The uncertain future of the jussara palm

Researchers investigate how climate variability and the extinction of bird species affect the genetic diversity and conservation of Euterpe edulis, the palm tree that symbolizes the Atlantic Rainforest.

By Peter Moon  |  Agência FAPESP – Several factors appear to affect the survival of the jussara palm (Euterpe edulis), from which the best and, thus, most valuable heart of palm is extracted. In addition to strong pressure from illegal harvesting and destruction of the Atlantic Rainforest, its main habitat, extinction of bird species and climate change may also lead to the disappearance of this particular palm species in the wild.

Scientists use the term ‘defaunation’ to refer to the extinction or severe decline of animal populations in natural ecosystems. Flora conservation policies typically ignore the problems of climate variability and loss of animal species responsible for seed dispersal. These two factors have been detected during years of research by biologist Mauro Galetti and his team in the Ecology Department of São Paulo State University (UNESP) at Rio Claro (São Paulo State, Brazil).

Heart of palm can be harvested from several palm species, but the products available in stores usually come from the jussara palm (E. edulis), peach palm (Bactris gasipaes) and açaí palm (Euterpe oleracea). E. edulis is native to the Atlantic Rainforest, while in Brazil, the other species are from the Amazon.

E. edulis almost always has a solitary stem, so the tree dies when the heart is extracted. The other species are multi-stemmed and resprout, like banana trees.

Another important difference is that E. edulis takes eight to 12 years to produce high-quality heart of palm, whereas hearts of B. gasipaes can be harvested only 18 months after planting.

The harvesting of heart of palm from E. edulis necessarily results in the felling of adult individuals, preferably the largest – they can reach 20 m in height. When adult trees are felled, there are fewer plants to produce seeds for dispersal and germination. The population declines, and the species may become locally extinct.

For this reason, Brazil’s National Flora Conservation Center (CNCFlora) includes E. edulis in its Red List of endangered plant species.

Conservation of E. edulis is directly linked to the maintenance of biodiversity in the Atlantic Rainforest biome. More than 48 bird species and 20 mammals feed on its seeds and fruits. Toucans, guans, thrushes and bellbirds are the main seed-dispersal agents, while agoutis, tapirs, peccaries, squirrels and many other animals benefit from its seeds or fruits. The fruits attract many animals because they are rich in fat and anti-oxidants.

Researchers at UNESP have found that the rapid decline in populations of seed-dispersing birds due to habitat fragmentation or destruction, as well as illegal trapping, is eroding the jussara palm’s genetic variability. When a species loses genetic variability, it becomes more fragile and less capable of surviving challenges, such as climate change. 

In a study published in the journal Conservation Genetics, researchers from UNESP, the Federal University of Goiás (UFG) and the University of Santa Cruz (UESC) conclude that the current pattern of genetic diversity in E. edulis in the Atlantic Rainforest is the result of a combination of climate variability over thousands of years and the impacts of more recent human activity, such as habitat destruction and the extinction of seed-dispersing bird species. 

The authors say that they found that the jussara palm’s genetic diversity has been reduced by both variations in climate during the last 10,000 years, which are part of a natural historical process, and defaunation, especially the loss of large-body frugivores (fruit-eating animals) due to anthropogenic factors (relating to the influence of human activity on nature).

These findings led the researchers to try to understand how frugivorous birds affect the process of genetic differentiation in E. edulis. 

Research conducted in Galetti’s laboratory had already confirmed that there was a link between a decrease in the size of the palm’s seeds (which vary naturally from 8 mm to 14 mm in diameter) and local extinction of large seed-dispersing birds.

A study published in the journal Science in 2013 described how the researchers investigated 22 areas of Atlantic Rainforest in Paraná, São Paulo State, Rio de Janeiro State, Minas Gerais and southern Bahia. They found that, in areas containing large frugivorous birds, such as toucans (Ramphastos spp.) and guans (Penelope spp. and Aburria jacutinga), the jussara palm’s seeds were larger, often exceeding 12 mm, whereas in areas where narrow-billed birds, such as thrushes (Turdus spp.), predominated, the seeds were never more than 9.5 mm in diameter.

In other words, in Atlantic Rainforest areas where toucan, guan and bellbird populations had become locally extinct owing to hunting, the larger seeds were no longer dispersed because they were too large to be eaten by thrushes and other small frugivores, which can only swallow small seeds. Seeds not eaten by birds do not germinate, so maintenance of the population of E. edulis depends on birds.

This difference in seed size may seem negligible, but it is important to the conservation of the jussara palm. “Smaller seeds more easily lose water because of their smaller surface area,” Galetti explained. “As a result, the palms are more sensitive to longer periods of drought, which will become more frequent owing to climate change.”

The researchers found that, in forest areas near Rio Claro (São Paulo State) in which jussara palms with small seeds predominated, the seeds simply failed to germinate after the severe drought in 2014.

“Selective pressure due to defaunation is so strong that, in some areas, the larger seeds of E. edulis disappeared in only 50 years. Is this selection perceptible at the genetic level? That’s the question we set out to answer in our new project,” said biologist Carolina da Silva Carvalho, whose PhD research is supervised by Galetti.

In a study published in 2016 in Scientific Reports, an online journal owned by Springer Nature, the UNESP group showed that defaunation not only alters the phenotypic variability of the seeds of E. edulis (i.e., their size), but also leads to evolutionary changes in its genotypes.

The study was supported by FAPESP under the aegis of the Thematic Project “Ecological consequences of defaunation in the Atlantic Rainforest” and via a regular research grant for the project “New sampling methods and statistical tools for biodiversity research: integrating animal movement ecology with population and community ecology”.

“In this project, we set out to investigate whether the extinction of large fruit-eating birds could lead to a genetic change in the jussara palm. However, we knew historical factors could also influence its genetic diversity, so we constructed a number of hypotheses and tried to see which process best explained the pattern of genetic diversity among populations of E. edulis,” Carvalho said.

The study took into consideration three key variables that could influence genetic diversity among populations of jussara palms. The first consisted of data regarding the loss of agents for dispersing large seeds (defaunation). 

The second key variable was biogeographical origin. The researchers investigated the differences between populations of E. edulis in ombrophilous forest (dense and wet, with perennial foliage) and in semi-deciduous areas (more open and dry, with vegetation that sheds leaves at certain times of year). 

They also investigated how forest fragmentation in the Atlantic Rainforest biome influenced the genotypic variability of E. edulis. Forest fragmentation can lead to a drastic decrease in the size of tree populations and an increase in their spatial isolation, reducing their genetic diversity.

“Our research clearly showed a genetic differentiation between palms in areas with and without large birds,” Carvalho said. “We concluded that the extinction of large frugivores is changing the jussara palm’s evolution.”

Is there a correlation between this genetic difference and seed size? “We don’t yet know,” Galetti said. “We haven’t reached the stage of analyzing the genome of E. edulis to find out which genes are responsible for seed size variation. What we can say is that defaunation changes natural selection, whereby smaller seeds are dispersed, and also affects the plant’s genetics.”

Given everything learned so far, can the situation be reversed? Is it possible to guarantee that populations that have only small seeds will survive climate change?

The researchers are now planning to restore the genetic diversity of E. edulis and the variability of seed size where it has been impaired.

“In many natural areas, E. edulis may disappear as a result of climate change if we don’t intervene, because small seeds lose more water and don’t germinate,” Galetti said. “In other words, the seeds won’t germinate in hot, dry years.”

“In this new phase of the project we want to see how best to restore genetic variability and seed size in areas where the large seed dispersers have become extinct,” Carvalho said. “There are areas with large and small seeds. However, only the large seeds aren’t being dispersed, owing to the absence of larger birds. There are also areas where large seeds have already disappeared. So we’re investigating whether simple reintroduction of large birds is sufficient to guarantee full recovery of jussara palm seeds or whether we need other, more effective restoration strategies.”

“Without the jussara palm the Atlantic Rainforest will become impoverished, because the jussara palm feeds the main seed dispersers in the forest,” Galetti said. “When I delivered a presentation on this problem to farmers and people who produce seedlings of E. edulis in nurseries, they told me that from now on they’re going to select larger seeds and produce seedlings from them.”

The study of the ecology of E. edulis occupies a central place in Galetti’s scientific career. “I began studying seed dispersal while I was an undergraduate in 1986, with a scholarship from FAPESP,” he said. “I investigated which birds dispersed and fed on jussara palm seeds. That was the basis for all our later studies. We have a solid natural history foundation on the interaction between frugivores and E. edulis and we can identify the best dispersers of its seeds with great confidence.”

Articles:

“Climatic stability and contemporary human impacts affect the genetic diversity and conservation status of a tropical palm in the Atlantic Forest of Brazil” (doi: 10.1007/s10592-016-0921-7) by Carolina da Silva Carvalho, Liliana Ballesteros-Mejia, Milton Cezar Ribeiro, Marina Corrêa Côrtes, Alesandro Souza Santos and Rosane Garcia Collevatti: link.springer.com/article/10.1007/s10592-016-0921-7.

“Defaunation leads to microevolutionary changes in a tropical palm” (doi:10.1038/srep31957) by Carolina S. Carvalho, Mauro Galetti, Rosane G. Colevatti and Pedro Jordano: nature.com/articles/srep31957.

“Functional extinction of birds drives rapid evolutionary changes in seed size” (doi: 10.1126/science.1233774) by Mauro Galetti, Roger Guevara, Marina C. Côrtes, Rodrigo Fadini, Sandro Von Matter, Abraão B. Leite, Fábio Labecca, Thiago Ribeiro, Carolina S. Carvalho, Rosane G. Collevatti, Mathias M. Pires, Paulo R. Guimarães Jr., Pedro H. Brancalion, Milton C. Ribeiro and Pedro Jordano, 2013: science.sciencemag.org/content/340/6136/1086.