The honeybee may be the best-known pollinator of plants, but there are thousands of pollinator species, including other bees, butterflies, moths, beetles, flies and even some birds and mammals. Pollinators help to produce food – their contribution to crops is worth billions of dollars. But they are also essential for the reproduction of plants generally, and thus for biodiversity.
This is why it’s a worry that certain pollinator species have fallen in numbers, and in some places the diversity of pollinators has decreased. These declines are due to the combined effects of pesticide use, destruction and degradation of habitats, invasive species, pollinator diseases and climate change.
More and more pollinators are becoming threatened with extinction. For example, the Loveridge’s sunbird is endangered by damage caused by people to the forest where it lives in Tanzania. Some species have already gone extinct, for example Libythea cinyras, a swallowtail butterfly from Mauritius. This species has died out entirely, probably due to the forests in which it lived being chopped down to make sugarcane plantations.
These declines motivated us to find out how important pollinators are for wild plants. Our paper provides the first global estimate of how many plant species mostly or completely rely on animal pollinators to make seeds and thus to reproduce. We found that it’s about 175,000 plant species – half of all flowering plants. This means declines in pollinators could cause major disruptions in natural ecosystems, including loss of biodiversity.
DIY versus animal pollination
We already knew that about 88% of flowering plants are pollinated by animals, as opposed to being wind-pollinated. But most animal-pollinated plants are also a bit auto-fertile. That means they can produce some seeds without pollinators, for instance by self-fertilisation. To estimate the importance of pollinators to seed production, we therefore had to account for auto-fertility.
We used the contribution of pollinators to seed production as an indicator of their importance to plants. We measured this by comparing seed production in the absence of pollinators to seed production with pollinators present.
There was already a lot of data, but it was spread out in hundreds of scientific papers, each focused on pollination of different plant species. Originally working in three separate groups, 21 scientists from five continents read these papers in languages including English, Spanish, Portuguese and Chinese to gather pollination information into databases. In our study, we combined forces and consolidated the data into a single database including 1,174 species from all over the world.
We estimate that, without pollinators, a third of flowering plant species would produce no seeds and half would suffer an 80% or more reduction in fertility. This means most plants would produce fewer seeds if they were less well pollinated by animal pollinators. In most plants, auto-fertility is very much plan B. Overall, we estimate only about 21% of plants are not vulnerable to pollinator declines at all. This includes 12% that are wind-pollinated and 9% that have very high auto-fertility.
We also found that in tropical areas, where there is a lack of information on trends in pollinator numbers, pollinators are especially important for plant reproduction.
Consequences for plants
As so many plants species are highly reliant on pollinators, if the numbers of pollinators decline, or different pollinators become the most numerous, reproduction of many plant species is likely to be affected. This would likely lead to declines in numbers of some of those plants. The endangered species Babiana odorata from the Western Cape in South Africa is an example of one of these vulnerable plant species, as it is completely dependent on pollinating bees to make seeds.
Even though some plants might evolve to be more auto-fertile on the one hand, or more attractive to the remaining pollinators on the other hand, this will not be possible for all plant species. Some plants will be at risk of extinction.
For example, in South Africa, in locations where special oil-collecting bees have disappeared due to habitat destruction, the numbers of some of the plants they pollinate – like the endemic witch orchid Disperis cucullata – have plummeted too.
Our results also suggest that pollinator declines could change the competitive balance between plant species, changing which plant species are most common. For example, many weeds are auto-fertile, raising the concern that they would gain an advantage because they would have less competition from more desirable pollinator-dependent plants.
Also, if ecosystems become dominated by auto-fertile plants, this could lead to further reductions in pollinators because auto-fertile plants typically have less of the pollen and nectar that pollinators feed on.
Seeds aren’t only important for plant reproduction, but also as food sources for animals. If plants are less well pollinated in future, there will be fewer seeds for some seed-feeding animals, which could also then be threatened with extinction.
What can be done
Our study provides grounds to hope that if pollinator declines can be reversed, the worst consequences can be averted. Most plants are quite long-lived, so it will take some years for these plant populations to start to decline and go extinct, even if pollinator declines reduce their reproduction.
Aside from their importance in crop production, we hope our findings will add motivation to conserve and restore pollinators for biodiversity itself. More research is especially important in tropical areas.
But there are already many things we can do, such as avoiding pesticide use and conserving their habitats.
Although pollinator decline has many potential knock-on effects, it is only one of a range of threats to biodiversity. Others are climate change, habitat destruction and invasive species. Our study shows that to mitigate the other threats to biodiversity, we must ensure that future ecosystems have enough of the pollinators they need.