Can ancient food webs help predict biodiversity collapse?

Our planet is no stranger to mass extinction events. Over the past 500 million years, five large-scale extinctions have taken place, with current predictions indicating that humans are rapidly driving towards a sixth.

The Permian-Triassic mass extinction was the largest of these events, and is believed to have wiped out up to 96% of all marine species.

Although scientists generally accept the causes of this extinction, how the impacts on Earth's ecosystems unfolded in its wake is still poorly understood.

A new study published in Current Biology examined marine fossils of different species that were discovered in nine sites across South China dating from before, during and after the Permian-Triassic mass extinction. 

Scientists sorted these species into groups based on predator-prey relationships to recreate how these ancient communities would have interacted.

They found that over half of the species went extinct with relatively little change to the overall ecosystem function. However, once the last species in each functional group began to disappear, this tipping point led to the rapid collapse of the ecosystem.

However, others have urged caution about recreating food webs of the past using this method. 

Professor Richard Twitchett, a palaeontologist at the Museum who studies mass extinctions and was not involved in the study, says, 'One key problem with the fossil record for any of these events is the chances of getting fossilised are really low, and it depends on whether the animals have hard parts. This makes recreating things like marine food webs impossible, as most marine organisms are soft-bodied.'

'Incorporating fossils from different sites spread across thousands of square kilometres in China makes it difficult to infer that certain organisms were eating each other in what we would understand as an ecosystem food web. They are separated by too much distance and time that the chances are they would never actually have interacted.'

What caused marine species to decline?

The Permian-Triassic extinction, known as The Great Dying, happened about 252 million years ago.

Large-scale volcanic eruptions in Siberia released huge amounts of greenhouse gases into the atmosphere, resulting in global warming, acid rain, oceanic poisoning, ocean acidification and marine anoxia.

This latest study suggests species loss and ecosystem collapse occurred in two phases, with the latter occurring 60,000 years after the initial decline in biodiversity. 

Ecosystems are often resilient to change as a number of different species fill similar roles within their environment. Therefore, ecosystem function can still be maintained if a single species goes extinct. But once all members of these functional groups start to disappear, the ecosystem can rapidly deteriorate.

'It's very difficult to remove all organisms that perform similar functional roles in marine ecosystems. It can happen locally, as is shown here in South China, but there is little evidence in the fossil record for it happening globally during these extinctions.'

'The key tipping point for marine ecosystems is the amount of oxygen dissolved in the seawater, which varies significantly depending on temperature,' says Richard.

'As the sea warms, it holds less dissolved oxygen, creating these 'dead zones'. Below a certain oxygen level, the only organisms that can cope are the ones that can metabolically survive in lower oxygen conditions.' 

What can previous mass extinctions tell us about the future? 

Earth is currently experiencing a rapid loss in biodiversity, with extinctions thought to be happening hundreds of times faster than would naturally occur. 

One million species of plants and animals are believed to be at risk of extinction, primarily because of human activities, such as deforestation, hunting and pollution. 

Climate change also presents a significant and long-term risk to biodiversity, with the burning of fossil fuels pumping billions of tonnes of carbon dioxide and other greenhouse gases into the atmosphere each year. 

Previous mass extinctions have shown the effects of rapid climate change. However, no mass extinction event in the past has happened in the same way because each has different starting conditions. 

For example, there might be variations in communities of species, the carbon cycle, and the position of the oceans and continents. Sometimes there is ice at the poles, and sometimes warming events occur during a natural warm period. 

'There are two lessons we can learn from the fossil record of these big extinction events,' says Richard. 'Firstly, we can see how bad it will get if we do nothing about climate change, and if we let carbon dioxide levels continue to rise to mass extinction causing levels.'  

'But second, we can see that life on Earth is far more resilient than is widely appreciated. Even the extreme global warming and biodiversity loss of the Great Dying could not sterilise our planet completely and species eventually recovered to a greater diversity than before.' 

While we cannot necessarily look at what has happened in the past and then apply it directly to the present or future, we can still learn lessons from the fossil record of previous mass extinction events. 

'A better way to use these events is to think of them as a series of 'natural experiments' that tell us how Earth's ecosystems have responded to past warming under different scenarios,' explains Richard.  

'If we see the same response time and again, then that is a powerful predictor that we'll see the same response in the future despite different starting conditions or rates of change.' 

It can also give us an insight into how specific groups of organisms might respond, particularly those that have weathered multiple events. For those organisms which have only recently evolved, their future is far less certain.  

'The other interesting thing is there are many groups of species in the oceans that have already been exposed to similar warming in their evolutionary history, so we have an idea of how they will cope,' says Richard.  

'But some species, such as whales and other cetaceans, have never experienced a warming-related mass extinction, so how they will cope remains to be seen.'