BirdLife species factsheet for Giant White-eye: http://datazone.birdlife.org/species/factsheet/Giant-White-eye
Giant White-eye (Megazosterops palauensis) is endemic to Palau, being common on the islands of Peleliu and Ngeruktabl (van Balen 2018). There is also one record from Babeldaob Island (Olsen and Eberdong 2009), but it is unknown why it is not known to occur on other neighbouring islands (van Balen 2018). It occurs in native limestone forest, in dense vegetation including Leucaena thickets (Engbring 1992), with the main current threat being from habitat loss. However, while it is not currently a threat, any possible introduction of Brown Tree Snake (Boiga irregularis) onto the islands could have as devastating an impact on the species as it has had to birds on Guam.
Currently listed as Near Threatened (BirdLife International 2018), an analysis of deforestation data by Tracewski et al. (2016) has shown that the amount of forest remaining within the species’s range is very limited, and as such it could warrant a change in Red List status. Therefore, it has been reassessed here against all criteria.
The initial topic on this analysis can be found here.
Criterion A – The population trend for this species has not been directly estimated. However, deforestation data from between 2000 and 2012 suggests that the area of suitable habitat for the species on average is declining by c.2.9% over three generations (13.2 years). Therefore, while it may be possible to consider the species to be in decline, the rate of decline is likely to be slow and would not approach the threshold for Vulnerable under this criterion. As such, Giant White-eye may be listed as Least Concern under Criterion A.
Criterion B – The Extent of Occurrence (EOO) for this species is 1,100km2. Tracewski et al. (2016) also estimate the maximum Area of Occupancy (AOO) (calculated as the remaining tree area within the species’s range) to be c.242km2. These values fall below the threshold for Endangered under criteria B1 and B2 respectively, but listing under these criteria requires at least two further conditions to be met.
The species is not known to undergo any extreme fluctuations, so it would not trigger condition c). Yet, given the deforestation data, we may be able to infer a slow ongoing decline in at least AOO, habitat quality and population size. Thus conditions b(ii,iii,v) are met.
For condition a), the species needs to be severely fragmented or to occur at a limited number of locations* (2-5 for Endangered; 6-10 for Vulnerable). The species is unlikely to fulfil the specifications for ‘severely fragmented’ (see IUCN Standards and Petitions Subcommittee 2017), but it does likely occur at a small number of locations. As a location depends upon the main threat to a species, if Brown Tree Snake were present then the number of locations would be two or three – dependent on the viability of the population on Babeldaob. However, with habitat loss as the main current threat, the number of locations will be larger than this, and indeed it could be larger than 6-10, but this is not certain. Despite this, the number of locations is likely to be fairly small, and so the species likely warrants listing at least as Near Threatened under Criteria B1ab(ii,iii,v)+2ab(ii,iii,v), although further information could show that it does warrant listing as Vulnerable under the same criteria.
Criterion C – Engbring (1992) estimated the population to be 13,876 individuals, although of course at the time that estimate will have been just for Peleliu and Ngeruktabl. However, given that there has only been one record from Babeldaob, the population there could be extremely small. The population estimate of Engbring (1992) roughly equates to a population size of 9,200-9,300 mature individuals, which falls below the threshold for Vulnerable. However, to warrant listing under this criterion requires other conditions to be met.
The rate of decline has not been accurately estimated or observed, and it is not known to undergo extreme fluctuations, so the species does not warrant listing under Criteria C1 or C2b respectively. The species also occurs in at least two viable populations, so it cannot be listed under Criterion C2a(ii). The only other option is C2a(i), which requires no subpopulation to contain >1,000 mature individuals. This is unlikely to be the case, given that the species is common on both Peleliu and Ngeruktabl. As such, the population may be fairly evenly split between the two islands. Therefore, it would not even approach the threshold for listing as Vulnerable under this criterion. Overall, Giant White-eye may be considered Least Concern under Criterion C.
Criterion D – The population size is too large to warrant listing under this criterion. The species does occur on only 2 or 3 islands, and if Brown Tree Snake should arrive on these islands then each could then represent separate locations, and given the high level of threat the snake poses the species could then warrant listing as Vulnerable under criterion D2. As the snake is not known to have reached these islands yet, though, it is instead thought that Near Threatened under criterion D2 may be more appropriate.
Criterion E – To the best of our knowledge there has been no quantitative analysis of extinction risk conducted for this species. Therefore, it cannot be assessed against this criterion.
Therefore, the species at least warrants listing as Near Threatened under criteria B1ab(ii,iii,v)+2ab(ii,iii,v); D2. However, we do request further information regarding the number of locations* where the species occurs, as it could warrant a higher threat status. Please note, though, that this topic is not designed to be a general discussion about the ecology of the species, rather a discussion of the species’ Red List status. Therefore, please make sure your comments are relevant to the information that is sought, or about the species’ Red List status.
*The term ‘location’ refers to a distinct area in which a single threatening event can rapidly affect all individuals of the taxon present, with the size of the location depending on the area covered by the threatening event. Where a taxon is affected by more than one threatening event, location should be defined by considering the most serious plausible threat (IUCN 2001, 2012).
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.
BirdLife International. 2018. Species factsheet: Megazosterops palauensis. Downloaded from http://www.birdlife.org on 25/09/2018.
Engbring, J. 1992. A 1991 survey of the forest birds of the Republic of Palau. U.S. Fish and Wildlife Service, Honolulu.
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. Available at: www.iucnredlist.org/technical-documents/categories-and-criteria.
IUCN Standards and Petitions Subcommittee. 2017. Guidelines for Using the IUCN Red List Categories and Criteria. Version 13. Prepared by the Standards and Petitions Subcommittee. Downloadable from http://www.iucnredlist.org/documents/RedListGuidelines.pdf.
Olsen, A. R.; Eberdong, M. 2009. Species richness and other noteworthy observations at an Important Bird Area in Palau. Micronesica 41(1): 59-69.
Tracewski, Ł.; Butchart, S. H. M.; Di Marco, M.; Ficetola, G. F.; Rondinini, C.; Symes, A.; Wheatley, H.; Beresford, A. E.; Buchanan, G. M. 2016. Toward quantification of the impact of 21st-century deforestation on the extinction risk of terrestrial vertebrates. Conservation Biology 30: 1070-1079.
van Balen, B. 2018. Giant White-eye (Megazosterops palauensis). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from https://www.hbw.com/node/60252 on 25 September 2018).