Changes to El Niño occurrence causing widespread tropical insect and spider declines

(Press-News.org) Arthropods, including insects and spiders, make up the vast majority of animal species on the planet.

Despite their small size they are irreplaceable contributors to the health of natural habitats, as well as vital food sources for birds and other larger animals.

But, arthropods may be declining globally. There is some evidence to support reduced numbers of species in temperate regions of the Northen Hemisphere. In the tropics, however, evidence for arthropod declines has so far been limited.

A recent international collaboration of scientists has attempted to find this missing evidence, with the findings published in Nature.

The team, including Professors Roger Kitching and Nigel Stork from Griffith University’s School of the Environment and Science, conducted a whole-of-tropics analysis on tropical forest insects and their relatives and the ecological roles that they perform.

Combining information from over 80 previous studies in tropical forest sites that have never been commercially altered by humans, the team found significant biodiversity loss in multiple types of arthropod, including butterflies, beetles and spiders.

The biodiversity loss matched drops in the amount of live leaf material consumed by arthropods over time, and substantial instability in the amount of dead leaves decomposed by arthropods.

“To find such large declines over many studies is really bad news,” said Dr Adam Sharp, first author and data analyst from Hong Kong University.

“Our results suggest strongly that the immense biodiversity of tropical forest arthropods is immediately threatened.

"Since all of the data we used comes from forest considered ‘untouched’, even the deepest and darkest tropical forests are likely to be heavily impacted.”

The team link climate change to the declines in arthropods and their respective ecological roles. The tropics experience natural but irregular year-to-year variation in climate, driven by an atmospheric phenomenon called the El Niño Southern Oscillation – ENSO. Long-term changes to the ENSO cycle, caused by climate change, are likely behind the observed arthropod declines.

Arthropods can be highly sensitive to ENSO, with different arthropod types coming and going during the opposing El Niño and La Niña stages of the cycle.

While there is considerable difference in effect across the tropics, El Niño conditions are often hot and dry while La Niña conditions are often cooler and wetter.

They should usually strike a balance such that no arthropods ever disappear completely - but the El Niño part of the ENSO cycle is becoming more frequent and more intense due to climate change.

“We believe that changes to El Niño occurrence are causing widespread arthropod declines,” said corresponding author Dr Mike Boyle.

“In these tropical forests that haven’t otherwise been physically modified by humans we can rule out habitat loss, pesticides, pollution and various other threats. In these places El Niño seems to be the prime suspect.”

Indeed, the team found the largest declines in arthropods occurred in those that favour La Niña conditions. If El Niño is becoming detrimental due to climate change, then its occurrence is sure to further chip away at arthropod biodiversity into the future.

“Arthropods are essential components of functioning ecosystems, carrying out vital processes including decomposition, herbivory and pollination,” said University of Hong Kong Associate Professor Louise Ashton.

“We must better understand how nature is shifting and what is happening to arthropods and their ecosystem processes in response to environmental change.

Co-author Professor Roger Kitching from Griffith University said: “The crucial message for Australia is the need to monitor the biodiversity in our rainforests – revisiting previous surveys is the key.”

The international team continue their research at forest sites across Hong Kong and Mainland China, Australia and Malaysia.

The study ‘Stronger El Niños reduce tropical forest arthropod diversity and function’ has been published in Nature.

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