Birds have always played an essential role in plant survival. As plants are generally unable to move on their own, birds carry their seeds and allow them to be distributed. Researchers have focused on the potential of plants to spread by birds in colder latitudes, to counteract the effects of global warming.
When plants use birds to spread
The phenomenon of seed spread by animals is called zoochory. Birds contribute to two types of zoochoria: first, epizoochoria, that is, the transport of semen that clings to feathers; second, endozoochoria, which occurs when birds ingest fruit and defecate from the seed that results from it. These processes can allow the seed to travel long distances and diversify the genetic composition of the resulting plants. Thus, migratory birds could help plants “track” climate change by scattering seeds over long distances. At least 18 researchers put it in a study published in the journal Nature.
The latter, focusing on the endozoochoria, explain that seeds can be systematically dispersed toward colder or warmer latitudes, depending on whether the fertility period of the plant species coincides with migrations to the north or south. In fact, according to lead author Juan P. González-Varo of the University of Cadiz (Spain): “In order for plant species to spread to birds migrating north, it must bear fruit between February and April. Plants that bear fruit in this period are characterized by either a very long period of fruiting (fruit production, editor’s note), as is the case with spruce (athorny trees), or until a very late period of fruiting, as is the case with ivyThey are based on phenology – variations of periodic occurrences of fauna and flora, depending on climate – and migration with data on 949 seed spread interactions between 46 bird species and 81 plant species. From 13 forest communities in Europe.
Global warming is a factor in the loss of plant biodiversity.
Indeed, “the climate changes we are currently experiencing are so rapid that many plants need propagation distances far greater than those that usually occur locally“Said Juan Pedro González-Varo. Migratory birds could allow this spread because they are able to carry viable seeds for tens or even hundreds of kilometers in short periods.”The most significant evidence comes from the Canary Islands in the Atlantic Ocean, where about 1.2% of birds have been found […] carried the seeds in their entrails from a continent more than 170 km away“, the researchers explain.
Too little migration to cold latitudes
Migratory birds from Europe migrate north in the spring and south in the fall. Those who migrate south scatter 86% of plant species from the studied forest communities. In contrast, birds that migrate north in the spring disperse only 35%. Some plants are transported both north in spring and south in autumn, which makes up more than 100%.
Juan P. González-Varo and his team also specify that “most of this critical north-expanding service is provided by only a few Paleactic migratory species (birds wintering in Central and Southern Europe or North Africa, editor’s note). Indeed, in Mediterranean communities, the black cap predominates with 49% seed dispersal. It is followed by European robin (15%) and thrush (9%). In temperate communities, blackbirds account for 44% of discharges, brown-headed thrushes for 15% and field thrushes for 10%. These species of birds are vaserines, that is, they are usually small in size, short-necked, and often sing and build nests. “Although these are common species on the European continent, only a few species have the potential to spread seeds in the north, some of which are (black hats and thrush, editor’s note) they are heavily hunted in the Mediterranean basin, both legally and illegally“, states the first author of the study.
An ecological envelope that is evolving at the rate of global warming
Whether plant or animal, species are distributed according to tolerance to ecosystem-specific conditions (altitudes, climates, habitats, etc.). These conditions qualify as an ecological envelope. Due to global warming, ecological envelopes – which create optimal conditions for plant life – are located towards higher (colder) latitudes. It remains to be seen whether migratory birds will be able to move fast enough to “follow” climate change, which will be necessary for plant species to colonize new, colder areas. “The average global speed at which organisms must move within range to maintain the same temperatures is estimated at 4.2 km per decade, although estimates exceed 100 km per decade in some areas.“, the researchers calculate.
“Our research suggests that migratory birds only help a minority […] plant species to spread to colder latitudes, while most species are scattered to drier and warmer areas“, concludes the research team. The conservation of these migratory species remains necessary, even if their contribution to the conservation of European flora biodiversity due to climate change may seem marginal. According to Beatriz Rumeu, co-author of the study, “this research is therefore necessary to understand the potential of European plants scattered by migratory birds to follow their ecological envelopes “. This could help halt or mitigate future biodiversity losses due to climate change. Other complementary studies might be of interest to understand seed propagation on a large scale.