Features

It's easy to think of mass extinctions only in terms of the impact on animal species but of course plants suffered too.

Stephen Hesselbo of Oxford's Department of Earth Sciences reports in this week's Science on his research studying fossils to see how plants fared in a major extinction event 200 million years ago.

I asked him about what these plant fossils can tell us about extinctions, biodiversity and climate change.

OxSciBlog: Why are these Greenland fossils of particular interest to those studying climate/biodiversity?
Stephen Hesselbo: The East Greenland fossil plant beds represent a uniquely detailed record of floral change across one of the 'big five' mass extinctions that characterize the history of life over the last 500 million years - that which occurred at the Triassic-Jurassic boundary, 200 million years ago.

The plant beds, which represent a warm temperate community, have been known about for about a hundred years, but have previously been worked on principally from the point of view of taxonomy and evolutionary relationships.

This new study takes advantage of the fact that a series of plant beds leading up to the mass extinction horizon were laid down under similar conditions on the banks of rivers in flood - changes in plant assemblages can therefore be interpreted in terms of changing ecosystems rather than vagaries of plant preservation.

A series of expeditions have shipped back more than a tonne of fossil material and so the statistics of the present study are based on large numbers.

OSB: What do they tell us about interactions between CO2 levels and plant life in the Triassic?
SH: Previous work on the density of stomata [the holes through which CO2 diffuses to the site of photosynthesis] on the fossil leaves  has indicated increases in CO2 over the Triassic-Jurassic boundary interval.  These are estimated to have risen from a starting value of about 600 ppm to a maximum of about 1800 ppm through the extinction event.

One effect of the flux of CO2 into the atmosphere is to leave a record in the carbon-isotope ratios of organic matter formed at the time.

Although the Triassic-Jurassic extinction was dramatic amongst animals, hitherto the effects on plants appear to have been much less intense.  However, the present study provides evidence for very marked collapse in ecosystem diversity in the run up to the extinction.

OSB: What do we think may have caused the sort of dramatic loss of biodiversity seen in this period?
SH: The Triassic-Jurassic boundary coincides with a period of volcanism on a massive scale - think Iceland but scaled up a hundred to a thousand times.  This happened as the super-continent Pangaea broke up (initiating formation of the Atlantic ocean in the process).

As well as directly contributing gasses such as CO2 and SO2 to the atmosphere, it is likely that molten rock baked older deposits of organic-rich sediments and salt to create large fluxes of CO2 and SO2 and other gases such as halocarbons. Other mass extinctions similarly coincided with periods of hugely enhanced volcanic activity.

OSB: Can we draw any lessons from this period about how rising CO2 emissions may affect plant/animal life in the future?
SH: To a certain extent yes - the present day flux rates for carbon are higher than anything that was likely at the Triassic-Jurassic boundary, but the total amount of carbon emitted is considerably less, so far.

Further complications are the unknown quantities and effects of the the additional gases produced during the mass extinction event.

Nevertheless, these so called 'flood basalt eruption' episodes do represent a natural pre-run of exactly the kind of release of fossil carbon, sulphur and other elements that we are currently subjecting the planet to.

OSB: How might further research in this area help us to better understand the impacts of climate change?
SH: This study shows how plant ecosystems responded to environmental change in the interval leading up to a mass extinction. Further work should tackle also the recovery interval, after the peak of the mass extinction, as ecosystems returned to normality.

Additionally we need to better constrain the changes in atmospheric carbon-dioxide that went alongside floral change, and also document similar changes that must have been occurring on different parts of the ancient Earth at the same time.

Finally an improved 'age model' (i.e. chronology of events) would be very useful. With these pieces of information we will be in a better position to describe the deterioration of plant ecosystems in response to greenhouse-gas forcing.

Professor Stephen Hesselbo is based at Oxford University's Department of Earth Sciences. The research was conducted with co-authors Jennifer McElwain of University College Dublin and Peter Wagner of the Smithsonian Institution.

Collecting guano halfway up a cliff may not be everyone's idea of fun but it has led to the discovery of the oldest nest of a bird of prey ever recorded.

Kurt Burnham, a DPhil student at Oxford University's Department of Zoology, was part of a team that collected 20 guano samples from 13 different gyrfalcon nest sites in cliffs in central-west and northwest Greenland.

Radiocarbon dating of these samples has found that one of the nests has been in continuous use for the last 2740-2360 years with three other nests likely to be over 1000 years old. The team report their findings in the journal Ibis. 

The nests themselves are little more than depressions or scrapes in rock ledges but the guano deposits reveal the long history of the birds' occupation.

'When analysing multiple samples from the same nest it was also possible to detect changes over time in the type of prey consumed, very likely an indicator of changes in the prey composition in the area,' Kurt told us.

'This study provides both information on the likely initial colonization of these areas by gyrfalcons in addition to probable paleoenvironmental conditions at the time and patterns of glacial retreat. ' 

So what makes the falcons return year after year? 

Kurt told BBC News Online: 'Something, be it nest ledge depth, or the amount of cliff overhang above the nest, is so attractive at these locations that gyrfalcons are re-using them for thousands of years.'

But, as he explains, this attachment makes them vulnerable to climate change.

'As a result of a warming and ameliorating climate other bird species, such as peregrine falcons, are moving further north.'

'As peregrine populations continue to increase in density they will likely use more and more of these traditional gyrfalcon nests, forcing gyrfalcons to find alternate locations to nest in which may not offer the same amount of protection from the harsh Arctic environment in Greenland.'

More in this Telegraph article. 

Scientists who saved the Large Blue butterfly are now turning their attention to some of the other 10,000 insect species that depend on ants for their survival.

Like the Large Blue these insects – ranging from butterflies and moths to beetles and hoverflies – all rely on ants in one way or another for food and protection and are known to be particularly vulnerable to the sort of environmental changes associated with climate change.

'Only seven or eight of the 10,000 or so insect species that we know depend on ants have been studied at all,' Jeremy Thomas of Oxford University’s Department of Zoology told us.

'Whilst the work done at Oxford and elsewhere on the Large Blue and its relationship with ants saved one species there are thousands more that are living on the edge and, if we don’t act quickly, could disappear from Britain’s meadows, woods and heathland.'

The insects in question range from beautiful butterflies such as the Silver Studded Blue [Plebejus argus], which co-habits with the Black garden ant [Lasius niger], to a hoverfly you might mistake for a bee [Microdon mutabilis], which lives off a type of wood ant [Formica lemani], to a beetle [Lomechusa] which specialises in getting ants to feed and protect it.

It's amazing to think that so many insects have found a way to infiltrate ant society and get a 'free ride' from their more socially-inclined neighbours. The nest parasite Lomechusa is a fascinating beast with a cuckoo-like lifestyle, the difference being this beetle never grows up and continues to trick ants into caring for it even as an adult.

But it's this relationship with ants that makes these insects particularly sensitive to small changes in habitat which, whilst not affecting them directly, can have a devastating impact if they affect the ant populations they depend on.

Jeremy explains: 'In the case of the Large Blue butterfly and the red ants [Myrmica sabuleti] it was a combination of less grazing by both human livestock and wild animals that caused the ants' 'microhabitat' to become overgrown, so that ant populations fell and there weren’t enough ants to nurture the next generation of Large Blues.'

'The worry is that these are just the sort of changes we can expect to see as a result of climate change.'

The researchers intend to study the impact human activity is having on 11 endangered insect species and the ants that support them. They will then use this information to predict how future changes in climate and land use will affect these populations and work out what can be done to mitigate any harmful effects.

Jeremy adds: 'our previous work has shown that if we can understand the complex relationships between these species and what they need to survive then we can take action to save them.'

'When you think that the number of insect species that rely on ants is twice the number of all mammal species in the world then it seems if we are serious about preserving biodiversity then we need to do all we can to save such endangered insects.'

Professor Jeremy Thomas is based at Oxford's Department of Zoology. 

Last week the scientists behind two recent publications on carbon emissions urged negotiators at the Bonn climate talks to make phasing out CO2 emissions altogether part of any future strategy.

Myles Allen and David Frame of Oxford University were amongst those who took the unusual step of writing an open letter to policy-makers.

In the letter the team ask those devising targets to take into account their research which shows an upper limit of one trillion tonnes of carbon. If cumulative carbon emissions exceed this limit the world is likely to suffer dangerous global warming of 2 degrees Celsius.

‘In addition to setting targets for emissions in 2020 and 2050, we feel the UNFCCC process should acknowledge that avoiding dangerous climate change will require emissions of the longest-lived greenhouse gases like carbon dioxide eventually to cease altogether,’ Myles Allen of Oxford’s Department of Physics commented. 

In the open letter the scientists warn: 'fossil carbon reserves substantially exceed the amount that can safely be released into the atmosphere. Net global carbon dioxide emissions will eventually have to decline towards zero leaving a substantial fraction of available fossil carbon stored, in some form, out of the atmosphere indefinitely.'

'We urge the participants in December's Conference of the Parties to the United Nations Framework Convention on Climate Change to acknowledge the need to limit cumulative carbon dioxide emissions as one element of their vision for long-term cooperative action to avoid dangerous climate change.' 

It's probably not something that governments around the world want to hear as they struggle to meet their modest emissions reductions commitments whilst trying to drag their economies out of recession.

Yet what the latest science is telling us is that, as Myles puts it, 'climate policy needs an exit strategy': in other words we need to imagine an end to emissions - a lower carbon economy doesn't actually solve the problem that the atmosphere is a finite resource which will, if we carry on emitting, run out of spare capacity.

It's perhaps a wake-up call to those who think that simply modifying our energy habits will get us out of our current predicament: we need to lower emissions now but we also need to plan an escape route to a zero carbon future.

Dr Myles Allen is based at the Department of Physics whilst Dr David Frame is Deputy Director of the Oxford University Smith School of Enterprise and the Environment

This week our fickle attitude to the wildlife around us was highlighted in two stories: one about the reintroduction of beavers to the US, the other about a proposed cull of grey squirrels in the UK.

In both cases, it seems, the line between a wildlife treasure that must be saved and an animal pest that must be eliminated is perilously narrow: become too successful as a species and you'll become an expensive inconvenience for humans - the lesson is: 'stay rare and we'll care!'

I think part of the problem is that we've failed to accept that, in a world where space and resources are at a premium, saving wildlife always comes at a cost.

This cost is more obvious in places such as Africa where, as highlighted by work by Oxford scientists on a beehive fence to deter elephants, coming up with solutions that enable wild animals and humans to coexist is often a matter of life or death for both parties - with elephants at risk of being shot and local people at risk of losing the crops they need to survive.

In countries such as the UK and the US humans have the upper hand having, over thousands of years, transformed much of the landscape - and its ecosystems - in a way that is convenient for us and getting annoyed with the animal 'pests' that dare to change it to suit themselves.

Maybe we could learn something from the efforts in Africa about how understanding animal behaviour can help us change the way we do things so that animals and people can get along.

For instance: what would scare away a beaver from building dams where we don't want them to? What should be eating grey squirrels but isn't? (probably because we haven't given such predators the space and resources to survive).

Perhaps, instead of blacklisting some species as 'pests', we need an animal version of détente in which we agree the zones of influence we're willing to give up to wildlife and be honest about the price we're willing to pay for biodiversity in our own backyard.