By Bethan Henderson, Freelance Writer

Published June 5th, 2022

In the 2010s, the locally extinct brown howler monkey (Alouatta guariba) was reintroduced to the Tijuca National Park in Rio de Janeiro, Brazil. This reintroduction demonstrated the importance of primates in seed dispersal and how the absence or presence of a single species has the potential to reshape an entire ecosystem.

Ecosystems are made up of populations of different species, intrinsically interconnected and affected by their interactions with one another. These trophic interactions may be negative, such as predation, or they may be beneficial and mutualistic, such as pollination. The mass species loss that we are experiencing during the anthropocene threatens many of these ecological interactions that are critical for a healthy and functioning ecosystem.

Therefore, when it comes to restoring ecosystems, it is important to consider restoring ecological interactions alongside reintroducing lost species. This process is called trophic rewilding—when animal species are reintroduced to restore these lost interactions.

A famous example of this is the reintroduction of wolves to Yellowstone National Park, an event that triggered a trophic cascade. The reintroduced wolves predated overpopulated elk, which allowed for overgrazed woodlands to recover, in turn bringing back species such as beavers. The beavers and the new tree growth altered the morphology of rivers and streams. Therefore, the reintroduction of a single species altered the physical landscape of Yellowstone, not just its wildlife.

The image of a predator chasing its prey is probably the first image that comes to mind when thinking of ecosystem interactions, however, plant-animal interactions are just as important. Seed dispersal is one of these processes, and is a vital ecological interaction that spans multiple species and trophic levels.

Seed dispersal

Seed dispersal is the transportation of seeds to new locations where they germinate, and is a process at risk because of the vulnerability of larger frugivores in many regions. Large frugivores consume larger fruit and therefore disperse large seeds that smaller frugivores cannot.

As small frugivores cannot fill the same niche as large frugivores, if the latter goes locally extinct, they need to be reintroduced in order to maintain this ecological interaction. Primates make up a large proportion of the large frugivore biomass in tropical forests, but are heavily impacted by hunting and habitat degradation.

If primate populations are lost, the consequential loss of seed dispersal would instigate ‘empty forest’ syndrome, where piles of fruit and seeds rot on the forest floor and large seeds rarely become trees. Plant recruitment and the composition of plant communities would be altered.

A study on howler reintroduction and seed dispersal

Tijuca National Park, nested within the Atlantic forest, has experienced the loss of large frugivores, namely the brown howler monkey. It was the site of a reintroduction of brown howler monkeys in 2010, whose aim was to restore and study seed dispersal processes.

Howler monkeys are folivore-frugivores (leaf-eating and fruit-eating) that swallow seeds whole, depositing them in their faeces. The faeces attract dung beetles, which unknowingly act as secondary dispersers, rolling the seed-filled droppings away and burying them for their young. This ecological interaction allows the seeds to sprout from the safety of deeper soil, less vulnerable to seed predation.

Howler monkey faeces are more attractive to dung beetles compared to the faeces of other frugivores because howlers defecate in groups, producing larger, more concentrated droppings. Dung beetle groups that are attracted to these droppings are also more diverse, and include larger dung beetle species that bury seeds deeper. Another side benefit of howler monkey seed dispersal is that their gut biome improves seed germination.

‘Howler monkey faeces are more attractive to dung beetles compared to the faeces of other frugivores.’

Therefore, the loss of the brown howler monkey directly threatens coprophagous dung beetles and the plants reliant on their secondary seed dispersal. A study by Landim and colleagues (2022) on the howler reintroduction tracked and compared seeds from the faeces of three primary seed dispersers during secondary dispersal by dung beetles. The primary seed dispersers included the brown howler monkey, the capuchin (Sapajus nigritus), and the rusty-margined guan (Penelope superciliaris).

The study found that howlers increased the number of large-seeded plants in the environment, because seeds were 40% more likely to be secondarily dispersed by dung beetles. Larger seeds also survived better.

Another factor is that the gut of the howler monkey benefits seed germination, whereas ingestion from species like the capuchin leads to 66% of seeds being nonviable.

Why does this matter?

Howler monkeys play a unique role in ensuring that large-seeded plants endure in the Atlantic forest. Without them, seeds would rot and dung beetle communities would be altered, affecting bioturbation and nutrient cycling. The lack of new saplings would affect other flora species and damage the carbon-storage capacity of tropical forests.

General extinctions represent issues that span ecosystems, which means they need a more holistic solution. The study results indicate that single species reintroductions can contribute to restoring ecological processes, something that actually has been evaluated very little.

While there are drawbacks to trophic rewilding and reintroduction and risks that need to be considered (such as scarcity of source populations, monitoring wildlife, and obtaining licences), they are relatively low-cost, effective ways to restore ecological functions in defaunated neotropical forests.

Conservation traditionally focuses on either animals or plants. Therefore, we need approaches that are complementary and consider the nuanced role of species. Worryingly, as populations of both are affected by environmental changes, their interactions could go extinct before any species extinctions actually occur.

As put by the authors of this study, ‘In order to restore ecological processes, one must look beyond interactions and consider the role of the species in the community in which it will be reintroduced.’

Featured Image: Miguel Rangel Jr. | Flickr

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