Archived 2018 topic: Sao Tome Thrush (Turdus olivaceofuscus): revise global status?

Sao Tome Thrush (Turdus olivaceofuscus) is endemic to the island of São Tomé, São Tomé and Príncipe. Favouring the lowlands, it inhabits predominantly forest habitats, mainly primary and secondary forest, but will also occur in shaded plantations, orchards, dry woodland in savanna, cloud forest and non-forest habitats like gardens (Atkinson et al. 1991, Christy and Clarke 1998, Dallimer et al. 2010, de Oliveira Soares 2017, Collar 2018). It does, however, appear to be more abundant in forest compared to degraded habitat like plantations (de Lima et al. 2014). Thus, given suspected ongoing habitat loss and degradation, as well as predation by introduced species and persecution by humans the species is currently considered to be undergoing a continuing decline (del Hoyo et al. 2005, R. F. de Lima in litt. 2010). With its restricted range and small population size, the species is therefore currently listed as Near Threatened (see BirdLife International 2018).

Conversely though, it has been suggested that the species may in fact be stable (R. F. de Lima in litt. 2018) and, as it is relatively frequent throughout the island, it may warrant downlisting (R. F. de Lima in litt. 2018). Therefore, in light of this, the species has been reassessed here against all criteria.

 

Criterion A – Even if the species is considered to be declining (see above) it is very unlikely that the species would approach the threshold for Vulnerable under this criterion (population size reduction of 30% over 3 generations or 10 years).

 

Criterion B – While the species has a restricted range (Extent of Occurrence = 930km2) and at least the area/quality of habitat may be declining, it is not thought to undergo extreme fluctuations and it is not severely fragmented. Additionally, as the main threat to the species is habitat loss/degradation, the number of locations* where the species is found is likely to be >>10. Therefore, the species likely does not warrant listing under this criterion.

 

Criterion C – The population size has not been directly estimated, but it is frequently observed (found at 1,111 out of 3,056 point count observations [de Oliveira Soares 2017]), and it is thought to number more than 2,500 individuals (R. F. de Lima in litt. 2010). Thus it is currently placed in the range 2,500-9,999 mature individuals. This is sufficiently low to meet the threshold population size estimate for listing as Vulnerable, but if the species were thought to be stable as per R. F. de Lima (in litt. 2018) then the species would not meet the other required conditions for listing under this criterion.

However, given that it is thought that there are ongoing threats to the species’s habitat, and it is less abundant in more degraded habitats compared to forest (de Lima et al. 2014) it may be more appropriate to suspect that the species is undergoing a continuing decline. The uncertainty over this though means that, at the moment it would likely not warrant listing at a higher level of threat than Near Threatened under criterion C2a(ii). We therefore request further information regarding the population trend.

 

Criterion D – The population size and range of the species are too large for the species to warrant listing under this criterion.

 

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.

 

Overall, therefore, Sao Tome Thrush could be downlisted to Least Concern. However, if the threat from habitat loss and or degradation is suspected to potentially be causing a slow population decline it may be more appropriate to retain the species as Near Threatened under criterion C2a(ii), given the uncertainty. Comment and further information are welcome, but 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 about the proposed listing.

 

 

*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: Turdus olivaceofuscus. Downloaded from http://www.birdlife.org on 01/03/2018.

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

Collar, N. 2018. Sao Tome Thrush (Turdus olivaceofuscus). 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/58286 on 28 February 2018).

Dallimer, M.; Melo, M.; Collar, N. J.; Jones, P. J. 2010. The Príncipe Thrush Turdus xanthorhynchus: a newly split, ‘Critically Endangered’, forest flagship species. Bird Conservation International 20(4): 375-381.

del Hoyo, J.; Elliott, A.; Christie, D. 2005. Handbook of the Birds of the World, vol. 10: Cuckoo-shrikes to Thrushes. Barcelona, Spain: Lynx Edicions.

De Lima, R. F.; Viegas, L.; Solé, N.; Soares, E.; Dallimer, M.; Atkinson, P. W.; Barlow, J. 2014. Can management improve the value of shade plantations for the endemic species of São Tomé Island? Biotropica 46: 238-247.

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.

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4 Responses to Archived 2018 topic: Sao Tome Thrush (Turdus olivaceofuscus): revise global status?

  1. Ricardo Faustino de Lima says:

    I propose that this species is downlisted for LC, for sake of consistency, namely because it is at least as widespread (it occurs pretty much across the entire island) and as frequent as other endemics currently listed as LC, such as the Sao Tome Paradise-flycatcher (the thrush was recorded in 1109 systematic point counts vs 876 for the flycatcher – based on de Oliveira Soares 2017).

    We lack reference data to assess this species in regard of population trends (criteria A).

    Despite being true that it is less abundant in non-forested areas and more sensitive than other non-threatened endemic species, the species still occurs there (see Fig. 1.6 in de Oliveira Soares 2017) and is abundant in forested areas. Bird ringing data even suggests that, in forest areas this species might be more abundant that what point count data suggests, being the second most frequent species caught in the mistnets (de Lima 2017).

    Taking into consideration this information and the assessment above, I argue that the remaining criteria do not apply either.

    de Lima, R.F. 2017. The Rufford Foundation Final Report: The Role of Bird Seed Dispersal on São Tomé Forest Dynamics. https://www.rufford.org/files/18618-B%20Final%20Report.pdf. Dowloaded 29 June 2018

  2. James Westrip (BirdLife) says:

    Preliminary proposals

    We would just like to confirm, the proposal to list the species as Near Threatened is based on a population size of 2,500-9,999 mature individuals, all in one subpopulation, where there is some evidence for a decline but this is only suspected, therefore, not quite triggering the conditions for listing as Vulnerable. The evidence for a decline is that the species is not as frequently seen in degraded habitat. Therefore, with continuing habitat degradation it is possible to suspect a continuing decline. How widespread the species is is therefore not relevant unless it would allow us to make a changed population size estimate. Given the evidence for a possible decline due to habitat degradation, in order to not approach the threshold for Vulnerable would require the population size estimate to be >20,000 mature individuals. Is this likely? If so the species could be listed as Least Concern.

    Given this requires further thought, our preliminary proposal for the 2018 Red List would be pend a decision on the species’s status until 2019 and retain the species’s current listing in the 2018 Red List.

    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 those in 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.

  3. Ricardo Faustino de Lima says:

    Dallimer et al. (2009) estimated an overall density of 4.28 São Tomé thrushes in native forest. Considering that there are 226 sqkm of native forest left in São Tomé (low estimate based on Soares 2017 – Jones et al. 2001 provided similar, but somewhat higher estimates), this would represent a population of nearly 100,000 individuals on the native forest alone.
    Data from Soares (2017) suggests that the frequency of the species in native forest is similar to that in secondary forest and shade plantations. Extrapolating to the 731 sqkm covered by those three land use types, using the population density estimates for native forest provides a population estimate over the 300,000 individuals.
    Therefore, I argue it is safe to assume that the population is >20,000 mature individuals and that the species can be listed as Least Concern straight away.

    Dallimer, M.; King, T.; Atkinson, R.J. 2009. Pervasive threats within a protected area: conserving the endemic birds of São Tomé, West Africa. Animal Conservation 12: 209-219.

  4. James Westrip (BirdLife) says:

    Recommended categorisation to be put forward to IUCN

    Following further review, the recommended categorisation for this species has been changed to Least Concern.

    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.

Comments are closed.