Essay by Eric Worrall

A natural adaption to an Australian ecological disaster exposes these claims of fragile food webs as nonsense.

Computer modelling predicts climate change causing cascading animal ‘co-extinctions’

By Eugene Boisvert and Anisha Pillarisetty

Computer modelling has shown the variety of vertebrate animal species found in locations across the globe could be cut by 27 per cent by the end of the century.

Key points:

• Models of the Earth were created populated by animal species and food webs
• Extinctions caused by other extinctions were also considered in the study
• One of the researchers involved says it shows biodiversity conservation and climate change mitigation go together

The simulation conducted on one of Europe’s most powerful supercomputers also found that one extinction caused a cascade of extinctions that have been coined “co-extinctions”.

The tool found that under the worst climate change prediction, 34 per cent more species would become extinct than would be predicted when not considering co-extinctions.

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To produce the study, the scientists created synthetic Earths complete with virtual species and more than 15,000 food webs to predict the interconnected fate of species.

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Read more: https://www.abc.net.au/news/2022-12-17/computer-modelling-shows-cascading-animal-coextinctions/101777762

The abstract of the study;

Coextinctions dominate future vertebrate losses from climate and land use change

GIOVANNI STRONA AND COREY J. A. BRADSHAW

SCIENCE ADVANCES
16 Dec 2022
Vol 8, Issue 50

DOI: 10.1126/sciadv.abn4345

Abstract

Although theory identifies coextinctions as a main driver of biodiversity loss, their role at the planetary scale has yet to be estimated. We subjected a global model of interconnected terrestrial vertebrate food webs to future (2020–2100) climate and land-use changes. We predict a 17.6% (± 0.16% SE) average reduction of local vertebrate diversity globally by 2100, with coextinctions increasing the effect of primary extinctions by 184.2% (± 10.9% SE) on average under an intermediate emissions scenario. Communities will lose up to a half of ecological interactions, thus reducing trophic complexity, network connectance, and community resilience. The model reveals that the extreme toll of global change for vertebrate diversity might be of secondary importance compared to the damages to ecological network structure.

Read more: https://www.science.org/doi/10.1126/sciadv.abn4345

The authors of the study admitted that gathering real world data is difficult, so they decided to create their own data.

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Apart from the obvious modeling and computational challenges to incorporate interactions among species, the main reason why there are few studies accounting for interactions is that obtaining sufficient data in most communities is intractable. Therefore, global-scale modeling of entire ecosystems appears to be the only viable solution, even if a challenging one (11, 22). 

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An important caveat is that while our virtual species are functionally realistic, they do not have taxonomic or phylogenetic meaning. Hence, our results reveal local changes in species diversity but do not provide information on global species extinctions per se. Neither does the model claim to produce an Earth replica, but instead aims to build an ecologically plausible Earth. Hence, the model cannot forecast Earth’s future but instead projects relative potential scenarios based on different assumptions (mainly carbon emissions) and reveals the underlying processes leading to those outcomes.

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Read more: Same link as above

So what is this evidence of adaptability I mentioned, which undermines claims that food webs are fragile?

My example is the story of Australia’s successful and less than successful ecological interventions.

Early colonists brought lots of ornamental plants and animals. Some of them, like prickly pear, became a serious nuisance because of a lack of local predators.

A beetle, the cactoblastis moth, was introduced in 1926 to control the pear plants, which were colonising valuable agricultural land. The moths successfully and rapidly eradicated the bulk of the prickly pear infestation. Today you can still see prickly pears by the sides of roads, but the plants tend to be very sparse, with large gaps between individual plants.

Australians were pretty excited by this successful biological control of a pest species, so in 1935 another species was introduced to manage cane beetles, which were threatening Australia’s sugarcane production – the cane toad.

There was a problem. Cane toads helped protect the sugarcane. But cane toads are toxic – no Australian predator species could cope with their venom. Across vast swathes of Australia predator species numbers crashed. The last meal of all the dead predators was a cane toad.

But the predators didn’t all die. This is important later in this sorry tale.

At the height of the crisis, gardens in badly hit states like Queensland were full of dinner plate size toads which were so fat they could barely move – I remember visiting my uncle in Brisbane, and barely being able to walk across his garden for all the toxic toads littering the lawn. None of the toads bothered moving out of the way of people – decades of safety from predators had taught them they had nothing to fear from anything in Australia.

Then Australia’s raptors figured out a solution. The venom of the toads is mostly in their skin, mainly on their backs. So the crows learned to flip the toads over and eat the non-venomous parts.

I first started hearing stories about crows eating toads a decade ago. Then I started seeing it with my own eyes.

Now there are hardly any toads to be found, except dead toads. After nightfall when the crows are roosting, the toads come out. But the much smaller toads which survived the crow apocalypse do not usually appear until after dark, and their behaviour is nothing like their dinner plate size ancestors – they frantically hop away and hide at the slightest noise or movement.

The predator species have recovered – the Aussie woodlands are full of large predatory lizards once again, and other predator species which were devastated by the toad plague. The recovered predator populations have either learned to avoid eating the toads, or there are so few toads thanks to the raptors, the toads are no longer a significant threat to predator populations.

My question – which part of this real world story of ecological disaster and recovery shouts fragile food web?

In my opinion the European supercomputer food web experiment is way too unrealistic to draw real world conclusions. New connections in the real world food web appear all the time, no food resource remains underutilised for long, even when the underutilised resource is a deadly toxic toad. Any breaks in the food web caused by climate change or disease or whatever, in the real world are rapidly filled.

There are a handful of species which are so specialised they actually would die if their food source was removed. For example, Koala Bears are so specialised at eating Eucalyptus leaves, they would likely all die if say a Eucalypt version of Dutch Elm Disease killed off all the Eucalyptus trees.

But are 17.6% of vertebrate species so specialised they cannot adapt to a small change in temperature? Are 27% of vertebrates about to die out? That seems highly implausible.

A few degrees of warming, if it occurs, is not an asteroid scale ecological catastrophe, or a million year duration volcanic eruption, it is a mild shift in climatic conditions, which life will have no problem adapting to if the paleo record is any guide. Just like life has already adapted to the many climatic shifts, introduced species and other disruptions which have occurred in Earth’s geological past.