Can birds assist the regeneration of tropical invaded forests?

By Dr Alba Costa, a researcher working on the effects of restoration ecology and biological invasions on plant-animal species interactions with Dr Chris Kaiser-Bunbury, Senior Lecturer in Conservation and Ecology at the University of Exeter

As a conservation scientist, my goal has always been to have a meaningful impact, even if small, in the fight against the degradation of the planet caused by humans. This is why I find it fascinating to study how entire ecological communities respond to human activities and to find the best way to restore them. Below I summarise a study that we recently published in Restoration Ecology, and include information on the frequently overlooked intense fieldwork experience behind these type of studies.

A major gap in ecological restoration monitoring

We know that the negative impacts of biological invasions on ecosystems are increasing at unprecedented rates worldwide, calling for urgent action to develop appropriate management strategies for the restoration of invaded areas. Invasive non-native plant species, for instance, can lead to water loss, changes in native plant diversity by competition for resources (e.g. light, nutrients and space) and modify species interactions and the functioning of ecosystems (e.g. herbivory, pollination, seed-dispersal). Ecological restoration, however, is frequently monitored at a species or taxon level, and we know very little about its effects on ecosystem processes and functions. But which functions should we focus on and which ecosystems should we prioritise?

The importance of islands

Islands are in serious need of restoration practices focused on recovering entire communities and ecosystem functioning. These ecosystems are particularly vulnerable to the impacts of invasive alien species due to their isolation and the simplicity of their biota. Moreover, islands are of high priority for biodiversity conservation since they are home to high concentrations of rare species and unique interactions. More than 20% of all known terrestrial vertebrates and plant species are on islands! On the main Seychelles island of Mahé, this unique biodiversity finds refuge in tiny patches of native vegetation or forest remnants on granitic mountaintops (inselbergs), hosting around 63% of endemic plant species! Inselbergs are surrounded by steep cliffs and highly invaded forests (80%) in the lowlands, consisting mainly of cinnamon (Cinnamomum verum). This combination of native vegetation and invaded forests makes the island an ideal system to study questions at the cross-section of biotic invasions, restoration ecology and conservation biology.

The link between seed dispersal and restoration

One of the key functions for the reproduction of plants is the dispersal of seeds. Flowers need to be pollinated to produce seeds, and these seeds must then be transported to a suitable site for germination. Since most tropical plant species have fleshy fruits and rely on animals for dispersal, the movement and behaviour of these animals are decisive for the persistence of native plant communities. Frugivorous birds can connect forest fragments and surrounding areas by transferring seeds between them. For instance, if frugivores transport more non-native seeds from invaded areas to native forest remnants and these propagules germinate and survive, these precious native forests would become invaded over time. If the opposite is true and there is a higher flux of native seeds from inselberg patches to the surrounding areas, forest remnants could facilitate native forest regeneration. So how did we test whether remnant forests on inselbergs could support natural recovery of the surrounding highly invaded forests?

Cliff-top mist-netting and assessing which way seeds are going: the truth

Recording how birds move around the landscape and in which direction seeds are transported is a challenge, not to mention assessing the fate of seeds after dispersal! To overcome this, we set up a large-scale experiment across eight inselbergs with different levels of plant invasion and caught birds in mist-nets placed parallel to the cliff edges (Fig. 1 & 2b). For each bird trapped we recorded whether it was arriving to or departing from the inselberg, and identified and counted the seeds in their droppings. We also recorded the number of seedlings and saplings both on and away from each inselberg.

Figure 1: Experimental setup. a) Experimental design used to record the direction of seed dispersal from and towards the Seychelles’ inselbergs. b) Study sites on Mahé. c) Bird’s-eye view of a study site: thick black line represents the boundary between the inselberg (native forest remnant) and the surrounding invaded forest.

It sounds quite simple right? Behind the scenes, this kind of fieldwork involved months of getting up at 3:45 am, driving to the site and pick up field assistants, climbing up the mountain in the dark with heavy equipment, rushing around the inselbergs to set up 8 mist-nets before dawn. Then, running between nets which needed checking every 20 minutes while conducting other experiments such as fruits and frugivore counts, reptile trapping, camera-trapping and faecal sample collection for around seven hours. And this in constant fear of rain coming and ruining all the effort invested. Granite on inselbergs is sharp and becomes slippery and dangerous when wet, and mist-nets need to be closed and removed quickly for the safety of birds! After sampling, the day wasn’t over; I had to do everything done in the morning, in reverse, plus process samples and data and prepare equipment once home. After a year doing this, several pairs of trousers ripped from falling, around 10 kg of weight loss, a lightning strike, a severe bout of dengue fever, and more than questionable mental health, I am finally able to write this, which is a good sign. Phew! This was by far the most challenging year of my life, and hopefully, will remain so! But was it all worth it? Let’s see what we found!

Figure 2 Fieldwork pictures a) View of an inselberg b) Mist-net on the inselberg boundary c) Me thinking in the field d) Indian myna bird (Acridotheres tristis) trapped in mist-net.

Are inselbergs ‘sources’ or ‘sinks’ of native plant seeds?

Six species of birds transported seeds of 18 plant species, of which only two were non-native (C. verum and Clidemia hirta). Birds clearly loved eating these two non-native species, since more non-native seeds were transported in both directions (Fig. 3a), particularly of Clidemia (98% of all seeds transported), which have 300 tiny seeds per fruit.

Moreover, birds transported a higher number of droppings containing either native or non-native seeds (dispersal events) from the inselberg to the invaded matrix (Fig. 3b). These results imply that these plant communities can act as sources of native seeds for the surrounding degraded forest and are more important food source for frugivorous birds than invaded forest.

Figure 3: Total a) number of droppings with seeds and b) number of seeds (log-transformed) per site. Different lowercase letters represent significant (p < 0.05) differences.

Inselbergs as refuges for native plants

The density of non-native seedlings was higher outside, and of native seedlings inside the inselbergs (Fig. 4a). Moreover, native saplings grew much more densely than non-native saplings in both forest types (Fig. 4b). This is despite the high proportion of non-native seeds arriving from the surrounding forest and confirms that inselbergs are refuges for native plant populations.

Figure 4: The mean number of a) seedlings and b) saplings per m2 and site. Different lowercase letters represent significance (p < 0.05).

The verdict

This study is one of the first to reveal the role of fruit-eating birds in transporting seeds between native and invaded vegetation areas and identify their potential to facilitate native species regeneration in degraded forests. We show that preserving small native forest patches can be effective to foster the natural regeneration of surrounding invaded forests via frugivore birds. However, we should treat this message with caution, since native forests on inselbergs, are not immune to invasion and non-native seeds are also heavily dispersed in both directions. Therefore, it remains crucial that we continue to protect and restore these native plant communities, not only for their own immense conservation value, but also to preserve and enable their role in the natural regeneration of surrounding areas. I must say that, despite the many challenges, I feel immensely lucky to be part of a story that hopefully will bring us closer to understanding how we can optimise restoration practices to regenerate and preserve native biodiversity not only on islands but worldwide.

Article information: Restoration ecology, 15 February 2022,

Twitter: @alba_costa_

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