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. 

Mass Extinction: An Introduction

This blog will discuss the causes and ramifications of mass extinction, and explore the debates surrounding the possibility of the onset of a sixth mass extinction. An understanding of the conditions preceding historical mass extinctions are important to our comprehension of the biological and evolutionary significance of our current climatic state on a potential forthcoming mass extinction.

As climatic conditions change, ecosystem variables shift, and environmental conditions alter, a certain number of species extinctions are likely to occur. Fossil evidence suggests that the majority of species have a lifespan of between 2 and 10 million years, this process of extinctions is considered to be ‘normal’. Although biodiversity loss occurs at a regional level, it influences the functioning of earth systems and leads to more wide reaching consequences. In contrast, mass extinctions can be defined as extinctions of a significant proportion of global biota in a geologically insignificant period of time (Sepkoski 1986).

Rockstrom et al (2009), amongst others, suggest that the current rate of extinction is between 0.2 and 0.5 per year per million species, somewhere between 100 and 1000 times greater than the ‘natural’ level. Discussion of the causes of mass extinctions, specifically whether they are due to climatic or anthropogenic influence will be tackled in later posts.

Mass extinction is a timely and relevant topic, and a rich source of academic debate. This blog will examine the complexities of mass extinction, and feels justified in doing so for the following reasons:

• A greater understanding of the causes of mass extinction will facilitate a more informed and committed approach to efforts to ensure the health of future ecosystems.
• Ocean feedbacks, and the development of their effects on life on earth, need to be understood in the context of mass extinctions.
• Levels of CO₂ and other greenhouse gases, the rates and causes of fluctuations, need further examination, as several theories suggest that they are contributing factors to mass extinction events.
• The level at which humanity can responsibly continue to make long-term social and economic developments needs to be understood in the context of biodiversity loss (Rockstrom et al 2009).