The value of pollinators for seed production

In a study to provide a global estimate of the importance of pollinators for plants in natural ecosystems, researchers from Stellenbosch University, led by Dr James Rodger, a postdoctoral fellow in mathematical sciences, found that, without pollinators, one-third of flowering plant species wouldn’t produce seeds at all.

The value of pollinators for seed production
Species such as Gorteria diffusa, Dimorphotheca sinuata and D. pluvialis are dependent on pollinators, such as the bee fly pictured here, for seed production, and on seed for persistence in the system, making them vulnerable to pollinator decline.
Photo: AG Elllis

Approximately 175 000 plant species (half of all flowering plants) rely mostly or completely on animal pollinators to develop seeds and thus reproduce. A decline in pollinators could therefore cause major disruptions in natural ecosystems, including loss of biodiversity.

This is the finding of a paper titled ‘Widespread vulnerability of plant seed production to pollinator declines’, published in the journal Science Advances on 13 October 2021.

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Dr James Rodger, a postdoctoral fellow in the Department of Mathematical Sciences at Stellenbosch University (SU) and lead author, says this is the first study to provide a global estimate of the importance of pollinators to plants in natural ecosystems.

The study, involving 21 scientists affiliated to 23 institutions from five continents, was led by Rodger and Prof Allan Ellis from SU. It is a product of the Synthesis Centre for Biodiversity Sciences in the German Centre for Integrative Biodiversity Research.

Prof Tiffany Knight of the Helmholtz Centre for Environmental Research in Leipzig, Germany, and a senior co-author of the report, says recent global assessments of pollination have highlighted a knowledge gap in our understanding of the degree to which plants depend on animal pollinators.

“Our synthetic research addresses this gap and enables us to link trends in pollinator biodiversity and abundance to consequences for plants at a global level,” she explains.

A global first
While most plants are animal-pollinated, most also have a small degree of autofertility. This means they can produce at least some seeds without pollinators, for example by self-fertilisation. However, until this study, the question “How important are pollinators for wild plants?” did not have a clear answer at the global level.

The researchers used the contribution of pollinators to seed production, measured by comparing seed production in the absence of pollinators to seed production with pollinators present, as an indicator of their importance to plants.

Data on this did exist, but was spread out in hundreds of papers, each focusing on pollination experiments on different plant species. To address this problem, researchers at various institutions began to consolidate the information in databases.

Rodger developed the Stellenbosch Breeding System Database as a postdoctoral fellow in SU’s Department of Botany and Zoology; Knight, Prof Tia-Lynn Ashman and Dr Janette Steets led the sPLAT working group that produced the GloPL database; and Prof Mark van Kleunen and Dr Mialy Razanajatovo produced the Konstanz Breeding System Database. All three databases were combined into a new one for the current study.

The database contains the information from 1 528 separate experiments representing 1 392 plant populations and 1 174 species from 143 plant families and all continents, except Antarctica.

The findings show that, without pollinators, one-third of flowering plant species would produce no seeds and half would suffer an 80% or more reduction in fertility. Therefore, even though autofertility is common, it by no means fully compensates for reductions in pollination service in most plant species.

“Recent studies show that many pollinator species have gone down in numbers, with some even having gone extinct. Our finding that large numbers of wild plant species rely on pollinators shows that declines in pollinators could cause major disruptions in natural ecosystems,” Rodger warns.

Knock-on effects
Van Kleunen, from the University of Konstanz and a co-author of the report, says it is not a case of all pollinators disappearing.

“If there are fewer pollinators to go around, or even just a change in which pollinator species are the most numerous, we can expect knock-on effects on plants, with affected plant species potentially declining, further harming the animal species and human populations dependent on those plants. Pollinators are important not only for crop production, but also for biodiversity.

“It also means that plants that don’t rely on pollinators, such as many problematic weeds, might spread even more when pollinators continue to decline,” he adds.

Dr Joanne Bennet, a co-author of the report from the University of Canberra who curated
the GloOL database, says another disconcerting factor is the positive feedback loop that develops if pollinator-dependent plants decline or go extinct. “If autofertile plants come to dominate the landscape, then even more pollinators will be negatively affected, because autofertile plants tend to produce less nectar and pollen.”

All is not doom and gloom, however, according to Rodger. Many plants are long-lived, opening a window of opportunity to restore pollinators before plant extinctions occur from a lack of pollinators.

“We lack high-quality, long-term monitoring data on pollinators in Africa, for example, including South Africa, although some work has been started in this regard. We hope that our findings will stimulate more of this kind of research, so that we can detect pollinator declines and mitigate their impacts on biodiversity,” Rodger concludes.

Phone Dr James Rodger on 060 574 8151, or email him at [email protected].