Invasive species present one of the biggest challenges for conservation biologists, with many endemic species threatened by the arrival of invasive predators or competitors. Such organisms typically arrive in an area where local species are naïve to their behaviour, and undergo an explosive ecological release, being freed from their own predators and pathogens whilst at the same time able to exploit local prey which have no defences against them. Species on oceanic islands are often particularly vulnerable to the arrival of invasive predators, and are also often home to highly endemic species, not found anywhere else, which means that the arrival of a novel predator on such an island can quite often lead to a cascade of extinctions. Among Vertebrates, island-dwelling Bird species appear to be particularly vulnerable to such effects, with widespread loss of native Bird species across island systems such as Hawai’i, New Zealand, and the Mascarenes.
However, where species are not completely wiped out, they can respond positively to predator control programs, with the recovery of Bird species recorded on many small islands where invasive Mammal species have been removed. Such programs are difficult to run on larger islands however, and in such cases species recovery is often limited to fenced-off areas where the predators have been removed, although the presence of wild populations in such areas greatly increases the availability of recruits to repopulate the rest of the island, should this later become possible.
The predatory Brown Treesnake, Boiga irregularis, was introduced to Guam, an island in the Mariana Archipelago, shortly after the end of World War II, since which time nine of the island's eleven indigenous Bird species have been wiped out. By the early 1990s, there were an estimated 5000-10 000 Brown Treesnakes per square kilometre on Guam, with 1-2 million thought to be living on island in total. Despite this, some Birds have managed to survive on the island, notably the endangered Yåyaguak (or Mariana Swiftlet), Aerodramus bartschi, which is a cave-roosting Bird, difficult for the Snakes to attack, and the locally endangered Såli (or Micronesian Starling), Aplonis opaca, a cavity-nesting Bird found across the Marianas. The omnivorous Såli is an important seed-disperser, and therefore critical to the ecology of the island, where it was formerly found in all habitats, but underwent a catastrophic decline after the introduction of the Brown Treesnake, with the last recorded survey of the species, carried out in the early 1990s, finding only 60-120 Birds remaining on Guam, almost all of which resided on Andersen Air Force Base on the northeast of the island.
Despite a lack of formal assessment since this time, the Såli is known to still be present on Guam, with the population around Andersen Air Force Base thought to have expanded due to Snake control measures. The species is known to be suffering high fledgeling mortality, again due to the Snakes, but has recently been seen in urban areas in northern and central Guam, where it has not been seen since the 1980s, leading to hopes that these Birds may be expanding back into parts of their former range, despite a lack of Snake control.
In a paper published in the journal Bird Conservation International on 1 March 2021, Henry Pollock of the School of Global Environmental Sustainability at Colorado State University, Martin Kastner of the Department of Ecology, Evolution, and Organismal Biology at Iowa State University, Gary Wiles of Olympia in Washington State, Hugo Thierry, also of the Department of Ecology, Evolution, and Organismal Biology at Iowa State University, Laura Barhart Dueñas of the Division of Aquatic and Wildlife Resources at the Guam Department of Agriculture, Eben Paxton of the Pacific Island Ecosystems Research Center of the U.S. Geological Survey. Nicole Suckow, also of the School of Global Environmental Sustainability at Colorado State University, Jeff Quitugua also of the Division of Aquatic and Wildlife Resources at the Guam Department of Agriculture, and Haldre Rogers, again of the Department of Ecology, Evolution, and Organismal Biology at Iowa State University, present the results of a study of an island-wide survey of the distribution and abundance of the Såli on Guam,
At 541 km², Guam is the largest island in Micronesia, as well as the most populated, with a population of about 160 000 people in 2010, and the most developed, with about 20% of the . The northern part of the island is covered by a limestone plateau, with karst forest, and the majority of the island's Human population and urbanised areas, while the southern part is dominated by volcanic geological features, with areas of savanna and ravine forest, and a more sparse Human population.
Previous studies have established that Såli roosting on Andersen Air Force Base range widely throughout the forested areas along the eastern and southern perimeter of the base during the day, but return to the developed area around 3.00 pm, where they are relatively sedentary and easier to count. In 2017-8 over 350 Birds from the Air Force Base population were colour-banded, providing a basis for future population estimates based upon resighting of these Birds; a number of Birds were also radio-tagged, all of which used forest extensively and travelled off-base but returned to the core roosting area at night.
Pollock et al. divided the main developed area of Andersen Air Force Base into 28 search areas of roughly similar size, comprising three habitat types: urban, residential housing, and golf course. Every day two areas were chosen at random, and each of these was observed by two fieldworkers, who traversed the search area together, to increase overall detection probability and the accuracy of colour-band identifications. Adjacent search areas were never searched on the same day, to minimise the risk of double-counting Birds. Observers also remained in constant contact during surveys, and communicated movements of any Birds throughout a given search area. All Birds detected in the search areas were logged, along with data on their ages and any banding information; the open nature of the landscape made it relatively easy to approach and visually observe Birds detected by sound.
In order to study the distribution of the Såli away from Andersen Air Force Base, Pollock et al. combined data from transect surveys with recorded opportunistic sightings of the Birds. They excluded the small, but stable, population of Såli known to be present on the nearby island of Cocos. Three data sources were consulted for data on opportunistic sightings, the eBird website, which enables members of the public to record Bird sightings anywhere in the world, a database if sightings maintained by the Guam Division of Aquatic and Wildlife Resources, and a personal database maintained by Martin Kastner. All of the sightings in the Division of Aquatic and Wildlife Resources database were confirmed by biologists from the department, and all of the sightings recorded in Martin Kastner's database were made by scientists familiar with the species.
A total of 46 transect surveys were carried out in April–May 2018. Each transect route was visited once, with 19 of the routes having been previously used in a Division of Aquatic and Wildlife Resources survey in 1985, ten surveys were carried out in rural areas with little development (two northern, three central, and five southern), and nine in suburban areas within a kilometre of forest (five northern and four central). The transects were an average of 5189 m in length, with ten-minute surveys being carried out at ten points spaced roughly evenly along these lines. A further 27 transects were carried out in areas where sightings of Såli had recently been recorded by the Division of Aquatic and Wildlife Resources; these surveys were only 500 m in length, and did not overlap with the longer surveys. All but one of these short surveys were carried out within 500 m of an area of forest, with surveys carried out at six points roughly 100 m apart on the transect.
Pollock et al. found 16 previous studies that mentioned either the abundance or distribution of Såli on Guam published between 1901 and 1995. Twelve of these were published prior to 1970, and invariably described the Såli as very common. By 1978–1979 the species was rare on the southern part of the island, and uncommon on the northern and central parts. The first attempt at assessing the population of the Birds on Guam was carried out in 1981, and counted 1667 Såli in a series of surveys, which was extrapolated to a total of 15 132–18 602 Birds on the island. By this time Såli were completely absent from the southern part of the island, and in the central part, only a single small population was found, around the village of Hagåtña. This survey only found an estimated 231 individuals living at and around Andersen Air Force Base.
Subsequent surveys found almost no Såli on the island, and by the early 1990s it was estimated that only 60–120 Birds remained, including 50–100 living on Andersen Air Force Base, and the nearby areas of Mt. Santa Rosa and Gayinero, Yigo. Smaller groups of Birds, totalling no more than five individuals, were present at the Conventional Weapons Storage Area (now called ‘Munitions Storage Area’), and Naval Computer and Telecommunications Area Master Station (now called ‘Naval Base Guam Telecommunications Site’), as well as a scattering of solitary Birds along the southern coast.
During their three week-long surveys of Såli around Andersen Air Force Base, Pollock et al. counted 683, 609, and 844 Birds, respectively. However, in forests along the southern and eastern peripheries of the base they only counted 3–6 Birds each week. The Såli appeared to be concentrated towards the centre of the base’s main developed area, with less Birds in peripheral search areas adjacent to forest edge. Less than 5% of the Birds counted were banded, including 42 unique individuals, with 13 re-sightings of Birds seen in week one in week 2, and four re-sightings of Birds seen in weeks one or two in week 3. Extrapolating from this, Pollock et al. conclude about 50 ringed Birds remain on Andersen Air Force Base, out of a total population of about 1391, 91.1% of which are adults and subadults.
Pollock et al. compiled records of sightings of Såli at 64 unique locations across Guam from 2005 to 2019, these were largely concentrated in villages of northern and central Guam, with a few sightings around the southernmost tip of the island. A total of 64 sightings were recorded in 12 of the island's 19 villages, representing 156 Birds. The Birds were more common in urbanised areas, including the island’s main business districts (particularly at large malls and shopping centres), as well as urban parks and residential areas. No Birds were sighted more than 2 km from a built-up area or major road. The Birds were most frequently seen perched on power lines, power poles, buildings, and trees. Ten nests were observed outside the Air Base, all on lamp posts or power poles.
Duromg the transect surveys, Pollock et al. made 91 observations of Såli on 20 of the 29 surveys. All sightings were on the northern and central parts of the island, with the majority around the Air Force Base and the island of Yigo. No Såli were detected in the southern villages of Merizo or Umatac, despite these being the closest points to the island of Cocos, with its own population of the Birds.
Based upon the results of the sightings records and survey results, Pollock conclude there are currently around 1450–1490 Såli living on the island of Guam. This includes 30–40 individuals living around the villages of Yigo and Dededo on northern Guam, 20–30 Birds in Hagåtña, and 10–20 in Tamuning-Tumon-Harmon, in the central part of the island, as well as up to ten further Birds living outside of these areas; the remainder of the population being resident at Andersen Air Force Base.
Pollock et al. have produced the first update on the distribution and abundance of Såli on Guam since the 1990s. They record a 15-fold increase in population size since the last population survey took place, with the population up from about 100 to about 1500, although 93-96% of the population is concentrated at a single location, Andersen Air Force Base. Despite this uneven distribution, the Såli do appear to be in the process of recolonising urbanised areas elsewhere in northern and central Guam, with a few isolated Birds being seen on southern parts of the island.
Away from Andersen Air Force Base almost all sightings of Såli occur in urbanised areas, with the overwhelming majority of such sightings. It is, however, unclear how well established the Birds are in areas away from the Base, as while the majority of these sightings were of pairs or small groups of Birds, indicating a potential for breeding, very few nests or juveniles were seen. This is further complicated by the fact that, apart from at Hagåtña, almost all of these sightings occurred at shopping malls, which might indicate that the (highly mobile) Birds are exploiting a new food source rather than settling in these areas.
For any Bird to survive on Guam today, they need to be able to avoid Brown Treesnakes. This means there are two major factors likely to influence the long-term survival of the Såli on Guam; Snake control measures, and the Birds increasing adaptation to urban areas. Snake control measures have been in place at Andersen Air Force Base since 1993, with thousands of Snakes being removed from the site each year, which has clearly benefitted the Såli. Similar Snake eradication programs are in place at other military installations on the island, but these sites are all much smaller, and do not seem to have created suitable safe environments.
Brown Treesnakes avoid brightly lit area, brightly lit areas, and open spaces such as lawns and car parks, and urbanised areas seem to have become refugia for the Såli on Guam. Recorded nesting by the Birds occurs in solitary trees, building cavities, lamp posts and artificial nest boxes.
Snake eradication programs on Guam are largely restricted to military instillations, and Guam International Airport, and although other urban areas clearly present some refuge from the Snakes, most also include patches of vegetation, where the Snakes are found. Most urban areas are known to have fairly high Snake populations, with the largest Snakes often found in such areas, where there is an abundance of prey. However, even the limited protection presented by these areas appears to be beneficial, with the Birds apparently becoming re-established here.
Whatever the current successes of the Såli, the Snake control measures currently present on Guam are unlikely to protect the species in the long run, as these are intended to protect the island's electrical infrastructure and prevent the spread of the Snakes to new islands, rather than to eradicate them. The number of Snakes captured at Andersen Air Force Base has remained steady since the 1990s, indicating the overall population of Snakes has been uneffected, and a constant supply of new recruits exists to replace any Snakes removed. In addition, studies have shown that, even on Andersen Air Force Base, very few fledgling Såli survive to adulthood, due to predation by both Brown Treesnakes and Domestic Cats, leaving the Bird population unusually skewed towards older Birds. The Såli have failed to recolonise the extensive suburban areas on the east-central Guam, despite the high Human population here. The Birds are also largely absent from the sparsely-populated central and southern parts of the island. This strongly suggests that the Birds will be unable to repopulate the island properly without a more extensive Snake control program being implemented.
One action that has clearly proven beneficial to the Såli has been the provision of nest boxes. These provide additional nesting opportunities for cavity-nesting Birds such as the Såli, and can offer protection from both the elements and predators. Such boxes have been placed on Andersen Air Force Base since the 1990s, with at least 50 predator-resistant boxes in place at any one time since 2015, which is thought to have allowed the fledging of about 900 Birds. However, fledgelings still suffer very high predation rates from both Brown Treesnakes and Domestic Cats, and there are still more nests in natural cavities than in nest boxes, so it is unclear how much of an impact this program has on the overall population.
The survival of the Såli is considered essential for ecosystem functioning on Guam. These omnivorous Birds are the only surviving native frugivores on the island, and as such are vital for the distribution of the seeds of many Plants, and subsequently the ability of the indigenous forests to regenerate. The ability of these Birds to survive in urban areas is beneficial for the species itself, but clearly of limited value to the island's forests. Thus, plans for rewilding efforts on Guam will require extensive application of Snake-control measures if they are to resume their natural ecological function.
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