Archived 2018 topic: Sao Tome Green-pigeon (Treron sanctithomae): request for information.

Sao Tome Green-pigeon (Treron sanctithomae) is now only found on the island of São Tomé, São Tomé and Príncipe having formerly also occurred on the nearby Ilhéu das Rolas. It is predominantly a forest species, but will tolerate some forest fragmentation and can occur in plantations (though it does not use these disturbed habitats as much as mature growth forest) (Gibbs et al. 2001, Carvalho et al. 2015, de Oliveira Soares 2017, Baptista et al. 2018).

Habitat loss is thought to have been the main factor behind its historical disappearance from Ilhéu das Rolas, and forest loss and degradation continues on São Tomé driven by human population growth and investment in agriculture (R. F. de Lima in litt. 2010). Currently, though, the major threat to the species is from hunting, which has led to large reductions in abundance or its complete disappearance from some sites (F. Olmos, M. Carvalho & R. F. de Lima in litt. 2009, 2011). This is thought to be causing rapid declines, and as such the species is currently listed as Vulnerable (see BirdLife International 2018b).

While conducting a review of the status of all bird species endemic to São Tomé, this species was considered to potentially warrant an update to its Red List category. Therefore, the species has been reassessed here against all criteria to engage discussion about whether a change in status is appropriate.

 

Criterion A – The species is suspected to be declining rapidly due principally to hunting, with habitat loss a potentially severe additional threat due to the species’s greater use of mature growth (Gibbs et al. 2001, Carvalho et al. 2015, Baptista et al. 2018). The species has disappeared from, or is now rarely seen in, accessible areas, with hunters reporting capture rates of less than half that of those only five years previously (F. Olmos, M. Carvalho & R. F. de Lima in litt. 2009, 2011). While the proportion of the range accessible to hunters is unclear, this appears to represent a real index of abundance that is of considerable concern. The overall rate of decline had been suspected to fall in the range 30-49% over three generations (12.6 years), which qualifies the species for listing as Vulnerable under criteria A2cd+3cd+4cd. If, however, those hunters’ capture rates changes accurately reflect the population trend for the species then the rate of population decline may be as high as c.83% over three generations, which would mean the species would be at the borderline of Endangered and Critically Endangered. Therefore, we request further information to assess how rapidly the population may be declining, but in the absence of such information it is very tentatively proposed that, under criterion A at least, the species would remain listed as Vulnerable.

 

Criterion B – The species has a restricted range, with an Extent of Occurrence (EOO) of 930km2, falling below the threshold for Endangered under criterion B1. Its Area of Occupancy has not been calculated per IUCN guidelines (see IUCN Standards and Petitions Subcommittee 2017), but it would still at least fall beneath the threshold for Vulnerable under criterion B2 (2,000km2). However, at least two further conditions (a, b and/or c) need to be met for a species to warrant listing under criterion B.

Given continued habitat loss/degradation, the impacts from hunting and information suggesting that the species has disappeared from certain sites it is likely that the species is undergoing continuing decline in AOO, habitat area/quality, and the number of mature individuals. Thus fulfilling subcriteria b(ii,iii,v).

The species is not believed to undergo extreme fluctuations (subcriterion c), and as of yet it is not thought to be severely fragmented. However, as the major threat to the species is hunting it is likely that the number of locations* where it occurs is <10, and potentially the island could be considered as only one location. Therefore, the species likely fulfils subcriterion a) to at least warrant listing as Vulnerable under criteria B1ab(ii,iii,v)+2ab(ii,iii,v), and could potentially warrant listing as Endangered under criterion B1ab(ii,iii,v), given the information currently available to us.

 

Criterion C – The population size is currently suspected to fall below 10,000 individuals (R. F. de Lima in litt. 2010) and so is placed in the range 2,500-9,999 mature individuals. De Oliveira Soares (2017) recorded the species from 784 out of 3,056 point counts, c.3.6 times the number of Columba thomensis, whose population size is currently placed in the range 250-999 mature individuals (see BirdLife International 2018a). Meanwhile, Carvalho et al. (2015) recorded c.10x as many T. sanctithomae individuals as for C. thomensis. Therefore, it is tentatively suggested, based on descriptions of abundance, that it should remain in the current population size range, although we welcome further information regarding this.

The species is considered to be undergoing a continuing decline, and occurs in one subpopulation so warrants listing as threatened under criterion C2a(ii). It is not known to undergo extreme fluctuations, so it would not warrant listing under criterion C2b; and the rate of decline is only suspected, so we cannot clearly asses it against criterion C1, based on our current information. Therefore, the species would merit listing as Vulnerable under criterion C2a(ii), and if the population size was considered to be smaller (e.g. 1,000-2,499 mature individuals) then it could warrant listing as Endangered under the same criterion.

 

Criterion D – If we retain the current population size estimate of 2,500-9,999 mature individuals, then the species would not approach the threshold for Vulnerable under criterion D1, but if it were to be moved to the range 1,000-2,500 mature individuals it may warrant listing as Near Threatened under this criterion.

Despite a small range size it is not suspected to have a restricted enough AOO to trigger criterion D2. However, as the main threat to the species is from hunting the number of locations* where it is found may approach or meet the threshold under this criterion. But to warrant listing as Vulnerable under this criterion also requires the species to have the potential to become Critically Endangered or Extinct within a short period of time (one or two generations [4.2-8.4 years]; IUCN Standards and Petitions Subcommittee 2017). This is possible, particularly given extrapolations from capture rate trends, but uncertain. Therefore, it could warrant listing as Vulnerable under criterion D2, but may more appropriately be listed as Near Threatened, given the uncertainty.

 

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.

 

Therefore, overall, the species at least warrants retaining its listing as Vulnerable, but under an expanded criteria string of A2cd+3cd+4cd; B1ab(ii,iii,v)+2ab(ii,iii,v); C2a(ii) (and also potentially D2 too). However, the species could warrant uplisting to Endangered with the strongest case being for it to be listed as such under criterion B1ab(ii,iii,v), although depending on further comments or information it also could warrant listing as Endangered under criteria A2cd+3cd+4cd, B2ab(ii,iii,v) and C2a(ii). Therefore, we request information as to whether the species may warrant uplisting; most importantly asking a) is the rate of decline implied by reduced capture rates may be typical of the whole population?, b) could the island could be considered just one location* given the main threat is from hunting?, and c) could the population size range be considered to be lower than currently listed?

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

Baptista, L. F.; Trail, P. W.; Horblit, H. M.; Boesman, P. 2018. Sao Tome Green-pigeon (Treron sanctithomae). 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/54302 on 1 March 2018).

BirdLife International. 2018a. Species factsheet: Columba thomensis. Downloaded from http://www.birdlife.org on 01/03/2018.

BirdLife International. 2018b. Species factsheet: Treron sanctithomae. Downloaded from http://www.birdlife.org on 01/03/2018.

Carvalho, M.; Fa, J. E.; Rego, F. C; de Lima, R. F.; Santos, G.; Palmeirim, J. M. 2015. Factors influencing the distribution and abundance of endemic pigeons on São Tomé Island (Gulf of Guinea). Bird Conservation International 25: 71-86.

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.

Gibbs, D.; Barnes, E.; Cox, J. 2001. Pigeons and doves: a guide to the pigeons and doves of the world. Pica Press, Robertsbridge, U.K.

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.

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4 Responses to Archived 2018 topic: Sao Tome Green-pigeon (Treron sanctithomae): request for information.

  1. Ricardo Faustino de Lima says:

    The lack of historical evidence-based population estimates makes it hard to estimate population trends (criteria A) for the Sao Tome Green-pigeon, even though hunting pressure on the species seems to be unsustainable (Carvalho 2015), and responsible for recent declines both in abundance and distribution area (Carvalho et al. 2015).

    The potential range of the species was recently estimated well below previous EOO estimates: 291 sqkm (based on de Oliveira Soares 2017) vs. 930 sqkm (BirdLife International 2018b). Hunting focusing on this species is driven by demand around the capital (Carvalho et al. 2015b) and therefore this species should be considered as occurring on a single location. There is also multiple sources of evidence that the range, quality of the habitat and mature population size are under continuing decline and thus the species could be classified as EN using criteria B1ab(ii,iii,v)+2ab(ii,iii,v).

    Provisional population estimates suggest that, despite the small range, this species is locally abundant and might have a population larger than the 10,000 mature individuals required to trigger criterion C for VU, and certainly well above the 2,500 required for EN (min. 37,007 individuals in total – Carvalho 2015). Recent data also suggests that the species is very frequent (recorded at 778 systematic bird point counts, almost one third of the total), despite being absent from almost two thirds of the island (based on de Oliveira Soares 2017). Considering this information, I do not think that this species warrants EN listing based on population size criteria (C & D1).

    Regarding the abovementioned questions:

    a) is the rate of decline implied by reduced capture rates may be typical of the whole population?
    I do not think we have enough data to support this, and therefore suggest we avoid using population trend criteria (A) to assess this species.

    b) could the island could be considered just one location* given the main threat is from hunting?
    Definitely yes.

    c) could the population size range be considered lower than currently listed?
    No. Despite the distribution range being even smaller than previously assumed, the species can be locally abundant and does not seem to trigger population size criteria (C & D1) not even for being classified as VU.

    Additional references:
    Carvalho , M. , J.M. Palmeirim, F.C. Rego, N. Sole, A. Santana & J.E. Fa. 2015b. What motivates hunters to target exotic or endemic species on the island of São Tomé, Gulf of Guinea? Oryx 49: 278-286.

    Carvalho, M. (2015) Hunting and conservation of forest pigeons in São Tomé (West Africa). PhD thesis. Lisbon University, Portugal. (https://www.repository.utl.pt/handle/10400.5/9265?locale=en)

  2. Jorge Palmeirim says:

    The estimates of the population numbers are still quite rough, and it seems safer to base the evaluation mostly on the range and area of occupancy. Thus I also believe that the species should be moved to EN but based on B1 and B2.

  3. 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 Endangered under criterion B1ab(ii,iii,v). We highlight though that the EOO has to be calculated using a Minimum Convex Polygon rather than representing an area of mapped range (which will therefore include areas of unoccupied range, hence our larger range value); and AOO has to be calculated using 2x2km2 grid, which has not been done accurately yet and so we cannot compare the species to the threshold values for B2 given the current data.

    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.

  4. Ricardo Faustino de Lima says:

    I have tried to assess AOO using a 2x2km grid. There are records of the species in 128 of those quadrats, meaning that the species has an AOO that surpasses the 500 sqkm threshold, thus not classifying as EN under criteria B2. Considering that some (few) areas of São Tomé have not been surveyed, that others are seldom visited, and that the species might have seasonal movements, it is likely that the species occurs in more than those 128 2x2km quadrats.
    Therefore, I agree with listing this species as Endangered under criterion B1ab(ii,iii,v) only.

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