Dapple-throat (Arcanator orostruthus): revise global status?

BirdLife species factsheet for Dapple-throat

This discussion was first published as part of the 2020 Red List update. At the time a decision regarding its status was pended, but to enable potential reassessment of this species as part of the 2021 Red List update this post remains open and the date of posting has been updated.

Dapple-throat (Arcanator orostruthus) occurs in Mozambique (Mt Namuli and Mt Mabu) and Tanzania (East Usambara Mountains and Udzungwa Mountains). The species inhabits undisturbed wet montane forest, especially near streams and in shrubby areas (Collar and Robson 2020). It is found at elevations between 900 and 1,800 m (Dinesen et al. 2001, Dowsett-Lemaire 2010, Collar and Robson 2020). The global population has not been quantified, but is preliminarily placed in the band 6,000-15,000 mature individuals.

Dapple-throat is thought to be in decline as habitat within its range are fragmented and degraded. While large tracts of forests remain intact, lower zones within the range are densely settled by now, and encroachment is taking place also above 1,500 m (Dowsett-Lemaire 2010). Forests are being cleared mainly for agricultural use and timber extraction (Dowsett-Lemaire 2010, J. Fjeldså in litt. 2012, J. Timberlake in litt. 2020).

The species has been considered Vulnerable under Criterion B1ab(i,ii,iii,v) (BirdLife International 2020). However, this is no longer tenable because this was based on an Extent of Occurrence (EOO) value calculated as the ‘area of mapped range’. This is no longer appropriate and the EOO should be calculated using a Minimum Convex Polygon (see IUCN 2001, 2012, Joppa et al. 2016), as EOO is a measure of the spatial spread of areas occupied by a species, not the actual area it occupies. The resulting EOO value now exceeds the thresholds required to maintain the species’s current listing, and as such it potentially cannot retain its current Red List status. Therefore, we have fully reviewed the species here against all criteria.

Criterion A – The population trend for this species has not been directly estimated. The only threat know to Dapple-throat is habitat loss, and Global Forest Watch (2020) report a loss of tree cover at a rate of 8% between 2001 and 2018 within its range. Assuming that the decline in tree cover is exponential and continuing at the same rate, this equates to a rate of habitat loss of 5% over ten years. Assuming further that the population declines at roughly the same rate as tree cover, we can tentatively place the population decline in the band 1-9% over ten years (one generation length being 3.2 years; Bird et al. 2020)*. This is too low to qualify for listing as threatened, and therefore Dapple-throat may be considered Least Concern under Criterion A.

Criterion B – The newly calculated Extent of Occurrence (EOO) for this species is 236,000 km2. This is too large to meet the threshold for Vulnerable under Criterion B1, and Dapple-throat may be listed as Least Concern under this criterion. The Area of Occupancy (AOO) has not been quantified following IUCN Guidelines (IUCN Standards and Petitions Committee 2019), and therefore the species cannot be assessed against Criterion B2.

Criterion C – The global population is estimated to number 6,000-15,000 mature individuals. Assuming that the true population size is closer to the lower end of the estimate, the species meets the threshold for Vulnerable under Criterion C. As the population decline is suspected however, the species does not fully meet the conditions for listing as threatened, and can thus at most be listed as Near Threatened under this criterion. The population decline is likely < 10% over ten years, and as such Dapple-throat does not approach the threshold for listing under Criterion C1. Moreover, even though it is conceivable that the species forms four subpopulations given the fragmentation of the range, it is highly unlikely that the largest subpopulation numbers less than 1,000 mature individuals. Therefore overall, the species does not meet sufficient conditions to list as threatened or Near Threatened under Criterion C, and may thus be considered Least Concern under this criterion.

Criterion D – The global population and range are too large to warrant listing as threatened, and thus the species may be considered Least Concern under this criterion.

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, it is suggested that Dapple-throat (Arcanator orostruthus) be listed as Least Concern.We welcome any comments to the proposed listing.

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’ Red List status. Therefore, please make sure your comments are relevant to the species’ Red List status and the information requested. By submitting a comment, you confirm that you agree to the Comment Policy.

*Bird generation lengths are estimated using the methodology of Bird et al. (2020), as applied to parameter values updated for use in each IUCN Red List for birds reassessment cycle. Values used for the current assessment are available on request. We encourage people to contact us with additional or improved values for the following parameters; adult survival (true survival accounting for dispersal derived from an apparently stable population); mean age at first breeding; and maximum longevity (i.e. the biological maximum, hence values from captive individuals are acceptable).

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

Bird, J. P.; Martin, R.; Akçakaya, H. R.; Gilroy, J.; Burfield, I. J.; Garnett, S.; Symes, A.; Taylor, J.; Šekercioğlu, Ç.; Butchart, S. H. M. (2020). Generation lengths of the world’s birds and their implications for extinction risk. Conservation Biology online first view.

BirdLife International. 2020. Species factsheet: Arcanator orostruthus. http://www.birdlife.org (Accessed 28 February 2020).

Collar, N.; Robson, C. 2020. Dapple-throat (Arcanator orostruthus). 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, Spain. https://www.hbw.com/node/59689 (Accessed 28 February 2020).

Dinesen, L.; Lehmberg, T.; Rahner, M. C.; Fjeldsa, J. 2001. Conservation priorities for the forests of the Udzungwa Mountains, Tanzania, based on primates, duikers and birds. Biological Conservation 99: 223-236.

Dowsett-Lemaire, F. 2010. Further ornithological exploration of Namuli and Mabu Mountains (northern Mozambique), and the urgent need to conserve their forests. Bulletin of the African Bird Club 17(2): 159-177.

Global Forest Watch. 2020. World Resources Institute. http://www.globalforestwatch.org (Accessed 28 February 2020).

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 Standards and Petitions Committee. 2019. Guidelines for using the IUCN Red List Categoreis and Criteria. Version 14. http://www.iucnredlist.org/documents/RedListGuidelines.pdf.

Joppa, L. N.; Butchart, S. H. M.; Hoffmann, M.; Bachman, S. P.; Akçakaya, H. R.; Moat, J. F.; Böhm, M.; Holland, R. A.; Newton, A.; Polidoro, B.; Hughes, A. 2016. Impact of alternative metrics on estimates of extent of occurrence for extinction risk assessment. Conservation Biology 30: 362-370.

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8 Responses to Dapple-throat (Arcanator orostruthus): revise global status?

  1. Red List Team (BirdLife International) says:

    Global Forest Change data on tree cover loss up to 2019 have now been released and made available via Global Forest Watch. Based on these data, over ten years approximately 3.6% of tree cover with 75% canopy cover was lost from within the species’s range (Global Forest Watch 2020). This does not affect the above assessment under Criterion A.

  2. Sam Jones says:

    The information here with regards to the Mozambican distribution is outdated. Arcanator orostruthus also occurs at the Njesi Plateau north of Lichinga in Niassa – (see Jones et al. 2017, 2020 – linked at bottom).
    This has been communicated to numerous individuals via KBA assessments etc quite exhaustively, but curiously still remains absent!

    https://www.researchgate.net/publication/320710912_The_Njesi_Plateau_expedition_a_biological_assessment_of_Mt_Chitagal_Mt_Sanga_and_the_Njesi_Plateau_in_Niassa_Province_Mozambique?_sg=Iiia5hswxFXsDzOuQWnmslE54zxChvWj9IuaXMIMKzxQ4iJxzX0B0wB9KfkiX2xiK0j89n08LOc9xq6pxLQHA3d8xW5B27_UHtq1IAGC.FIf3h7ZDp4LUUQKuS5-W5elCwa0mDmbDSknFRrePUj2X_dRNkvG1brxcJmc4EJCWuijCrFqbBVkSR3ugcvFAUg
    https://www.tandfonline.com/doi/abs/10.2989/00306525.2019.1675795

  3. Red List Team (BirdLife International) says:

    It has come to our attention that information kindly provided by F Dowsett-Lemaire and J Timberlake was mistakenly omitted from the initial assessment above. It is therefore provided below, and has the following effect on the proposal:

    The current population size for Dapple-throat is estimated to be in the band of 6,000-15,000 mature individuals (BirdLife International, 2020). The population on Mt Namuli was estimated to be 300-500 pairs (600-1,000 mature individuals) in 2007 (Dowsett-Lemaire, 2010).

    The forest loss on Mt Namuli between 2007 and 2017 is estimated to be up to 80% in some areas, and the situation is continuing to deteriorate (Timberlake, 2017). If we take a precautionary approach and assume that the Mt Namuli population is declining at the same rate, an 80% reduction from 600-1,000 mature individuals equates to a loss of 480 – 800 mature individuals. If we assume the true population size is near the lower end of the estimate, a loss of 480-800 individuals from an estimated population of 6,000 equates to a rate of loss of 8-13% over the last 10 years, based on the decline at Mt Namuli alone.

    In Tanzania, this species is found in the East Usambara and Udzungwa mountains. According to data in Hall et al., (2009), forest cover in the East Usambara mountains in 1975 was 299km2. By 2000, this had reduced to 263km2. In Udzungwa, there was 1402km2 of forest cover in 1975, which had fallen to 1354km2 in 2000. Combined, this equates to a reduction from 1701km2 in 1975, to 1617km2 in 2000, a reduction of roughly 5% over the 25 years. If we assume that the rate of decline is continuing, and that the decline is exponential, then this equates to a rate of decline of roughly 2% over 10 years. The data from Global Forest Watch for most of the last two decades indicated a rate of loss of 5% over 10 years across the whole range, but it is plausible that the actual rate of forest loss in Tanzania is higher than detected by Global Forest Watch, as it is at Mt Namuli. Additionally, while the population in Tanzania was estimated to be c.10,000 (L. Dinesen in litt. 2007), this is likely to be an overly optimistic estimate (L. A. Hansen per J. Fjeldså in litt. 2012) as the population at Udzungwa is apparently common in only a few areas (L. Dinesen in litt. 2007) . As such, it is not completely clear what proportion of the global population is represented by the population on Namuli.

    If we assume that the global population size is close to the lower estimate of 6,000 mature individuals, of which 600-1,000 are at Namuli, and a maximum of 200 are on Mt Mabu (Dowsett-Lemaire, 2010), then we can assume that the Tanzania population may be 4,800 – 5,200 mature individuals. Assuming also that this population has declined by at least 5% in the last 10 years (as suggested by Global Forest Watch) then the population may now be 4,560-4,940 mature individuals. Combined with the estimated remaining population at Namuli (assuming an 80% decline and 120-200 mature individuals remaining), and a remaining 200 individuals at Mabu, the global population could be 4,880 – 5,340 mature individuals remaining from the original 6,000. This equates to a possible ten-year decline of 11-19%.

    Therefore, in the absence of more conclusive data from Tanzania, the 10-year rate of decline is tentatively placed in the 10-20% band, and further information may result in a more accurate decline estimate.

    This does not affect the proposal under Criterion A, but if further information indicates that the population is likely to have undergone a reduction of more than 20% over the past ten years, the species may be assessed as a higher category of threat under this Criterion.

    Criterion C has been reassessed as follows:

    Criterion C: The global population is estimated to number 6,000-15,000 mature individuals. Assuming that the true population size is closer to the lower end of the estimate, the species meets the threshold for Vulnerable under Criterion C. In order to fully qualify as threatened under this criterion, other conditions must be met. As this species is a forest specialist, and such a large area of forest has been lost at Namuli, a continuing population decline can be inferred. Even though it is conceivable that the species forms four subpopulations given the fragmentation of the range, it is highly unlikely that the largest subpopulation numbers less than 1,000 mature individuals. Additionally, it does not meet the requirement that 100% of mature individuals exist in one subpopulation, which therefore precludes the listing of this species under Criterion C2. A rate of decline of 10-20% over ten years is suspected for this species. Because this rate is only suspected, it does not fully meet the criteria for classification as Vulnerable, however it may be considered Near Threatened, approaching classification as threatened under criteria C1.

    References:

    Dowsett-Lemaire, F., 2010, Further ornithological exploration of Namuli and Mabu Mountains (northern Mozambique), and the urgent need to conserve their forests, Bulletin African Bird Club, Vol 17, No 2, pp 161-162, 168.

    Hall, J., Burgess, N.D., Lovett, J., Mbilinyi, B., and Gereau, R.E., 2009, Conservation implications of deforestation across an elevational gradient in the Eastern Arc Mountains, Tanzania, Biological Conservation, 142 (11), pp:2510-2521

    Timberlake, J., 2017, Mt Namuli ‒ a conservation update, report for Legado, Mozambique, Unpublished report.

  4. Red List Team (BirdLife International) says:

    Many thanks to everyone who has contributed to this discussion. We greatly appreciate the time and effort invested by so many people in commenting. The window for consultation is now closed. We will analyse and interpret the new information and post a preliminary decision on this species’s Red List status on this page in early July.

    Thank you once again,
    BirdLife Red List Team

  5. Red List Team (BirdLife International) says:

    Preliminary proposal

    Based on available information, our preliminary proposal for the 2020 Red List would be to list Dapple-throat as Near Threatened under Criterion C1.

    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 2020 Red List categories will be published on the BirdLife and IUCN websites in December 2020/January 2021, following further checking of information relevant to the assessments by both BirdLife and IUCN. The final publication date will be publicised by IUCN here: https://www.iucnredlist.org/assessment/updates

  6. Ben Phalan says:

    I forward some additional comments on this species from Françoise Dowsett-Lemaire:

    “My conclusion on Dapplethoat in Mozambique is that Mabu is of very secondary importance (cf. my 2010 paper). The bird is rare and limited to altitudes above 1400 m. With warming climate, it will likely go extinct there. Had I told you that I discovered that two montane species had become extinct on Mt Mangochi in Malawi? (my visit was in Oct 2017), and yet the forest on Mangochi was completely intact. Both Abyssinian Hill Babbler (Pseudoalcippe abyssinica, now a Sylvia) and Cholo Alethe have disappeared. Our last surveys were in 1983, when the alethe was rare (the forest was rather dry for this sp.) and the alcippe then in very small numbers (2-3 territories located in the best part of the forest). So extinctions can happen even when the habitat is preserved.

    Namuli forest was far more important for Dapplethroat (and of course for Namuli Apalis), given that most forest was at high altitude there (the big block of Manho forest).”

  7. Ben Phalan says:

    Further comments from Ben Phalan:

    I don’t have personal experience of this species, but have gone through the discussions here in detail, as well as the reports by Jones et al. and Timberlake, and discussed this case with Françoise Dowsett-Lemaire. To me, there seems a reasonable probability that the true population size is substantially lower than the band in which it is estimated. Considerable declines have taken place in parts of the species’ distribution, and in the absence of more concrete evidence from other parts, a conservative assumption would be to assume the same degree of decline has occurred in those too.

    The total population in Mozambique is known from expeditions to be <1000, with most of them at Mt Namuli, and smaller populations at Mt Mabu and Mt Chitagal.

    In Tanzania, there are few recent data. The guesstimate of 5000 mature individuals in that country is mostly based on decades-old data and may well be far too high now. The BirdLife species account says that in recent years "the total population in the East Usambaras was considered to possibly only number a few pairs (Borghesio et al. 2008)", having been estimated at as much as "several thousand" in 1990. That only leaves the Udzungwa Mountains, where there seems to be little new information since the 1990s. As the species is quite local, loss or degradation of key forest areas, even if not adding up to a high deforestation rate overall, could have caused substantial declines. I would be cautious using Global Forest Watch data to estimate loss of habitat, because forest cover is a highly imperfect proxy for habitat, and those data do not account for the effects of forest degradation on habitat quality.

    My suggestion would be to leave this species at Vulnerable until there is more concrete information on population size and trend, especially from the Udzungwas. There is also a strong case for calculating Area of Occupancy before any further revision of this species' status, as it is restricted to a few small patches of moist montane forest scattered across a much larger Extent of Occurrence.

  8. Red List Team (BirdLife International) says:

    Recommended categorisations to be put forward to IUCN

    Based on the possible but unconfirmed population reduction in Tanzania, our proposal for the 2020 Red List is to pend the decision on this species, awaiting further information on the size of the Tanzanian population. The discussion for Dapple-throat will be kept open until 2021, while the current Red List category will remain unchanged in the 2020 update.

    Final 2020 Red List categories will be published on the BirdLife and IUCN websites in December 2020/January 2021, following further checking of information relevant to the assessments by both BirdLife and IUCN.

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