BirdLife species factsheet for Dapple-throat
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.
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.