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Rising CO2 Levels Threaten Dung Beetles

Extreme weather conditions, as seen in recent flood, drought and fire events worldwide, are some of the most obvious ways in which rising atmospheric CO2 levels are changing our world. But there are some serious effects that CO2 is having on our ecosystems that are less easily observed.

A new study led by Wits University post-doctoral researcher, Dr. Claudia Tocco, provides evidence that elevated CO2 levels directly affects the development and survival of tunnelling dung beetles (Euoniticellus intermedius). The study, published in the international journal, Global Change Biology, presents a possible explanation for the current ‘insect apocalypse’ — a global decline in insect populations that is still not well understood.

Serendipitous science

“The idea to investigate the effects of elevated CO2 levels on dung beetles was a result of ‘serendipitous science’,” says Tocco. “My labmate and colleague at Wits, Mr. Nic Venter, was growing cacti under different CO2 conditions to investigate how these plants may be affected under future scenarios in our changing world.” Venter was looking at CO2 levels under four scenarios: pre-industrial (~1750), modern-day, 30 years into the future, and 50 years into the future. “We thought, why not put some dung beetles under the same conditions and see what happens?,” says Prof. Marcus Byrne, senior author of the paper and Dr. Tocco’s postdoctoral advisor. What they found came as a surprise.

Beetles grown under heightened levels of atmospheric CO2 experienced lower survival rates, and were smaller in size. “When raised under CO2 levels predicted for the year 2070, a third fewer beetles emerged and were 14% smaller in size when compared to pre-industrial CO2 levels,” says Tocco.

“When we first found this result, we were surprised!” says Byrne. “We were not expecting such a drastic effect. In fact, we were not convinced at first that this result was real, and so we repeated the experiment – but we kept getting the same result.”

“We knew that increased CO2 levels can affect insects indirectly by changing plant quality,” says Venter, “but did not expect such a direct effect on the beetles themselves.”

The proof is in the soil

“Dung beetles like many insects, spend a large portion of their lives in the soil — as larvae, pupae and as adults,” says Dr Blair Cowie, another of Tocco’s colleagues and fellow labmate in Prof. Byrne’s research group. “Most people perhaps do not realise that increases in atmospheric CO2 levels also affect the soil, and our study shows that this can in turn affect animals that live in soil.”

The team suspects that the negative effects experienced by dung beetles under scenarios of heightened CO2 in this study may be a result of increased competition between the beetles and bacteria in the soil. “Our next steps are to conduct further experiments to tease apart whether it is the CO2 levels in the dung ball, the brood balls, or the soil in general that is affecting dung beetle development,” says Cowie.

“It is the fact that the lives of dung beetles are so closely tied with the soil that makes them such excellent model organisms to investigate changes in soil ecology,” says Tocco. “If atmospheric CO2 is affecting dung beetles, it is affecting other insects too.”

Explaining the insect apocalypse

Across the world, insect numbers are collapsing, and the findings from this study may provide new insight into the cause. Because changes in climatic conditions vary across the globe, and some temperature changes may in fact be beneficial to insects, changes in weather patterns don’t seem to adequately explain population declines. While insecticides can cause declines, their use is patchy globally, so they don’t appear to be causing the worldwide problem. But, points out Dr. Tocco, “Our findings of how heightened CO2 levels affect dung beetles presents a plausible explanation for the insect apocalypse, since the increases in CO2 are consistent across the planet,” says Tocco.

Impacts of Dung Beetle Declines

Not only is elevated CO2 affecting the climate, it is also affecting animals who are delivering ecosystem services to us. In this 3:36 video about the group’s research, Dr. Blair Cowie says, “Insects play a crucial role in our ecosystems and in the case of dung beetles, losing them to increasing levels of CO2 is likely to cost us dearly in terms of ecosystem services. With their major role as recyclers likely to be decreased or reduced all together, this could have potentially disastrous consequences as our ecosystem shifts out of balance, making us more concerned about rising CO2 levels.”

Dr. Nic Venter also points out the decline in forage nutritional values. That’s something we’ll be taking an in-depth look at in coming issues.

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