An international research team, with the participation of the Doñana Biological Station – CSIC, has concluded that waterfowl have a key role in allowing European plants to compensate for climate change by moving them to cooler latitudes during spring migration. The study has been published in the high-impact journal Global Ecology and Biogeography.
It has long been known that fruit-eating birds, such as thrushes and other songbirds, disperse the seeds from berries in their droppings, but plants lacking a fleshy fruit have until now been widely assumed to be incapable of dispersing by birds. "Ducks such as mallards were known to disperse pondweeds, but this new study shows they are vital for dispersing the seeds of many terrestrial plants, and especially during spring migration when they head northwards to breeding areas", said Professor Andy Green, a researcher at the Doñana Biological Station – CSIC. This role for non-frugivorous waterbirds is particularly significant because the great majority (92%) of European flowering plant species do not have a fleshy fruit.
Spring migration, crucial for plants to face climate change
The spring migration is precisely the movement that allows plants to shift their distributions in line with climate change. The ability of fruit-eating birds to carry seeds in this direction is limited because they take fruits from bushes and trees at the wrong time of year, and particularly during autumn migration on the way to wintering grounds. In contrast, according to this study, ducks carry seeds of many species in spring, when they head towards cooler areas. Until now, they were not thought to be capable of this since spring migration occurs before seed production.
"The conclusions of this study disprove this belief because ducks do not take the seeds off the plants, but instead swallow them months after they are produced, especially when filtering them out from lake sediments", explained Adam Lovas-Kiss, a researcher at the Institute of Aquatic Ecology of the Hungarian Ecology Research Centre. Only in the case of submerged plant species were seeds dispersed soon after production. In the case of emergent and terrestrial plant species, they found the opposite trend.
During the study, the team sampled mallard droppings throughout the annual cycle in a major lake in Hungary, Velence Lake, where mallards are known to migrate up to 2,300 km between breeding and wintering sites. Compared to other seasons of the year, more plant species were dispersed per dropping in spring, although there were more seeds per dropping in winter. Furthermore, there were more seeds of terrestrial than aquatic plants in spring, whereas in other seasons aquatic seeds were dominant. Over 600 droppings were collected and over 5,000 seeds recovered, demonstrating that an impressive number of seeds can survive passage through the mallard gut. In addition, 40% of all these seeds were subsequently germinated in the laboratory, proving that seeds dispersed by these birds are viable.
The key role of waterfowl in seed dispersal
"Mallards are the best-known duck species for the public. It is a widely distributed duck species, often found in urban parks. Most people think about them as a recreational partner, or a quarry for hunters, but there is rising evidence that they are also important seed sowers", explained Adam Lovas-Kiss. Ducks have mutualistic relationships with many plants that were previously overlooked: the birds get energy by digesting some of the seeds, and at the same time the plants have some of their seeds dispersed into new habitats.
"Until now, most of these plant species have been thought to have no natural means of dispersing more than a few hundred metres at best, which would not be enough to keep pace with climate change. However, mallards can indeed disperse seeds over hundreds of kilometers when migrating. Ducks are therefore providing a vital ecosystem service that is helping ecosystems to adjust to the drastic changes associated with global heating", concluded Andy Green.
Urgyán, R., Lukács, B.A., Fekete, R., Molnár V., A., Nagy, A., Orsolya, V., Green, A.J., Lovas-Kiss, Á. 2022. Plants dispersed by a non-frugivorous migrant change throughout the annual cycle. Global Ecology and Biogeography. DOI: 10.1111/geb.13608
Andy J. Green and Adam Lovas-Kiss are joint senior authors of this research.
Estación Biológica de Doñana – CSIC
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