Archived 2015 topics: Seychelles Warbler (Acrocephalus sechellensis): Downlist to Near Threatened?

Seychelles Warbler (Acrocephalus sechellensisBirdLife species factsheet) is currently classified as Vulnerable under Criterion D2, since it was confined to just four tiny islands with a very small range and was thought to be at risk from introduced predators.

A. sechellensis was present on several islands in the Seychelles until human disturbance in the 20th century reduced the species to a single population on the tiny (0.3 km2) island of Cousin between 1920 and 1988. In 1968 the species numbered fewer than 30 individuals. Conservation efforts were directed towards the species, which was translocated to the islands of Aride, Cousine and Denis between 1998 and 2004 and recovered to an estimated 2,500 individuals by 2007 (Bristol 2005; Richardson et al. 2006; G. Rocamora in litt. 2007). A species action plan produced in 2001 recommended that the range of the species be increased to five islands through translocations to secure the future of the species (Richardson 2001).

In December 2011, 59 adult Seychelles Warblers were captured on Cousin Island and translocated to Frégate Island (Wright et al. 2014). At the time of the translocation, Frégate Island possessed 0.37 km2 of habitat suitable for A. sechellensis and this was predicted to increase to 0.42 km2 by 2016 as a result of regeneration work. By June 2013, the A. sechellensis population on Frégate had increased to 80 individuals, which included 38 of the original translocated birds and 42 birds which had hatched on Frégate. Six birds were the offspring of birds that had themselves hatched on Frégate, indicating that a second generation had hatched on the island since the translocation. The source population on Cousin recovered to carrying capacity by October 2012.

The total population was estimated in 2014 at approximately 2,800 adults (N. Shah in litt): 320 on Cousin (Brouwer et al. 2006), 2010 on Cousine (Van de Crommenacker and Richardson 2007), 1,850 on Aride (Orchard 2004), 300 on Denis (J. van der Woude, unpub. data) and 80 on Frégate (Teunissen 2013). It has been predicted that as the newer populations reach carrying capacity, the total population will exceed 7,000 birds (Wright et al. 2014).

Although the range of A. sechellensis remains very small, the species now has five populations on five islands and its population has further increased to an estimated 2,800 individuals. It may therefore be considered that the risk of introductions of predators is no longer likely to drive the species to Critically Endangered or Extinct in a very short time. As a result, it may no longer be justified to list the species as Vulnerable under Criterion D2 and it may be more appropriate to downlist the species to Near Threatened.

Additional information and comments on this proposal are welcomed.

References

Bristol, R. 2005. Conservation introductions of Seychelles Fody and Warbler to Denis Island, Seychelles. Re-introduction News 24: 35-36.

Brouwer, L., Richardson, D.S., Eikenaar, C. & Komdeur, J. 2006. The role of group size and environmental factors on survival in a cooperatively breeding tropical passerine. Journal of Animal Ecology 75: 1321-1329.

Orchard, D. 2004. Aride Seychelles Warbler Census. In: Betts, M. (ed.) Annual Report 2003. Aride Island Nature Reserve, Seychelles. Royal Society of Wildlife Trusts, Newark.

Richardson, D.S. 2001. Species Conservation Assessment and Action Plan: Seychelles warbler (Timerl Dezil). Joint report of Nature Seychelles, University of East Anglia, Norwich.

Richardson, D.S., Bristol, R. & Shah, N.J. 2006. Translocation of the Seychelles warbler Acrocephalus sechellensis to establish a new population on Denis Island, Seychelles. Conservation Evidence 3: 54-57.

Teunissen, N. 2013. A newly translocated population of the Seychelles warbler: is there evidence for interspecific competition with Seychelles white-eyes? MSc thesis. University of Groningen.

Van de Crommenacker, J. & Richardson, D.S. 2007. Monitoring and Studying the Seychelles warbler: fieldwork on Cousine Island. University of Groningen Report, Groningen.

Wright, D.J., Shah, N.J. & Richardson, D.S. 2014. Translocation of the Seychelles warbler Acrocephalus sechellensis to establish a new population on Frégate Island, Seychelles. Conservation Evidence 11: 20-24.

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3 Responses to Archived 2015 topics: Seychelles Warbler (Acrocephalus sechellensis): Downlist to Near Threatened?

  1. Andy Symes (BirdLife) says:

    Preliminary proposals

    Based on available information, our preliminary proposal for the 2015 Red List would be to pend the decision on Seychelles Warbler and keep this discussion open until 2016, while leaving the current Red List category unchanged in the 2015 update.

    There is now a period for further comments until the final deadline of 31 August, after which recommended categorisations will be put forward to IUCN.

    The final Red List categories will be published on the BirdLife website in late October and on the IUCN website in November, following further checking of information relevant to the assessments by both BirdLife and IUCN.

  2. The Seychelles warbler is currently listed as vulnerable D2. I.e. ‘….population very small or restricted…. with a very restricted area of occupancy (typically less than 20 km2) or number of locations (typically five or fewer) such that it is prone to the effects of human activities or stochastic events within a very short time period in an uncertain future, and is thus capable of becoming Critically Endangered or even Extinct in a very short time period.’

    The objective set out in the 2001 action plan (Richardson 2001) was to increase the number of populations to five with a population of more than 3000 mature adult individuals. By the April 2013 the population was estimated at ca. 2800 adults and rising; Cousin = 320 (Brouwer et al. 2006), Cousine = 210 (Van de Crommenacker & Richardson 2007), Aride = 1850 (Orchard 2004), Denis = 300 (J. van der Woude et al 2013) and Fregate = 80 (Teunissen 2013). Given the growth trajectories in the newly established population on Denis and Fregate we now estimate those two populations at ca 600 and ca 200 respectively, thus taking the total to >3000 adult birds as of August 2015. A census on Denis is currently being undertaken to verify the population numbers there, while another one is planned on Fregate in early 2016. Furthermore, given the positive growth trajectories and the habitat available we predict the populations on Denis and Fregate may eventually exceed 2000 and 2500 respectively (Hammers and Richardson 2011; DS Richardson Pers. Comm), thus taking the global population to ca > 6500.

    Recent studies have provided evidence that the Seychelles warbler existed in a large population (potential estimates of ca 20,000 individuals) across the inner granitic islands in its recent evolutionary past (pre-human colonisation) (Spurgin et al 2014). This species then lost 25% of its genetic variation during the bottleneck that occurred during the 1900’s (Spurgin et al 2014). Importantly, the translocations undertaken to help conserve this species have been very successful in translocating almost all the remaining genetic variation to each of the new populations (Wright et al 2014), thus avoiding potential problems with increased inbreeding and/or loss of adaptive potential on those islands. Of equal importance is that the translocations have not impacted on the original population on Cousin, either in terms of population size or genetic variability (Wright et al 2014).

    The consequence of the work outlined above is that we now have five well established, genetically maximised, populations containing a total of ca 3000 birds and rising. While it would always be good to have more populations of the SW if suitable locations are available, the data indicate that the SW has already reached a threshold whereby it could legitimately be downgraded to ‘near threatened’.

    Brouwer, L., Richardson, D.S., Eikenaar, C. & Komdeur, J. 2006. The role of group size and environmental factors on survival in a cooperatively breeding tropical passerine. Journal of Animal Ecology 75: 1321-1329.
    Hammers, M, & Richardson, D.S. 2011. Assessment of the suitability of Frégate Island for
    Seychelles warblers (Acrocephalus sechellensis) 2011. University of Groningen/ University of East Anglia Report.
    Orchard, D. 2004. Aride Seychelles Warbler Census. In: Betts, M. (ed.) Annual Report 2003. Aride Island Nature Reserve, Seychelles. Royal Society of Wildlife Trusts, Newark.
    Richardson, D.S. 2001. Species Conservation Assessment and Action Plan: Seychelles warbler (Timerl Dezil). Joint report of Nature Seychelles, University of East Anglia, Norwich.
    Richardson, D.S., Bristol, R. & Shah, N.J. 2006. Translocation of the Seychelles warbler Acrocephalus sechellensis to establish a new population on Denis Island, Seychelles. Conservation Evidence 3: 54-57.
    Spurgin LG, Wright DJ, Velde M, Collar NJ, Komdeur J, Burke T, Richardson DS (2014) Museum DNA reveals the demographic history of the endangered Seychelles warbler. Evolutionary Applications 7(9):1134-1143.
    Teunissen, N. 2013. A newly translocated population of the Seychelles warbler: is there evidence for interspecific competition with Seychelles white-eyes? MSc thesis. University of Groningen.
    Van de Crommenacker, J. & Richardson, D.S. 2007. Monitoring and Studying the Seychelles warbler: fieldwork on Cousine Island. University of Groningen/ University of East Anglia Report.
    Van der Woude, J. Apperloo, R. & van Marrewijk, M (2013) Seychelles warbler population census on Denis Island, August –September 2013. University of Groningen Report, Groningen
    Wright, D.J., Shah, N.J. & Richardson, D.S. 2014. Translocation of the Seychelles warbler Acrocephalus sechellensis to establish a new population on Frégate Island, Seychelles. Conservation Evidence 11: 20-24.

  3. Andy Symes (BirdLife) says:

    Recommended categorisation to be put forward to IUCN

    Based on further information received, we have been able to change the preliminary proposal and revise the recommended classification on the 2015 Red List to Near Threatened (criterion D2) .

    The final categorisation will be published on the BirdLife website in late October and on the IUCN website in November, following further checking of information relevant to the assessment by BirdLife and IUCN.

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