A study co-authored by Dr Bernard Coetzee, a senior lecturer in the Department of Zoology and Entomology at the University of Pretoria (UP), has detailed how chance events could alter the Living Planet Index (LPI), a measure of the world’s biological diversity based on population trends.

Caption: Dr Bernard Coetzee in the field

The LPI tracks over 20 000 vertebrate species and has shown a precipitous decline for most species globally. This study, which was published in Nature Ecology & Evolution, set out to test the effects of random population fluctuations on the LPI. A random event, like a bird flying off or a chance encounter with a predator, can mean there is less chance of seeing species in future. Random events, therefore, have implications for efforts to monitor biodiversity on a global scale.

“We are losing most of the species on the planet,” Dr Coetzee said. “One of the ways that scientists and policymakers around the world are trying to measure such biodiversity loss is by the use of global indicators. The LPI is used a lot. However, like most indicators, they have biases, which means they are not as accurate as they can be.”

Caption: A roan antelope is rare and endangered. Picture by Charles J Sharp.

The quality of the data collated by the LPI on vertebrate populations differs: some species are sampled each year, while others are done every few years. This has created gaps in the information, which makes it difficult to combine population data in a simple index. The designers of the LPI were able to get around this limitation by fitting statistical models to fill the gaps caused by missing data.

“Our study showed how this crucial step of filling in data gaps has made the LPI sensitive to random population fluctuations,” said Dr Falko Buschke of the University of the Free State and lead author of the study. “Models of chance events show that they can change estimates of declines in the global LPI by 9.6%,” Dr Coetzee added.

The findings provide an estimate of the LPI that is robust to random population fluctuations and shapes of temporal trends. Ultimately, this will allow for population trends to be monitored more accurately to track progress towards the goals of the Post-2020 Global Biodiversity Framework, a plan for the conservation and management of biodiversity for the next decade and beyond.

According to Dr Coetzee, it is imperative that the LPI represents real changes to populations, and not just random fluctuations. “Our work shows that random fluctuations in populations can alter the outcome by a few percentiles. It may not seem like much, but it’s critical for policymakers to have as accurate an assessment of the decline of life on the planet as possible. Our work can help ensure that.”