5.9 million in 1947 to 2.44 million in 2008.

Colony Collapse Disorder is explained in this clip:

For beekeepers, colony loss is upsetting. But from a large-scale environmental perspective, the repercussions of permanently losing these industrious creatures could be devastating. Indeed the reproductive decline of wild plants has been attributed to pollination failure (Ghazoul 2005).

Le Conte (2008) notes that although A. mellifera has shown remarkable resilience to past changes in climate, there is doubt as to whether it will be able to adapt to the level of environmental change that Earth is currently undergoing.


References

Ghazoul J. (2005) ‘Buzziness as usual? Questioning the global pollination crisis’, Trends in Ecology and Evolution, 20, 7, 367-373.

Klein A.-M., Vassiere B.E., Cane J.H., Steffan-Dewenter I., Cunningham S.A., Kremen C. and Tscharntke T. (2007) ‘Importance of pollinators in changing landscapes for world crops’, Proceedings of The Royal Society Biological Science, 274, 303-313.

Le Conte Y. and Navajas M. (2008) ‘Climate change: impact on honey bee populations and diseases’, Revue Scientifique et Technique Office, 27, 2, 499-510.

Levy S. (2011) ‘The Pollinator Crisis: What’s best for bees’, Nature, 479, 164-165.

Olroyd B. (2007) ‘What’s killing American Honey Bees’, Public Library of Science Biology, 5, 6, 1195-1199.

Ratnieks F.L.W. and Carreck N.L. (2010) ‘Clarity on honey bee collapse’, Science, 327, 152-153.

Sammataro D., Gerson U. and Needham G. (2000) ‘Parasitic Mites of Honey Bees: Life, History, Implications, and Impact’, Annual Review of Entomology, 45, 519-548.

Soland-Reckeweg G.S., Heckel G, Neumann P, Fluri P and Excoffier L. (2009) ‘Gene flow in admixed populations and implications for the conservation of the Western honeybee, Apis mellifera’, Journal of Insect Conservation, 13, 317-328.

vanEngelsdorp D., Hayes J., Underwood R.M. and Pettis J. (2008) ‘A survey of honey bee colony losses in the US, Fall 2007 to Spring 2008’, Public Library of Science, 3, 12, 1-6.

A sixth mass extinction; are humans to blame?

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.

References

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. 

A Sixth Mass Extinction?

There is much debate as to whether a sixth mass extinction event is currently underway. A recent paper by Barnosky et al (2011) argues we are now irrevocably in the midst of species loss which could rival that of the ‘big 5’. Papers by Rockstrom et al (2009), Raup and Sepkoski (1986) and Wakeand Vredenberg (2008) also discuss this issue, and will be used to detail some of the complexities of the ‘sixth’ mass extinction.

·       Comparison with ‘normal’ extinctions. Under normal background extinction conditions, those taxa that go extinct tend to be from small populations in restricted geographic ranges. Hence if certain species can be seen to decline rapidly in number, large creatures in particular, then extinction selectivity may be changing to enter a mass extinction state. Although 99% of all species that have ever lived on earth are now extinct, this species loss is usually balanced by speciation. Given the current rates of species loss, however, teamed with the fact that evolution of new species takes many thousands of years and revival from mass extinction spans millions of years, no meaningful biodiversity recovery is likely to take place in our lifetime (Barnosky et al 2011).

·       Estimations of species loss. Species-area relationships can be used to relate species losses to habitat area losses, these suggest that future species extinctions will be between 21 and 52% of all current species. Major problems can arise because most species have not yet been formally described; fossil remains are biased and incomplete, not all species fossilize well, if at all, and fossil analysis is often carried out at genus rather than species level which can lead to species being lumped together. Thus, estimations are likely to be under-representative as if one species in a genus becomes extinct, the genus as a whole will remain relatively untouched.

·  Approaches to reconstructions. Using an E/MSY, extinctions per million species years, approach, Barnosky et al (2011) observe that current extinction rates are notably higher than both background rates and those of the previous half-millennia. Alarmingly, comparison of historical and recent extinction rates using a 500-year rate approach has shown that if all threatened species went extinct within the next hundred years, bird and mammal extinction would take somewhere between 240 and 500 years to match the level of the big five extinctions, and in 2,265 years 75% of species would be lost. Hitherto, there have been discrepancies in the assessment of species loss through the use of rate and magnitude, and questions have been raised as to effectiveness of extrapolation of extinction rates of well-studied taxa. These rates are highly dependent on the length of time for which they are measured, so current short-term extinction measurements are not accurately comparable with long-term data. A solution to this is the estimation of species extinction rates by maximising fossil background rates and minimising current extinction rates, and the use of combined rate-magnitude comparisons.

Figure 1. Current extinction magnitudes, expressed as percentage of species. This illustrates the severity of extinctions within taxonomic classes, and suggests that extinction levels comparable to those of the big 5 are not far off. Source: Barnosky et al (2011).

·       Climate models. Climate change has progressed to the extent that some Earth systems have now exceeded their stable Holocene state. This has resulted in retreat of summer sea ice in the Arctic Ocean, retreat of mountain glaciers, loss of mass from Greenland and Antarctic ice sheets and rapid rates of sea level rise. Climate models don’t take long-term feedbacks into account, however, so may underestimate the severity of long-term climate change by ignoring considerable threats to ecological life support systems.

Conclusion

Although extinction is a necessary part of evolution, species losses are occurring at a much higher rate than is considered normal. This topic requires further discussion, and the focus of the next post will be the extent to which humans are to blame for the species loss that is arguably underway.

References

Rockstrom et al (2009) ‘A safe operating space for humanity’, Nature, 461, 472-475.

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

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’, Proceedings of the National Academy of Science of the United States of America, 105, 1, 11466-11473.

Raup D.M. and Sepkoski J.J. (1986) ‘Periodic extinction of families and genera’, Science, 231, 4740, 833-836. 

Species loss

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?

http://www.telegraph.co.uk/earth/wildlife/8847946/Top-10-endangered-species.html

http://www.guardian.co.uk/environment/2011/oct/25/french-losing-wildflower-harvest-environment?intcmp=122

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.