Seychelles White-eye (Zosterops modestus): revise global status?

BirdLife species factsheet for Seychelles White-eye

The Seychelles White-eye (Zosterops modestus) has populations on the islands of Mahé and Conception in the Seychelles, with translocated populations on Frégate, Ile du Nord (North Island) and since 2018, Grande Soeur Island. The species was previously listed as Endangered on the basis of having a very small population, but the population estimate was increased to 500-650 individuals in 2013 (Safford and Hawkins 2013) and consequently the species was downlisted to Vulnerable in 2016 (IUCN 2016).

In 2017 it was discovered that rats had colonised the species’s main stronghold, Conception Island, and the population on the island had declined from c.340 individuals in 2009 (Rocamora and Labiche 2009) and c.310 individuals in 2014 (G. Rocamora in litt. 2019) to around 15 individuals (G. Rocamora in litt. 2019, IBC-UniSey unpubl.). We are therefore carrying out a reassessment of this species’s Red List status.

BirdLife has recently produced new estimates for all bird generation lengths, with the new figure for this species now 2.3 years (BirdLife International in prep.). Our current information on the species’s conservation status will now be compared to all Red List Criteria.

Criterion A – The revised Seychelles White-eye generation length of 2.3 years means that reductions should be assessed over a period of 10 years for the application of Criterion A.

The population on Mahé was estimated to number c.60 birds in 2006, but declined down to 40 birds in 2011-2013 (G. Rocamora in litt. 2014) and has now declined to c.20 birds by 2018 (G. Rocamora in litt. 2019, IBC-UniSey unpubl.).

The population on Conception was estimated at c.340 individuals in 2009 (Rocamora and Labiche 2009) and c.310 individuals in 2014 (G. Rocamora in litt. 2019). In 2017 it was discovered that Black Rats Rattus rattus had colonised the island, probably about two years before in view of the rat distribution and abundance observed during visits. Subsequently, the population of Seychelles White-eye on Conception was almost eliminated, estimated at 10 to 15 individuals in late 2018 (G. Rocamora in litt. 2019, IBC-UniSey unpubl.).

The transfer of 37 individuals from Conception to Frégate Island in 2001 and 2003 resulted in the establishment of an estimated population of c.100 individuals there in 2007 (G. Rocamora in litt. 2007), c.150 individuals in 2010/2011 (G. Rocamora in litt. 2014) and c.240 individuals in 2017 (G. Rocamora in litt. 2019; Rocamora et al. 2018). Thirty individuals were translocated from Frégate Island to Grande Soeur Island in 2018 (G. Rocamora in litt. 2019, IBC-UniSey unpubl.).

In 2007, 25 birds were transferred to Ile du Nord, and 23 to Cousine (Rocamora and Henriette-Payet 2009). The population on North Island increased to approximately 100 individuals in 2014 (Havemann and Havemann 2014) and 125-140 individuals in 2017 (Pietersen, 2017), 17 of which were translocated to Grande Soeur Island in 2018 (G. Rocamora in litt. 2019). However, the population on Cousine has apparently failed, with no more than five individuals recorded there in 2013 (G. Rocamora in litt. 2014), all of which vanished shortly after (G. Rocamora in litt. 2019, IBC-UniSey unpubl.).

The newly-translocated population on Grande Soeur was reported to number c.40-45 individuals and to have produced four fledglings in 2019 (G. Rocamora in litt. 2019). However, according to IUCN guidelines, a reintroduced population should not be included in a Red List assessment until at least five years have passed (IUCN 2017), so the new population on this island is not included in this assessment.

Combining these figures and assuming linear trends, the overall population size is estimated to have undergone a reduction of between 30% and 49% over the last three generations, which meets the threshold for listing as Vulnerable under Criterion A2.

Since the fast-declining population on Conception is now very small and the populations on Frégate and Ile du Nord are currently increasing, it is unclear whether the overall population size will continue to decline in the future. If we assume that ongoing trends will continue, the overall population size is not projected to decrease over the next three generations and the species would be assessed as Least Concern under Criterion A3. If we cautiously assume that the populations on Frégate and Ile du Nord will remain stable, the overall population is projected to decline by 1-9% over the next three generations, and the species would again be assessed as Least Concern under this Criterion.

Over a period of three generations which extends from the past and into the future, the maximum population reduction is estimated to be 30-40% if ongoing trends continue, or 40-50% if we cautiously assume that the populations on Frégate and Ile du Nord will remain stable. These ranges meet the thresholds for listing as Vulnerable or Endangered under Criterion A4. Given that the population on Frégate and Ile du Nord have recently been growing, and both islands have been estimated to have carrying capacities of several hundred individuals (Rocamora in litt. 2019), it would be reasonable to assume that these subpopulation will continue to grow by at least some degree, and the most appropriate category under Criterion A4 would therefore be Vulnerable.

Criterion B – The species’s Extent of Occurrence (EOO) is estimated at 1,500 km2. This meets the threshold for Endangered under Criterion B1. According to IUCN Guidelines (IUCN 2017), a species’s Area of Occupancy (AOO) should be calculated at the level of 2km x 2km grid squares. Although this species has been estimated to occupy a very small area of habitat (<20km2 in total [Rocamora in litt. 2019]), this estimate was not calculated at the spatial scale of 2km x 2km grid squares and hence we are unable to use it to assess the species under Criterion B2. However, based on a 4km2 grid placed over the area of mapped range, the AOO must be smaller than 88 km2, so it meets the threshold for Endangered under Criterion B2. To list the species as threatened on the Red List under Criterion B, two of conditions a-c must also be met.

The species is not severely fragmented, since less than 50% of its total area of occupancy is in habitat patches that are smaller than would be required to support a viable population. The establishment of Black Rats on Conception Island recently resulted in the largest subpopulation of Seychelles White-eye almost being eliminated. According to the IUCN definition of the term ‘location’ as a ‘geographically or ecologically distinct area in which a single threatening event can rapidly affect all individuals of the taxon present’ (IUCN 2017), and based on the most significant threat being invasive rats, the species is assessed as having between 3 and 10 locations. If we consider the number of locations to be 3-5, the species meets condition a at the level of Endangered.

Since the fast-declining population on Conception is now very small and the populations on Frégate and Ile du Nord are currently increasing, it is uncertain whether the overall population size will continue to decline into the future. If the species is expected to be undergoing a continuing decline in the number of mature individuals, which is liable to continue into the future, then condition b would be met. However, based on recent population trajectories, and the modelled carrying capacities for Frégate and Ile du Nord of several hundred individuals (G. Rocamora in litt. 2019), it is unlikely that the overall population size will continue to decline into the future. Even if current population trajectories do not imply a continuing decline, if it were likely that there will be another introduction of rats onto one of the remaining rat-free islands within the species’s range within the next 10 years, then we could infer a continuing decline and condition b would be met. However, reinvasion of Frégate and Ile du Nord by rats is also considered unlikely as there are efficient biosecurity protocols and no reinvasion has occurred since rats were eradicated in 2000 and 2005 respectively (G. Rocamora in litt. 2019). Lastly, if it can be inferred that there is a continuing decline in the area or quality of habitat as a result of invasive species or other threats, then condition b would be met. If the best evidence suggests that neither the species’s population or its habitat area or quality are likely to continue to decline into the future, then condition b would not be met.

There is no evidence that the species’s population or range size are undergoing extreme fluctuations. Condition c is not met.

The species’s EOO and AOO fall beneath the thresholds for listing the species as Endangered, and condition a may be met at the level of Endangered, but it is unclear whether condition b is met. If the species is undergoing a continuing decline, then it may be assessed as Endangered under Criterion B1+2. If there is not a continuing decline, as it is likely to be the case, then the species may be assessed as Near Threatened under this criterion.

Criterion C – Based on the figures described above, and according to G. Rocamora (in litt. 2019), the species’s population size is estimated at c.340-400 individuals. This roughly equates to 227–267 mature individuals, rounded here to 220–270 mature individuals. This does not include individuals that have been translocated to establish a new population on Grande Soeur Island (40-45 birds are estimated to be present by April 2019; Rocamora in litt. 2019), since five years have not yet passed since this translocation. This range of population size estimates meets the threshold for listing the species as Critically Endangered or Endangered under Criterion C. To list the species as threatened under Criterion C, further conditions must also be met.

The population has recently undergone a rapid decline. Nevertheless, since the fast-declining population on Conception is now very small and the populations on Frégate and Ile du Nord are currently increasing, it is unclear whether the overall population size will continue to decline into the future. If the species is expected to be undergoing a continuing decline in the number of mature individuals, which is liable to continue into the future, then the species could qualify as threatened under Criterion C. However, based on recent population trajectories and the modelled carrying capacities for Frégate and Ile du Nord (G. Rocamora in litt. 2019), it is unlikely that the overall population size will continue to decline into the future. Even if current population trajectories do not imply a continuing decline, if it is likely that there will be another introduction of rats onto one of the remaining rat-free islands within the species’s range within the next 10 years, then we can infer a continuing decline. However, reinvasion of Frégate and Ile du Nord by rats is considered unlikely as there are efficient biosecurity protocols and no reinvasion has occurred since rats were eradicated in 2000 and 2005 respectively (G. Rocamora in litt. 2019). If we therefore consider that the species’s population is unlikely to continue to decline into the future, and a further rat introduction within the species’s range is not probable within the next ten years, then the species would not qualify as threatened under this Criterion, but may qualify as Near Threatened.

If there is a continuing decline, and all relevant populations are predicted to continue at their current trajectories, then the population would be assessed to undergo a continuing decline at a rate of up to 30-49% over 10 years and 5 years and up to 31% over 3 years (from 2016-2019). This could qualify the species as Critically Endangered under Criterion C1.

The species has at least four subpopulations, the largest being on Frégate Island, which had c.210 individuals in 2018 (G. Rocamora in litt. 2019, IBC-UniSey unpubl.), roughly equivalent to 140 mature individuals. If there is a continuing decline, then the species would qualify as Endangered under Criterion C2a(i). The population on Frégate Island does not hold more than 90% of the total population, so Criterion C2a(ii) is not met and the species is assessed as Least Concern under Criterion C2a(ii). There is no evidence that the species’s population size is undergoing extreme fluctuations so the species is not assessed as threatened under Criterion C2b.

In order to determine the species’s status under Criterion C, it is therefore pivotal to determine whether the species’s population decline is likely to continue into the future, or a similar event (introduction of rats) is likely to happen again within the next ten years. If it is, then the species could qualify as Critically Endangered under Criterion C1. If not, the species may qualify as Near Threatened, approaching threatened under Criteria C1+2a(i).

Criterion D

As described above, and according to G. Rocamora (in litt. 2019), the species’s population size is estimated at c.340-400 individuals. This roughly equates to 227–267 mature individuals, rounded here to 220–270 mature individuals. This does not include individuals that have been translocated to establish a new population on Grande Soeur Island, since five years have not yet passed since this translocation. This qualifies the species for listing as Endangered under Criterion D or Vulnerable under Criterion D1, depending on where the true population size is most likely to fall. The species has a small number of locations, and further introductions of invasive species could plausibly drive the species to Critically Endangered within one generation (2.3 years). This qualifies the species 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.

In this case, we have very good data on which to assess the qualifying criteria over short timeframes. However, there is a key question that requires a judgment that greatly influences the conclusion: Is there a continuing decline?

The recent catastrophe for the population on Conception appears to be a one-off reduction. In this case, Seychelles White-eye would remain listed as Vulnerable, under Criteria A2ace+4ace;D1+2.

Should there be convincing evidence that this event is part of a continuing decline, the species could qualify as Critically Endangered under Criterion C1.

How likely is another introduction of rats onto one of the remaining rat-free islands within the species’s range, within the next 10 years?

Is there any reason to expect a continuing decline in the overall population for any other reason?

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.

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.

References

BirdLife International (in prep) Generation lengths of the birds of the world. Unpublished data set. Available from science@birdlife.org.

Havemann, C.J. and Havemann, T. (2014) North Island. Seychelles Wildlife News 14(October-December): 9-11.

Island Biodiversity and Conservation centre of the University of Seychelles (IBC-UniSey), Unpublished data, 2016-2019. CEPF project: Advancing Ecosystem Management and Threatened Species Recovery through Partnerships with Private sector.

IUCN (2001) IUCN Red List Categories and Criteria: Version 3.1. IUCN Species Survival Commission. IUCN, Gland, Switzerland and Cambridge, U.K.

IUCN (2012) IUCN Red List Categories and Criteria: Version 3.1. Second edition. IUCN Species Survival Commission. IUCN, Gland, Switzerland and Cambridge, U.K. www.iucnredlist.org/technical-documents/categories-and-criteria

IUCN (2016) The IUCN Red List of Threatened Species. Version 2016-3. Available at: www.iucnredlist.org. (Accessed: 07 December 2016).

IUCN (2017) Guidelines for Using the IUCN Red List Categories and Criteria. Version 13 March 2017. Available at: https://www.iucnredlist.org/resources/redlistguidelines.

Pietersen, D. (2017) Assessment of the Seychelles White-eye Zosterops modestus Newton, 1867 population on North Island, Seychelles. 5–21 October 2017. Percy FitzPatrick Institute of African Ornithology. 12 p.

Rocamora, G. and Henriette-Payet, E. (2009) Conservation introductions of the Seychelles white-eye on predator-free rehabilitated islands of the Seychelles archipelago, Indian Ocean. In: Pritpal S. Soorae (ed.), Global re-introduction perspectives: Re-introduction case-studies from around the globe.

Rocamora, G. and Labiche, A. (2009) Variation of abundance of land birds and reptile on Conception between the pre and post rat eradication periods (2005 to 2008). Annex 12. In: Rocamora, G; Jean-Louis, A. (ed.), Réhabilitation des Ecosystèmes Insulaires. Rapport annuel au secrétariat du FFEM. Quatrième année d’opérations 1/05/08 au 30/06/09, pp. 66-74. Island Conservation Society, Seychelles.

RocamoraG., HenrietteE., Sorry A. & Labiche  R. 2018. Conservation introduction of the globally threatened Seychelles White-eye Zosterops modestus to Grande Soeur Island (Seychelles).  Project proposal. CEPF Project ‘Advancing Environmental Management Practices & Threatened Species Recovery through Partnerships with Private Sector – First Phase’. Island Biodiversity & Conservation centre, University of Seychelles. Safford, R. J. & Hawkins, A. F. A. (2013) The Birds of Africa. Volume VIII: The Malagasy Region. Christopher Helm, London.


 

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1 Response to Seychelles White-eye (Zosterops modestus): revise global status?

  1. GERARD ROCAMORA says:

    Thank you for this reassessment of the global status of the Seychelles white-eye. I broadly agree with your analysis and your conclusion that the species continues to qualify as Vulnerable, for meeting criteria A2ace+4ace;D1+2.

    The species population is unlikely to continue to decline despite the terrible set-back of the invasion of the only viable source population on Conception, which has been almost wiped out (but may recover at least to some extent through some rat control until eradication can be performed). Instead, the two healthy populations of Frégate and Ile du Nord (totalling 350-400 birds), are likely to continue to increase, and hopefully the newly established small breeding population of Gde Soeur (40-45 birds) too, even though both the relictual populations of Conception and Mahé (less than 20 birds each) could become extinct any time. Although invasion by rats is like a Damocles Spade that can never be ruled out, it is unlikely that rats may recolonise Frégate or Ile du Nord (North Island Ltd) as both islands have efficient biosecurity procedures and have remained rat free since 2000 and 2005 respectively.

    Regarding distribution, the species’s area of occupancy was estimated in 1997 at less than 4km2 (10 squares of 0.25km2 of regular presence on Mahé, plus 0.7km2 on Conception; Rocamora, 1997). Distribution has now been reduced by half on Mahé (so less than 2km2) where it is permanently present (breeding) in 2 locations only, but this has been largely compensated by the translocations on Frégate and Gde Soeur that have added c. 3km2. In total, the distribution of the species is less than 5km2 in 6 locations: Mahé-La Misère, Mahé-Gde Anse, Conception, Frégate (since 2001), Ile du Nord (since 2007), and Gde Soeur (since 2018).

    All recent information provided to BirdLife International to update the conservation status of the species results from team work coordinated by the Island Biodiversity and Conservation centre of the University of Seychelles (IBC-UniSey). It has been gathered from fieldwork conducted since 2016 as part of our CEPF funded project ‘Advancing Ecosystem Management and Threatened Species Recovery through Partnerships with Private sector’ on all islands where the Seychelles white-eye is present in collaboration with our island partners. These activities have been led by IBC-UniSey team (Elvina Henriette, André Labiche, Abel Sorry, Nathachia Pierre, Jessica Perrette, Lucile Gagnor and Gerard Rocamora), with contributions from the Seychellles Ministry of Environment, Energy and Climate Change (Dora Radegonde), Frégate Ecology department (led by Janske van de Crommenacker and Richard Baxter) and the University of East Anglia Seychelles warbler team (led by David Richardson) on Frégate, Cape Town Fitzpatrick Institute of Ornithology (Darren Pietersen) and the North Island Environment department (led by Carl Havemann and Tarryn Retief) on Ile du Nord.

    The two following publications, derived from the PhD of Dr Elvina Henriette, should be added to the bibliography as they provide precise data on the population size and growth of the Frégate population between 2001 and 2010.

    – HENRIETTE, E. & ROCAMORA, G. (2012). Survival rates of a tropical island endemic following conservation introduction on a rehabilitated island: the case of the endangered Seychelles White-eye. Rev. Écol. (Terre Vie), 67: 223-236.

    – HENRIETTE, E. & ROCAMORA, G. (2011). Comparative use of three methods for estimating the population size of a transferred island endemic: the endangered Seychelles White-eye (Zosterops modestus). Ostrich, vol. 82 (2): 87-94.

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