More than 60 billion leaf litter invertebrates died in the Black Summer fires. Here's what that did to ecosystems

Source: The Conversation, Heloise Gibb, Nick Porch

The Black Summer megafires engulfing south-eastern Australia in 2019–2020 were so intense they burned habitats rarely exposed to fire, such as southern warm temperate rainforest.

These rainforests range from East Gippsland in Victoria up to just south of Sydney. Usually, they stay moist enough to prevent major fires. But in that unprecedented summer of fire, 80,000 hectares burned. Our new research estimates more than 60 billion invertebrates in the soil and leaf litter died too.

While our hearts went out to the burned koalas and kangaroos, this was a silent tragedy. These tiny creatures are enormously important in ecosystems. They eat dead leaves, create rich soil, and provide a key food source for bandicoots and lyrebirds. Many species have very small ranges, putting them at real risk of decline or even extinction from fire.

As renowned naturalist E. O. Wilson once said, invertebrates are the "the little things that run the world". But because they are small and out of sight, we still underestimate their significance in ecosystems and their contribution to Australia's biodiversity. They're all but forgotten when ecological disasters strike.

How did we find out how many invertebrates died?

In warm temperate rainforests, there's a layer of moist leaf litter which is home to an abundance of ancient lifeforms. These include the macroinvertebrates big enough to see with the naked eye, such as velvet worms, snails, land hoppers, millipedes, slaters and beetles.

Many of these groups include species with very small ranges, putting them at particular risk from bushfire and other changes to their environments. 

Back in the lab, we ran the samples through Tullgren funnels, which sort leaves from creatures, then counted the macroinvertebrates. We excluded the tiny springtails and mites, which are hugely abundant mesoinvertebrates. We found every hectare of unburnt rainforest had 2.5 million litter macroinvertebrates, while severely burnt forests had a quarter as many.

If we look at all temperate rainforest burned at different severities across the south-east that means 60 billion tiny deaths. But of all the forest that burned during that summer, rainforests made up only about 1%. The total loss might be closer to 6 trillion individuals. Then to get to truly extraordinary numbers, we can include mites and springtails which account for around 95% of individual invertebrates. That would give us an estimate of 120 trillion.

Why are these tiny creatures so important?

Invertebrates account for fully 99% of all animal species and most of the weight of animals on the planet. Renowned Australian scientist Baron Robert May is famously quoted as saying "to a good approximation, all species are insects". Even now, an estimated 70% of all Australian invertebrate species remain undescribed. Many will go extinct before we have time to document them. 

Although we know little of the ecology of most invertebrate species, collectively we know they play crucial roles in ecosystems. Losing this rich food source is likely to slow the recovery of key ecosystem engineers such as lyrebirds and bandicoots, which turn over large volumes of dirt in search of them.

When we try to replant forests without invertebrates, many plants and trees struggle. That's why conservationists are using leaf litter transplants to move vital invertebrates from healthy forests to new ones.

These critters are a vital way nutrients cycle through our forests by breaking down leaves and other organic matter. Globally, they're directly responsible for converting about 40% of all leaf litter into soil. By turning over leaves or shredding them into pieces, they make it possible for microbes to help decompose organic matter. Without this work, leaf litter would begin to pile up, setting the scene for more fires.

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