Over the last 30 years, up to a quarter of all known bee species have fallen off international global records, despite a major increase in the number of such records available.

Around 20,000 species of bee are known to science, but analysis of a detailed international data set suggests around 5,000 of these species have not been seen since 1990.

The loss of the bees may not mean they are entirely extinct, researchers said, but represents a sharp collapse in thousands of populations, meaning these species have become so rare that no one is observing them in nature.

“With citizen science and the ability to share data, records are going up exponentially, but the number of species reported in these records is going down,” said lead author of the research Eduardo Zattara, a biologist at the Institute for Research on Biodiversity and the Environment at the National University of Comahue in Argentina.

“It's not a bee cataclysm yet, but what we can say is that wild bees are not exactly thriving.”

Declining bee populations have been widely reported in various countries around the world, but the researchers said previous studies have tended to focus on a specific area or a specific type of bee. But this study aimed to identify more general, global trends in bee diversity.

“Figuring out which species are living where and how each population is doing using complex aggregated datasets can be very messy,” said Dr Zattara.

“We wanted to ask a simpler question: what species have been recorded, anywhere in the world, in a given period?”

To obtain the answer, the researchers pored over the Global Biodiversity Information Facility (GBIF), an international network of databases, which contains over three centuries’ worth of records from museums, universities, and private citizens, which together hold data on the 20,000 known bee species from around the world.

In addition to finding a quarter of total bee species are no longer being recorded, the researchers observed the decline is not evenly distributed among bee families.

Records of halictid bees - the second most common family - have declined by 17 per cent since the 1990s. Meanwhile, records for melittidae bees - a much rarer family - have gone down by as much as 41 per cent.

“It's important to remember that ‘bee’ doesn't just mean honeybees, even though honeybees are the most cultivated species,” said Dr Zattara.

“Our society's footprint impacts wild bees as well, which provide ecosystem services we depend on.”

The research team cautioned that though their study provides a close look at the global status of bee diversity, it is too general an analysis to make any certain claims about the current status of individual species.

“It's not really about how certain the numbers are here. It's more about the trend,” said Dr Zattara.

“It's about confirming what’s been shown to happen locally is going on globally. And also, about the fact that much better certainty will be achieved as more data are shared with public databases.”

But the researchers warn this type of certainty about global declines might not come until it is too late to reverse the trend.

They said urgent action needs to be taken to help protect the remaining populations of these vital pollinating insects.

“Something is happening to the bees, and something needs to be done,” said Dr Zattara.

“We cannot wait until we have absolute certainty because we rarely get there in natural sciences.

"The next step is prodding policymakers into action while we still have time. The bees cannot wait."

The study is published as the UK government has approved the use of neonicotinoid insecticides, which were banned in the EU after they were linked to the decline of bees and birds.

Recent research has reinforced the link, after scientists found the substance - the most common insecticide worldwide, had a “big effect on the amount of sleep taken by both flies and bees.”

Around a third of wild bees are in decline in the UK, due to habitat loss, climate change and the use of toxic pesticides, according to a 2019 study.

The new research is published in the journal One Earth.