In this blog post, Tamara Burgos, a postdoc at University of Cádiz, presents her paper “Apex predators can structure ecosystems through trophic cascades: Linking the frugivorous behaviour and seed dispersal patterns of mesocarnivores”. Tamara explains how rewilding apex predators can affect key ecosystem functions such as seed dispersal and asks: why are mammal carnivores the main actors involved in this story?
About the paper and research
The case of the grey wolf (Canis lupus) reintroduction in Yellowstone National Park investigated by William J. Ripple and collaborators during more than 15 years has been a reference for my research. They revealed a tri-trophic cascade where wolves, elks and riparian trees interacted between each other in a landscape of fear. After the reintroduction of the grey wolf, elk populations decreased and riparian plants recovered and grow up in height. New research has been appearing progressively involving different trophic levels in the study of trophic cascades up to date, but the cascading effects revealed in our research have been novel for science.
The Iberian lynx (Lynx pardinus) is one of the most endangered feline species in the world and it was close to extinction in the 90’s decade. Nowadays, Iberian lynx populations are recovering thanks to conservation actions such as reintroductions or habitat management. Like it has been documented with other apex predators such as bears and wolves, the Iberian lynx can control the abundance and behaviour of smaller carnivores (i.e. mesopredators), such as foxes or martens. For this reason, I realised that the restoration of this top predator could trigger cascading effects on lower trophic levels and ecosystem functions where mesocarnivores are involved in, such as seed dispersal. But, are mammal carnivores actual seed dispersers?
In Mediterranean ecosystems of Southern Spain, mammal carnivores are generalist and opportunistic predators which often feed on fruit when other resources are scarce. Moreover, mammal carnivores can disperse viable seeds to long-distances and suitable environments for germination and establishment, playing a key role in forest expansion processes. Therefore, they are key seed dispersers for many plants, especially for large-fruited plants which have lost their main and legitimate seed dispersers in defaunated ecosystems.
One of my preferred Mediterranean landscapes offered the opportunity to assess the hypothesis that lynx recovery can trigger trophic cascades down to plants: Sierra de Andújar Natural Park in Southern Spain. The problem was that this Natural Park is formed by multitude of private lands, and we needed to get the permissions from tens of landowners to carry out the research. After a long process of social dinners and bar meetings with hunters and landowners, my colleagues and myself were allowed to work in these lands. However, there were still some obstacles to overcome. Working with wild mammal carnivores can be challenging because these are elusive and nocturnal animals difficult to observe, especially interacting with plants. The best solution that we found to this issue was to design a natural experiment by using a non-invasive sampling technique not suitable for all audiences: collecting faeces. Thus, we collected more than 1500 mesocarnivore faeces during two years and analysed their seed content at the laboratory. We compared the number of scats with seeds, the fruit biomass content and the diversity of the dispersed seeds between areas with lynx presence and lynx absence across the Natural Park.
The study reveals a novel trophic cascade from an apex predator to the whole fleshy-fruited plant community dispersed by carnivores. They found that the quantity and diversity of dispersed seeds by medium-sized carnivores drastically decreased in the presence of lynx. We found that this effect was stronger for the stone marten, with marked reductions in the number of carnivore scats with seeds (93% lower) and the diversity of seeds they dispersed (46% lower) within lynx territories. Moreover, the red fox dispersed 68% fewer seeds in open habitats when coexisting with lynx. This behavioural alteration is probably related to an anti-predatory response to avoid high-risk habitats, but can contribute to modify forest regeneration dynamics and landscape structure in the long-term.
The altered seed-dispersal patterns of mesocarnivores are probably related to their lower abundance and their anti-predatory behaviour in the presence of lynx. While restoring apex predators is certainly a successful conservation tool to save them from extinction, this study demonstrates that conservationists should pay attention to cascading impacts prior planning reintroduction programs of large predators. This is especially important in altered and defaunated ecosystems, where specialised frugivorous species are missing and medium-sized carnivores provide unique services to plants.
Although we have assessed trophic cascades at local scale, we consider that our study can have broadly application and we encourage rewilding programs to consider cascading impacts as powerful mechanisms which are able to alter key ecosystem functions in myriad ways. However, there are still big questions to answer, such as the ultimate effect of the reduced seed dispersal by mesocarnivores in rewilding scenarios on plant demography and survival. A reduction in the seed rain may be not translated to lower rates of seedling emergence and plant establishment, since the microsite of seed arrival is a key factor affecting plant survival. Hopefully, we can reveal some of this questions in the next story.
About the author
I am currently working as a post-doc researcher at University of Cádiz in Spain. My research interests focus on plant-animal interactions and the study of ecological functions involving carnivorous mammals. I am a passionate biologist broadly interested on the effects of global change on relevant ecosystem services such as seed dispersal. My fascination by the world of mammal carnivores since I was a child has led me to recently finish a PhD to understand the strength of trophic cascades under rewilding scenarios of apex predators.
Like the blog post? Read the research article here.
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