Såli (Micronesian starling –Aplonis opaca) as a key seed dispersal agent across a tropical archipelago

Abstract

Seed dispersal is an important ecological process that structures plant communities and influences ecosystem functioning. Loss of animal dispersers therefore poses a serious threat to forest ecosystems, particularly in the tropics where zoochory predominates. A prominent example is the near-total extinction of seed dispersers on the tropical island of Guam following the accidental introduction of the invasive brown tree snake (Boiga irregularis), negatively impacting seedling recruitment and forest regeneration. We investigated frugivory by a remnant population of Såli (Micronesian starling – Aplonis opaca) on Guam and two other island populations (Rota, Saipan) to evaluate their ecological role as a seed disperser in the Mariana archipelago. Using a combination of behavioural observations, nest contents and fecal samples, we documented frugivory of 37 plant species. Native plants comprised the majority (66%) of all species and 90% of all seeds identified in fecal and nest contents. Diet was highly similar across age classes and sampling years. In addition, plant species consumed by Såli comprised 88% of bird-dispersed adult trees and 54% of all adult trees in long-term forest monitoring plots, demonstrating the Såli’s broad diet and potential for restoring native forests. Overall, we provide the most comprehensive assessment to date of frugivory by the Såli and confirm its importance as a seed disperser on Guam and throughout the Marianas.

Functional robustness of seed dispersal by a remnant frugivore population on a defaunated tropical island

“…There was a consistent presence of zoochorous seedlings under perch and roost trees at the site with Såli present, whereas seedlings were very scarce at the site without Såli. All 15 species of seedlings found at AAFB have been previously documented in the Såli's diet (Pollock et al, 2020), strengthening the evidence that they were the primary dispersers, and that they are dispersing at least some viable seeds.…”

“…Various predator control devices, such as traps, bait tubes and aerial bait delivery , may soon allow snake suppression throughout parts of the landscape, and predatorresistant nest boxes have been deployed successfully within the species' range (Savidge et al, 2018). Therefore, conditions on the island are ripe for rewilding (sensu Seddon et al, 2014) (Pollock et al, 2020). However, to build resilience within this system it will be important to strive toward the reintroduction of a full complement of native dispersers (Fricke et al, 2018).…”

“…However, a remnant population of about 1400 birds of a single frugivorous species (Såli or Micronesian starling, Aplonis opaca ) persists in northern Guam, concentrated mainly within the urbanized landscape of a military installation. The Såli's broad diet (Craig, 1996; Jenkins, 1983; Pollock et al, 2020) and frequent movements between habitat types (Rehm et al, 2018) make it a good candidate for restoring seed dispersal across the landscape.…”

“…Therefore, conditions on the island are ripe for rewilding ( sensu Seddon et al, 2014) through the restoration of seed dispersal by Såli. Even a single effective seed disperser can support a large proportion of interactions within a tropical forest ecosystem (Kaiser‐Bunbury et al, 2010), and Såli have been recorded as dispersing at least 88% of zoochorous species found in forest plots on Guam (Pollock et al, 2020). However, to build resilience within this system it will be important to strive toward the reintroduction of a full complement of native dispersers (Fricke et al, 2018).…”

Recent recovery and expansion of Guam’s locally endangered Såli (Micronesian Starling) Aplonis opaca population in the presence of the invasive brown treesnake

“…Nevertheless, some bird species have managed to persist in the presence of the brown treesnake (Wiles et al 2003), including the endangered Ya ˚yaguak (Mariana Swiftlet) Aerodramus bartschi (Apodidae) which roosts in caves that may be relatively inaccessible to snakes, and the locally endangered Sa ˚li (Micronesian Starling) Aplonis opaca (Sturnidae). The Sa ˚li is a cavity-nesting omnivore and important seed disperser in the Marianas (Rehm et al 2017, Pollock et al 2020, with a broad geographic distribution across much of Micronesia (Craig and Feare 2018). Although historically common-to-abundant throughout Guam across all habitat types (Jenkins 1983, Craig andFeare 2018), the Sa ˚li's distribution and abundance on Guam declined precipitously along with the rest of the avifauna after the introduction of the brown treesnake (Savidge 1987, Wiles et al 2003.…”