Could apex predators limit the seed dispersal of fleshy-fruit plants? A rewilding scenario involving mammal carnivores.

This blog post is provided by Tamara Burgos and tells the #StoryBehindThePaper for the paper “Predation risk can modify the foraging behaviour of frugivorous carnivores: implications of rewilding apex predators in plant-animal mutualisms”, which was recently published in the Journal of Animal Ecology.

Tamara Burgos is carrying out her PhD in Ecology at the University Rey Juan Carlos, Madrid, Spain. Her research interests focus on trophic cascades and the ecological effects of apex predator reintroductions on key ecosystem functions which carnivores are involved in, such as seed dispersal. Her fascination by the world of mammal carnivores have led Tamara Burgos to be involved in several conservation projects where the main goal was the study of Iberian lynx (Lynx pardinus) populations in Spain. This feline is one of the most endangered in the world and it was near extinction at the end of the 20th century. Nowadays, lynx populations are recovering largely thanks to conservation and reintroduction programs where several stakeholders are involved, from private landowners to governments.

The Iberian lynx, like other apex predators such as bears and wolves, is known for controlling the abundances of smaller mesopredators, such as foxes or Egyptian mongooses and altering their behaviour and activity patterns. For this reason, cascading effects triggered by the rewilding process of apex predators worldwide became the main focus of Tamara’s research.

Iberian lynx (Lynx pardinus) just released into the wild in Central Spain with a monitoring collar via GPS. Credit: Tamara Burgos.

William J. Ripple and Robert L. Beschta planted the seed, studying these cascade effects from wolves to primary producers in Yellowstone National Park, after the reintroduction of the grey wolf in the 90’s. New research studies were appearing progressively involving different trophic levels in the study of trophic cascades, however nobody thought of mesocarnivores as potential direct controllers of plant populations and vegetation structure.

During her Master thesis on the effects of releasing apex predators on mesopredator and prey abundances, Tamara and her supervisor, Emilio Virgós realised that reintroductions of large predators could affect plants, not only via control of large herbivore populations but also via mesopredator suppression. Many medium-sized carnivores play a key role in ecosystem functioning as agents of seed dispersal, by consuming a great quantity and variety of fleshy-fruits. Foxes, badgers or martens have a generalist and opportunistic diet and they often feed on fruit, especially in ecosystems where this is an abundant resource. Therefore, if apex predators can control mesopredator abundances, this top-down effect could have indirect consequences for many fleshy-fruit plants whose seed dispersal depends mostly of mesocarnivores.

Red fox (Vulpes vulpes) feeding on Iberian pear fruits. Foxes are effective seed dispersers because they spread the seeds across large distances and bring them to vacant habitats, thus playing an important role in the recolonization of old or abandoned labour fields. Video recorded by camera-trapping. Credit: Tamara Burgos.

Meanwhile, Jose M. Fedriani studied the seed dispersal patterns of the Iberian pear (Pyrus bourgaeana) in Southern Spain and discovered that carnivore mammals were the main seed dispersers for this tree in Mediterranean ecosystems. Tamara and Emilio saw a perfect opportunity for collaboration and together they designed Tamara’s thesis project on trophic cascades and plant-disperser mutualisms, which she is currently working on.

The research team found the perfect place to develop this study: Sierra de Andújar Natural Park. After a long process of social dinners and bar meetings with hunters and landowners, Tamara and colleagues were allowed to carry out their research study on their lands. However, there were still some obstacles to overcome. Working with mutualist interactions where carnivores are involved can be challenging because these animals are elusive and difficult to observe when they are mainly interacting with plants. The best solution that the authors found was to design a natural experiment by using technology. Camera-traps are useful devices to study carnivores in many aspects of their biology such as behaviour or demography. Thus, the authors offered pear fruits beneath fruiting Iberian pear trees and analysed the foraging behaviour by the frugivorous carnivores recorded by camera-traps. They compared the number of visits, fruits consumed and feeding behaviour of individuals which coexisted with the Iberian lynx, and individuals which inhabited outside the lynx distribution range by selecting sites with lynx and sites where the lynx was extinct inside the Natural Park.

Small Iberian pear tree (Pyrus bourgaeana) probably dispersed by a badger (you can see the badger latrine beneath the tree). Credit: Tamara Burgos.

The study reveals a potential trophic cascade from an apex predator to a fleshy-fruit plant mostly dispersed by carnivores. They found that 70% and 100% of fox and stone marten visits, respectively, occurred at pear trees located outside Iberian lynx territories. Moreover, the study showed that foxes co-existing with lynx consumed 38% less fruit and were therefore less efficient frugivores. They consumed less fruit per unit of time and made shorter visits to pear trees, both behavioural features typically linked to an anti-predatory response. A larger competitor can easily predate foxes and martens and the predation risk perceived inside lynx ranges could lead them to use surrounding areas more intensively to avoid conflictive encounters with lynx.

Therefore, understanding the ecological interactions among the different levels of food webs is essential to design suitable conservation strategies and predict potential cascading effects in altered ecosystems. Rewilding programs should consider trophic cascades as a powerful mechanism, which can alter key ecosystem functions in contrasting ways. However, this is only the starting point and future research is necessary to shed more light on this issue, for what they hope will be more papers and research projects approaching cascading effects from reintroductions on mutualist interactions where carnivores are involved. 

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