Mammalian frugivores represent a major group of seed dispersers in tropical ecosystems. Large frugivores are especially important as they are the only ones able to disperse large-seeded plants. They also disperse seeds over long distances, which is important for facilitating genetic exchange among plant populations. The new study shows that the loss of large-bodied mammals can have severe consequences for tropical ecosystems, especially for plants that depend on large mammals for seed dispersal.
The extinction of large fruit eaters is a severe problem in tropical ecosystems. While extinctions nowadays accelerate due to human activities such as overhunting, deforestation and forest fragmentation, humans have also driven many large mammals extinct during their early history. For instance, many giant sloths and other elephant-like creatures, which have inhabited the rainforests of the past, disappeared at the end of the last Ice Age, probably due to human hunting or habitat modification.
‘In our study we mapped not only the distribution of present-day frugivores but also those species that went extinct in the last 130,000 years’, says Dr. Jun Ying Lim, the lead author of the study. ‘This allowed us to quantify the relationship between mammalian frugivores and palms for both the present and the past. We then used simulations to make projections into the future under different extinction scenarios, and predicted which mammals are likely to go extinct in the next 100 years and how this might affect palms’.
An important baseline of the study is that the researchers could demonstrate a relationship between the maximum body sizes of fruit-eating mammals and the sizes of the largest palm fruit across the world. ‘To quantify this, we compiled the necessary data on traits and species distributions for hundreds and thousands of mammals and palms over many years, mainly from existing databases, published articles, books, museum records, specialized websites and other scientific sources’, says Dr. W. Daniel Kissling, researcher at the UvA Institute for Biodiversity and Ecosystem Dynamics and senior author of the study.
Such global analyses with big datasets are only possible because generations of scientists have measured and collected data on palms and mammals all over the world. ‘We synthesize and integrate such measurements into large databases and use statistical models and simulations to infer ecological relationships and to detect global change’, continues Kissling.
An important finding of the study is that the relationship between palm fruit size and body size of fruit-eating mammals is strongest when considering the body size of present-day mammals only. This suggests that palm floras have adjusted their fruit sizes over thousands of years to fit the composition of present-day frugivore assemblages. However, one notable exception exists in the New World where several large-fruited palm species have persisted despite the extinction of many large-bodied seed dispersers tens of thousands of years ago.
‘The extinction of giant-sloths and other megafauna, and with them their seed dispersal functions, seem to be partly compensated by smaller mammals such as agoutis, which can play an important role in secondary seed dispersal’, says Lim. ‘This suggest that conservation efforts should also consider secondary seed dispersers as they are crucial for the survival of these palms.'
The study uses the global frugivore-fruit size relationship of the present day together with scenarios of mammal extinctions to simulate the potential future impact of the extinction of animal seed dispersers on palms. ‘Our model suggests that fruit sizes of palm assemblages would have to decrease by about 4% to maintain the global frugivore-fruit size relationship over the next 100 years’, says Lim. He adds ‘this seems modest, but our estimates are at the level of whole palm assemblages rather than predictions for individual palm species’.
The authors argue that the impact of future defaunation will be greatest on palms with the biggest fruits because small fruited palms are likely to be dispersed by a wider range of animals compared to large fruited palms. ‘Together with other evidence our results suggest that large-fruited palms and similar other tropical plants will face severe problems in the future’, warns Kissling. He thinks dispersal limitation caused by megafauna extinctions will reduce gene flow among plant populations and ultimately lead to the co-extinction of plant species, even if the final disappearance of trees can take some time due to their long generation times. ‘We must put more focus on the conservation and restoration of large-bodied frugivores and their interactions because they are essential for keeping the functioning of tropical ecosystems alive’, concludes Kissling.
‘One of the major avenues of research that I hope to pursue is to develop better mechanistic models of seed dispersal and forest regeneration, which will allow us to better predict species-level responses and ecosystem change caused by the loss of animal dispersers’, says Lim. Driven by the findings of this study and after his postdoctoral stint at the University of Amsterdam, he now conducts research in Southeast Asia, where many forests already have a depauperate fauna and thus change in structure and functioning due to the loss and reduction of animal seed dispersal.
Lim, J.Y., Svenning, J.-C., Göldel, B., Faurby, S. & Kissling, W.D. (2020): Frugivore-fruit size relationships between palms and mammals reveal past and future defaunation impacts. Nature Communications, 29 September 2020. DOI: 10.1038/s41467-020-18530-5