The Tahiti Reed-warbler (Acrocephalus caffer) is endemic to Tahiti in the Society Islands, French Polynesia. The species is absent from the Tahiti peninsula (Tahiti Iti) and has been rare and local throughout the 20th century, being recorded in 19 valleys during 2017-2018 (Lazzari et al. 2018). It occurs in bamboo thickets and second growth forests in river valleys and hillsides to 700 m and feeds on insects as well as other small animals and nectar (Pratt et al. 1987, Thibault 1988).
The main threats to the species are habitat degradation and loss through the development of roads and tracks (P. Raust in litt. 2007) bamboo exploitation and the invasion of the plant Miconia, and the presence of invasive species including Common Myna Acridotheres tristis, and the Red-vented Bulbul Pycnonotus cafer (Thibault 1988, Seitre and Seitre 1991, Lazzari et al. 2018).
The current spread of Little Fire Ant Wasmannia auropunctata over the past 10-15 years in several of the Tahiti Reed-warbler’s range valleys, including Papenoo valley where the largest population concentration is found, may also threaten the species (Bouysseroux et al. 2018, SOP-Manu 2019, C. Blanvillain in litt. 2020). Tahiti Reed-warbler was first officially recorded in 2004, but it is likely to have been present on the island at least a decade earlier (Bossin and Padovani 2010, Bousseyroux et al. 2018) and it continues to expand its range on the island (SPREP 2014). Studies have shown that colonisation of areas of Tahiti by Little Fire Ants has led to a reduction in invertebrate diversity and may have caused Tahiti Monarchs to abandon their territories (Bouysseroux et al. 2018, SOP-Manu 2019).
Tahiti Reed-warbler has a small range and population size, and it was previously thought to declining, so the species is currently listed as Endangered. However, recent surveys across 39 valleys appeared to show a range expansion (SOP-Manu 2019). The use of a different survey methodology from that of the previous survey meant that the population trend was uncertain and it was precautionarily hypothesised to be stable (Lazzari et al. 2018, SOP-Manu 2019). Hence, we are undertaking a review of the species’s Red List Category:
Criterion A – Surveys have been carried out in 39 valleys during 1986-1991 (Monnet et al. 1993) and during 2017-2018 (Lazzari et al. 2018, SOP-Manu 2019). The 1986-1991 surveys recorded the species in 12 valleys and estimated the population to number a few hundred individuals (Monnet et al. 1993). The 2017-2018 surveys detected it in 19 valleys, including several valleys where it was previously reported to be absent, and estimated a population size of at least 372 adults (Lazzari et al. 2018, SOP-Manu 2019). Due to access limitations, the 2017-2018 survey did not cover one of the valleys previously surveyed, but all resurveyed valleys that were previously occupied were still occupied (SOP-Manu 2019). The 2017-2018 survey appeared to show a territorial expansion, but differences in survey methodologies, including the use of playback in the later survey, mean that the trend is uncertain and is precautionarily hypothesised to be stable (SOP-Manu 2019). Since there is no evidence that a reduction in population size has taken place over the past ten years (one generation length has been estimated at 2.8 years; Bird et al. 2020*), or that a reduction approaching 30% will occur over the next ten years, the species is assessed as Least Concern under Criterion A.
Criterion B – The Extent of Occurrence (EOO) is inferred to be 420 km2, based on a minimum convex polygon around the mapped range. This meets the initial threshold for Endangered under Criterion B1.Based on a 2 km x 2 km grid placed over the mapped range, the maximum AOO is inferred to be 456 km2. Since this figure is larger than the EOO, the figure is adjusted downwards to match the EOO, and is also placed at 420 km2. This meets the initial threshold for Endangered under Criterion B2. To list the species as threatened under Criterion B, two of conditions a-c must also be met.
The species is not severely fragmented. According to the IUCN Red List Categories and Criteria, ‘the term ‘location’ defines a geographically or ecologically distinct area in which a single threatening event can rapidly affect all individuals of the taxon present’ (IUCN 2012). The main threats to the species are habitat degradation and destruction through infrastructure development and the exploitation of bamboo, and invasive species such as the continued range expansion of the Little Fire Ant. None of these threats are likely to rapidly impact all individuals, and the number of locations is likely to be at least equal to the number of valleys where the species is present, and probably more. Therefore, condition a is not met.
As described under Criterion A, there is no evidence that the population size or range are undergoing a continuing decline. There may be a continuing decline in the quality or extent of habitat due to degradation caused by infrastructure development and the exploitation of bamboo, and due to the continued expansion of the range of the Little Fire Ant. However, since data has not yet indicated a decline in the population size of Tahiti Reed-warbler, it is not known whether these threats are causing true declines in the quality or extent of the habitat. Unless it is considered that these threats are affecting the habitat such that its population size will be affected, condition b is not met.
There is no evidence that the species’s population or range size are undergoing extreme fluctuations. Condition c is not met.
Although the EOO and AOO both fall beneath the initial thresholds for listing the species as Endangered under Criterion B, only one of the three conditions is possibly met. Therefore, the species may be assessed as Near Threatened under Criterion B1b(iii)+2b(iii) or Least Concern under this criterion, depending on whether the quality and/or extent of its habitat are judged to be truly declining.
Criterion C – Surveys by Lazzari et al. (2018) in 2017-2018 recorded a minimum of 181 breeding territories and estimated at least 372 adults, noting that they were unable to access all valleys where the species was previously recorded (Lazzari et al. 2018, SOP Manu 2019). The population size is placed in the band 250-999 mature individuals. The species meets the initial population size threshold for listing as Endangered under Criterion C.For a listing as threatened under Criterion C, there must also be evidence that the population size is undergoing a continuing decline. As described under Criterion A, this is not the case. Therefore, the species qualifies as Least Concern under Criterion C.
Criterion D – Based on the population estimate described above, Tahiti Reed-warbler qualifies as Vulnerable under Criterion D1. Although the species has a restricted area of occupancy, it is not likely that current or likely future threats could drive the species to Critically Endangered or Extinct within a very short time (within one or two generations, or three-six years). The species does not therefore meet the criteria for listing as Vulnerable under Criterion D2.
Criterion E – To the best of our knowledge no quantitative assessment of the probability of extinction has been conducted for this species, and so it cannot be assessed against this criterion.
Based on the above assessment, it is proposed to list the Tahiti Reed-warbler (Acrocephalus caffer) as Vulnerable under Criterion D1. We welcome any comments to the proposed listing. Information is particularly requested on the species’s population trends and the likely impact of threats to the species.
Please note that this topic is not designed to be a general discussion about the ecology of the species, rather a discussion of the species’s Red List status. Therefore, please make sure your comments are about the proposed listing. By submitting a comment, you confirm that you agree to the Comment Policy.
*Bird generation lengths are estimated using the methodology of Bird et al. (2020), as applied to parameter values updated for use in each IUCN Red List for birds reassessment cycle. Values used for the current assessment are available on request. We encourage people to contact us with additional or improved values for the following parameters; adult survival (true survival accounting for dispersal derived from an apparently stable population); mean age at first breeding; and maximum longevity (i.e. the biological maximum, hence values from captive individuals are acceptable).
An information booklet on the Red List Categories and Criteria can be downloaded here and the Red List Criteria Summary Sheet can be downloaded here. Detailed guidance on IUCN Red List terms and definitions and the application of the Red List Categories and Criteria can be downloaded here.
Bird, J. P.; Martin, R.; Akçakaya, H. R.; Gilroy, J.; Burfield, I. J.; Garnett, S.; Symes, A.; Taylor, J.; Šekercioğlu, Ç.; Butchart, S. H. M. (2020). Generation lengths of the world’s birds and their implications for extinction risk. Conservation Biology online first view.
Bossin, H., and Padovani, E. 2010. Audit des actions menees depuis 2006 en matiere de lutte contre la petite fourmi de feu Wasmannia auropunctata sur l’ile de Tahiti. Technical Report. Institut Louis Malarde.
IUCN. 2012. IUCN Red List Categories and Criteria: Version 3.1. Second edition. Gland, Switzerland and Cambridge, UK: IUCN. Available at www.iucnredlist.org/technical-documents/categories-and-criteria
Lazzari G., Blanvillain, C, Ghestemme T., Butaud. J-F., Pirard M. et Beaune D. 2018. Investigation on the Tahiti reed warbler’s (Acrocephalus caffer) distribution. Rapport SOP Manu. SOP Manu, Taravao Tahiti.
Monnet, C.; Thibault, J.; Varney, A. 1993. Stability and changes during the twentieth century in the breeding landbirds of Tahiti (Polynesia). Bird Conservation International 3: 261-280.
Pratt, H. D.; Bruner, P. L.; Berrett, D. G. 1987. A field guide to the birds of Hawaii and the tropical Pacific. Princeton University Press, Princeton.
Seitre, R.; Seitre, J. 1991. Causes de disparition des oiseaux terrestres de Polynésie Française. South Pacific Regional Environment Programme, Nouméa.
SOP Manu. 2019. Etude des Rousserolles a Long Bec a Tahiti. Te Manu 95.
SPREP. 2014. Managing the impacts of the little fire ants (Wasmannia auropunctata) in French Polynesia. Apia, Samoa.
Thibault, J. -C. 1988. Menaces et conservation des oiseaux de Polynésie Française. In: Thibault, J.-C.; Guyot, I. (ed.), Livre rouge des oiseaux menacés des régions françaises d’outre-mer, pp. 87-124. Conseil International pour la Protection des Oiseaux, Saint-Cloud.