Around the world, animals that pollinate flowering plants are in decline.
An increasing number of pollinating mammal and bird species are moving towards extinction, according to the first study of its kind.
Other, so far unpublished studies, also suggest that pollinating insect species are also heading towards extinction.
If these trends continue, say the studies' authors, key species will be lost, with potentially significant impacts on how ecosystems function.
The latest assessment is published in the journal Conservation Letters, by ecologist Eugenie Regan of the United Nations Environment Programme World Conservation Monitoring Centre, based in Cambridge, UK, and an international group of colleagues.
In their research, the scientists point out that animals pollinate more than 87% of flowering plant species, and humans use many of these plants for food, livestock forage, medicine, materials and other purposes.
A number of studies have indicated that pollinator numbers might be falling. But despite this, until now there has been no investigation of how they are faring at a global level.
The new study tackled the issue by examining how bird and mammal species known to pollinate plants are faring on the IUCN Red Lists.
The lists, produced by the International Union for the Conservation of Nature, are regarded as the most authoritative and objective system for collating information on the risks of species going extinct.
Overall, pollinating bird and mammal species are deteriorating in status, with more species moving towards extinction than away from it, the study found.
We will lose key, irreplaceable species in our ecosystems
On average, 2.4 species per year have moved one Red List category towards extinction in recent decades, representing a substantial increase in extinction risk across this set of species, report the scientists.
“These measures show which species are threatened with extinction and show that more and more species are becoming at risk of extinction over time,” Dr Regan told BBC Earth.
“In the real world, it means that pollinating species will become extinct and we will lose key, irreplaceable species in our ecosystems.”
If pollinators disappear, there will be huge ecological impacts. The number and diversity of wild plants would fall and ecosystems would become unstable.
There would also be a huge economic impact as well.
One study in 2005 estimated the total economic value of wild and managed pollination services worldwide at US$215 billion.
Fewer pollinators would affect crop yields, food security and ultimately the welfare of people, say the researchers.
It is possible that, if pollinating species do go extinct, others might fill the ecological voids they have left, mitigating the problem.
But things are unlikely to be that simple, says Dr Regan.
Ecosystems are our life support system, she points out.
At first, losses of species may make little difference, because there are so many. But if enough are removed or a few are taken away at crucial spots, the whole system will crash.
And the story may not stop with birds and mammals.
“The best data we have on insect pollinators is at the national level and mainly in Europe. These data are showing worse trends than mammals and birds, unfortunately,” she warns.