Archived 2018 topic: Sao Tome White-eye (Zosterops feae): revise global status?

Endemic to the island of São Tomé, São Tomé and Príncipe, Sao Tome White-eye (Zosterops feae) was recently split from Principe White-eye (Zosterops focedulinus) (del Hoyo and Collar 2016). It is predominantly a forest species, preferring mid- to high-altitude areas, but also occurring in the lowlands. It occurs in a range of forest habitats (Atkinson et al. 1991, Christy and Clarke 1998); and forest clearance for agriculture, timber and fuel (Atkinson et al. 1991) is thought to be the main threat to the species at the moment. The development of road infrastructure on the east and west coasts is increasing access to remote areas, which could increase forest loss (A. Gascoigne in litt. 2000). However, de Oliveira Soares (2017) found the species most commonly in areas of intermediate disturbance (secondary forest and plantations).

Surveys have shown the species to be most abundant in the central massif of the island, although it may show fluctuations in its distribution (and presumably abundance) (see Atkinson et al. 1991, Christy and Clarke 1998). It is thought to have undergone declines throughout the 20th Century, with the species described as common in the 1920s, uncommon in the 1970s and only locally common in 1990 (van Balen 2018), but there is now little evidence for continued declines and as such the species is considered to be stable. The population size is currently placed in the range 250-999 mature individuals, and so the species is currently listed as Vulnerable under criterion D1 (see BirdLife International 2018).

A recent survey found the species more frequently than other species placed in this population size range (de Oliveira Soares 2017), and the species can be found in groups of multiple individuals, suggesting an even greater population size (R. F. de Lima in litt. 2017). Therefore, we have reassessed the species here against all criteria to assess whether it would warrant a change in Red List category based on this information.

 

Criterion A – The species is currently considered to be stable, yet habitat loss could have an impact on the species. The species does, though, appear to favour areas of intermediate disturbance (de Oliveira Soares 2017, R. F. de Lima in litt. 2018) and so even if the population trend was changed to decreasing it is very unlikely that the species would approach the threshold for Vulnerable under this criterion (reduction of 30% over three generations [10.5 years]).

 

Criterion B – The species has a restricted range (Extent of Occurrence = 430km2), yet it appears to favour areas of intermediate disturbance (de Oliveira Soares 2017, R. F. de Lima in litt. 2018), so it is not certain to what extent forest loss/degradation affects the species, and so we cannot reasonably assess the species as undergoing any continuing declines (although see criterion C). The species does appear to undergo some fluctuations (see criterion C), although it is not certain whether these are in the extent of an order of magnitude, so they cannot be reliably assessed as ‘extreme’ per IUCN guidelines (IUCN Standards and Petitions Subcommittee 2017). Finally, the species is not thought to be severely fragmented and, given that the main threat to the species is thought to be from habitat loss/degradation, it likely occurs at >>10 locations*. Therefore, the species does not warrant listing under this criterion.

 

Criterion C – The population size is currently placed in the range 250-999 mature individuals based on descriptions of abundance, known records and its range size, and this fits with population density estimates for congeners or closely related species of a similar size (BirdLife International 2018). However, de Oliveira Soares (2017) found the species over twice as frequently as other São Tomé endemics currently placed in this range (e.g. Sao Tome Short-tail, Amaurocichla bocagii, and Sao Tome Scops-owl, Otus hartlaubi). Sao Tome White-eye can also be found in groups of up to 10 or 20 individuals (Atkinson et al. 1991, R. F. de Lima et al. 2018), and so the population size could be even greater (assuming that these groups consist of mature individuals capable of reproduction), potentially warranting moving the population size estimate to the range 1,000-2,499 mature individuals.

That said, based on the sightings overview in Atkinson et al. (1991) the species does appear to undergo fluctuations, and so the reporting rate found by de Oliveira Soares (2017), could have found the species during a peak. In this case, if the population is fluctuating we should use a lower estimate per IUCN guidelines (IUCN Standards and Petitions Subcommittee 2017). Thus, it could be that the 250-999 mature individuals estimate is appropriate.

From the point of view of criterion C, this debate is immaterial as both population size estimates fall within the range for listing as Endangered. To warrant listing under this criterion still would require the species to be undergoing a continuing decline and meet other subcriteria. The species is thought to have undergone historical declines due to habitat degeneration, but these are now considered to have halted. The fact that the species prefers slightly degraded habitat (secondary forest and plantations), lends further support to this view that the species may be stable. However, this could show inconsistency in the assessment of this threat type, and it could be that the species is more reliant on forest in poorer years (if the species is considered to be fluctuating). Therefore, in effect the population trend is uncertain.

There seems to be no strong evidence to infer a population decline though, so the species would not warrant listing as threatened. However, the species may still warrant listing as Near Threatened under this criterion as a precautionary measure, given the uncertainty over the trend. While the species may fluctuate, the combined uncertainty around this and the trend, and the fact these potential fluctuations are not ‘extreme’ means it likely wouldn’t warrant listing under criterion C2b. The lack of clear quantified population trend information precludes the use of criterion C1. But the species does likely occur in only one subpopulation so it could warrant a precautionary listing as Near Threatened under criterion C2a(ii).

 

Criterion D – If we accept an increased population size estimate of 1,000-2,499 mature individuals then the species would approach the threshold for Vulnerable under criterion D1, and thus would warrant listing as Near Threatened. If, however, we take the precautionary approach based on the fact that the population may undergo fluctuations, with the population size periodically falling below 1,000 mature individuals, then it would warrant listing as Vulnerable instead.

The species’s range is insufficiently restricted to warrant listing under criterion D2.

 

Criterion E – To the best of our knowledge no quantitative analysis of extinction risk has been conducted for this species. Therefore, it cannot be assessed against this criterion.

 

The listing of the species therefore depends on how precautionary we should be. The most precautionary approach would be to assume that the species does undergo fluctuations such that its population size should be kept in the range 250-999 mature individuals. This would mean that the species retains its current Red List status of Vulnerable under criterion D1.

A slightly less precautionary approach would be to consider that the species does warrant a change in population size estimate. Placing it in the range 1,000-2,500 mature individuals, yet being precautionary about the uncertainty over the population trend, would mean that it would warrant listing as Near Threatened under criterion C2a(ii); D1.

The least precautionary approach would be to assume the population is stable, and doesn’t warrant listing under criterion C. The species would still warrant listing as Near Threatened though, but solely under criterion D1.

We therefore welcome any comments and further information that would improve our confidence in our appraisal, but please make sure any comments are relevant to the discussion outlined in the topic, as it is not designed to be a general discussion about the ecology of the species, rather a discussion of the species’ Red List status.

 

 

*Note that 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. The size of the location depends on the area covered by the threatening event and may include part of one or many subpopulations. 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).

 

References

Atkinson, P.; Peet, N.; Alexander, J. 1991. The status and conservation of the endemic bird species of Sao Tomé and Príncipe, West Africa. Bird Conservation International 1: 255-282.

BirdLife International. 2018. Species factsheet: Zosterops feae. Downloaded from http://www.birdlife.org on 02/03/2018.

Christy, P.; Clarke, W. V. 1998. Guide des Oiseaux de Sao Tome et Principe. ECOFAC, Sao Tome.

del Hoyo, J.; Collar, N. J. 2016. HBW and BirdLife International Illustrated Checklist of the Birds of the World. Volume 2: Passerines. Lynx Edicions and BirdLife International, Barcelona, Spain and Cambridge, UK.

De Oliveira Soares, F. M. C. 2017. Modelling the distribution of São Tomé bird species: Ecological determinants and conservation prioritization. Masters Thesis, Universidade de Lisboa.

IUCN. 2001. IUCN Red List Categories and Criteria: Version 3.1. Gland, Switzerland and Cambridge, UK: IUCN Species Survival Commission.

IUCN. 2012. Guidelines for Application of IUCN Red List Criteria at Regional and National Levels: Version 4.0. Gland, Switzerland and Cambridge, UK: IUCN.

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.

van Balen, B. 2018. Principe White-eye (Zosterops ficedulinus). 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/60225 on 2 March 2018).

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2 Responses to Archived 2018 topic: Sao Tome White-eye (Zosterops feae): revise global status?

  1. Ricardo Faustino de Lima says:

    I agree that this species most likely does not approach the threshold for VU under population trend (A) or distribution range criteria (B). We lack evidence-based population estimates to use population size criteria (C) with confidence. However, considering frequency data, it seems highly unlikely that this species would trigger criteria D1.

    I would argue that this species might have more than 10,000 mature individuals, but as a precautionary measure I would include it in the 2,500-10,000 mature individuals range. This means that it would classify as VU in case it undergoes a continuing decline. We lack information on population trends, but its preference for areas of intermediate disturbance might warrant being classified as NT, to be upgraded to VU in case there is any indication that it might be undergoing a continuing decline and simultaneously meet other subcriteria, relevant for criteria C.

    I consider that the historical fluctuations suggested by the bibliography might be linked with the difficulty in detecting this species, which is very small and often found in the canopy.

    Therefore, I agree with the proposal of classifying this species as NT under criterion C2a(ii), as a precautionary measure that is neither too conservative nor too risky.

    Together with the Giant Weaver, the Sao Tome White-eye seems to be the only endemic preferring areas of intermediate disturbance (secondary forest and shade plantations – de Oliveira Soares 2017). Both species present similar distribution ranges (777 sqkm for the weaver and 816 sqkm for the white-eye), but the white-eye seems to be significantly more frequent (it occurred in 566 systematic point counts vs 267 for the weaver – based on de Oliveira Soares 2017). In comparison with other species, it would be consistent to list both as NT. Considering frequency data, that the white-eye is much smaller and hard to detect, and that it can occur in large flocks (> 20 individuals), if one of these species is to be attributed a higher threat status, I would clearly elect the weaver rather than the white-eye.

  2. James Westrip (BirdLife) says:

    Preliminary proposals

    Based on available information, our preliminary proposal for the 2018 Red List would be to list this species as Near Threatened under criterion C2a(ii).

    There is now a period for further comments until the final deadline in mid-July, after which the recommended categorisations will be put forward to IUCN.

    Please note that we will then only post final recommended categorisations on forum discussions where these differ from the initial proposal.
    The final 2018 Red List categories will be published on the BirdLife and IUCN websites in November, following further checking of information relevant to the assessments by both BirdLife and IUCN.

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