Many plants rely on the help of birds to spread. By eating their seeds and spreading them elsewhere with their droppings, new trees and bushes can grow. In areas deforested by humans, this job is primarily done by large birds such as pigeons and starlings, a new study shows. Together with smaller forest birds, they could now help to gradually reforest non-cultivated areas, the study authors suggest.
We have significantly changed much of the landscape. Where extensive forests once grew, today in many regions only isolated “forest islands” remain, surrounded by a sea of fields, pastures and settlements. This also influences the composition of the birds that feed on the fruits of the trees and spread their seeds. But how exactly the communities of fruit-eating bird species in forests differ from those in open areas was previously unclear.
3,000 fecal samples in the DNA test
Researchers led by Juan González-Varo from the Spanish University of Cádiz have now investigated for the first time which fruit-eating birds are increasingly found in areas modified by humans and which ones are found in Europe's forests. To do this, they examined the density of seeds lying on the ground and bird droppings in seven European landscapes: in England, Germany, Poland, but also in the Mediterranean countries of Spain and Italy. The sample areas, each measuring one to 3.8 square kilometers, included both forest areas surrounded by fields and cattle pastures with individual trees that were used as “resting places” by flying birds and which therefore also contained bird droppings.
The researchers examined the seed density at each sampling site and collected around 3,000 fecal samples over the course of a year. Using so-called DNA barcoding, González-Varo and his colleagues were then able to determine which bird left behind each scat sample and which tree seeds it had previously eaten. The researchers were able to compare which birds are active as “winged foresters” in which areas and which types of trees they prefer to spread via their seeds.
Birds as passive foresters
The result: In each landscape studied, González-Varo and his team identified an average of 16 fruit-eating bird species that spread the seeds of 14 different plant species. The biodiversity in the forest areas and in the open spaces of the pastures and fields was almost identical. Only the composition of the different birds and seeds varied. “Fruit-eating birds that are larger and more mobile than corresponding forest species can be found in largely deforested areas. They also spread the seeds of plants that are larger, bear more seeds and only produce fruit late in the year,” explains co-author Sascha Rösner from the University of Marburg.
In settlements and fields, it was primarily pigeons and starlings that took care of the distribution of tree seeds. The planting performance of starlings alone was 60 times higher in anthropogenic areas than in forests, as the researchers report. In the forest, however, smaller birds such as blackcaps and robins had their beaks in front. However, due to their smaller body size and mobility, they primarily spread the seeds of smaller plants, as DNA analyzes revealed. According to the scientists, these results underline that both forest and field-dwelling birds make an important contribution to the distribution of tree seeds and thus to reforestation. In the future, they could therefore specifically help with the reforestation of agricultural land that is no longer actively cultivated.
"Based on our study, we suggest that reforestation measures should focus on planting isolated trees as 'starting areas' for forests and on those plant species that are otherwise difficult to spread in open landscapes," recommends co-author Jörg Albrecht from Senckenberg Biodiversity and climate research center in Frankfurt am Main. The trees planted for this purpose should also be located near intact forests so that as many bird species as possible can eat their fruits and then distribute the seeds. By allowing birds from both habitats to “work together” as foresters in the long term, ecologically valuable forests would be created again on fallow land.
Source: Senckenberg Society for Natural Research; Specialist article: Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2302440120