Human action negatively affects frugivorous birds in Doñana
In the last 40 years, birds frugivorous, that feed on fruits, from the Doñana environment have experienced changes in their abundance, physical condition and calendar, according to a recent study carried out by research staff associated with the Doñana Biological Station (EBD CSIC).
These birds are essential for the natural regeneration of many species of trees and shrubs that depend on them to disperse their seeds. The work reveals the vulnerability of birds that consume fruits for the purposes of climate change and modification of land use, with repercussions for the regeneration of fruit-bearing plants, which are also the ones that provide them with food.
These birds are essential for the natural regeneration of many species of trees and shrubs that depend on them to disperse their seeds.
Finding uninterrupted information on birds in recent decades is very complicated, but this work takes advantage of a historical sampling carried out in the years 1981-1983 by Peter Jordan, research professor at the CSIC, repeating it between 2019 and 2021. In this way, the team has been able to verify the negative trend of frugivorous birds in the Doñana environment. The area, a known transit area for migratory species, has experienced changes in temperature and vegetation as a consequence of human action.
Measurement of the tarsus of a song thrush by an expert bander. These measurements are taken to document changes in the weight of the bird relative to its size / José A. Sencianes
Fewer species and in worse conditions
The work, carried out by specialists in ecology and a ringing team, shows worrying changes in frugivorous bird species. Currently, there is a lower abundance of wintering and seed-dispersing species (66% and 13% less, respectively), and they have been replaced by others more typical of forested areas, residents (non-migratory) and with diets more insectivorous.
Researchers have found that frugivorous birds today have fewer fat stores and lower body weight than they did in the 1980s
Likewise, on a seasonal scale, certain frugivorous species (9 out of 11) have advanced their maximum abundance date by a month compared to the 1980s, which could correspond to a response to the earlier appearance of fruits as an effect of climate change.
Apart from this change in its calendar, the study has analyzed the physical condition of these frugivorous birds, and has discovered that, in addition to being increasingly rare, they also present worse conditions than in the past: fewer fat reserves and lower body weight than would correspond to their size.
Irene Mendoza, postdoctoral researcher associated with the Doñana Biological Station and author of the article, highlights the importance of the study: “Although the ideal would have been to have 40 years of uninterrupted data, unfortunately this type of long-term ecological information is very expensive to obtain and was available to us.”
“Luckily, we had Pedro Jordano’s data from the 1980s, which was a golden opportunity to compare it with today. To do this, we repeated exactly the same sampling protocol in the framework of my project funded by a Marie Curie grant. In this way, we were able to convincingly verify that the frugivorous birds of Doñana are negatively affected by human action”, adds the expert.
Part of the scientific and technical team during one of the samplings carried out in October 2019 in the town of Hato Ratón (Doñana). / Carlos Ruiz
Effects of climate change
These changes probably have their explanation in the current context of global change. On the one hand, the vegetation of the Doñana area studied (Hato Ratón, Villamanrique de la Condesa, Seville) is now more wooded and is more dominated by the stone pine, compared to the 1980s, when the thicket of fleshy fruit dominated , especially mastic and olive trees.
On the other hand, the increase in temperature associated with climate change It is also related to changes in the arrivals and departures of the migratory birds, as well as with the abundance peaks of most of them. The lack of synchronization between the arrival of the birds and the production of fleshy fruits that they consume can mean that they have less accumulated fat, which is critical for their survival during migration and winter, which requires a large energy reserve.
The loss of frugivorous birds can cause such a decrease in seed dispersal that the life cycle of plants with fleshy fruit is interrupted.
These results are dramatic and highlight the vulnerability of the species that consume fruits to the action of the human being. The loss of frugivorous birds can cause such a decrease in seed dispersal that the life cycle of plants with fleshy fruit is interrupted, which in turn affects the availability of food for these birds that feed on they.
However, the extent of these effects remains to be seen. According Maria Campo-Celada, first author of the article, “although we have quantified these alterations at a local scale, they probably reflect situations that currently occur in many ecosystems, as documented in numerous studies. Although the evidence continues to accumulate, the difficulty in tackling the engines of global change leaves us with an uncertain future”.
“The study shows how frugivorous birds and plants depend on each other to persist and complete their life cycles. We are witnessing faster and more negative changes than we might think; changes that occur in short periods of time and that have great importance. The Mediterranean forest depends for its conservation and functioning on the persistence of these mutualistic interactions between plants and animals”, he concludes. Peter Jordan, National Research Award (2018) and co-author in the work.
Field-Headpiece et al. “Assessing short and long-term variations in diversity, timing and body condition of frugivorous birds”. OIKOS. https://doi.org/10.1111/oik.08387
Rights: Creative Commons.