The Silver-capped Fruit-dove (Ptilinopus richardsii) is found only on Uki, Three Sisters, Santa Ana, Santa Catalina, Bellona and Rennell in the Solomon Islands. It occurs in lowland forest including hurricane-damaged forest, and on Uki visits isolated trees away from main forested areas (Baptista et al. 2019). Its population size has not been estimated, but it has been reported to be abundant on Uki in the 1950s and common on Rennell (Baptista et al. 2019) and is currently listed as Least Concern.
We have received reports that Bauxite mining has recently been underway in Rennell (Hughes and Tuhanuku 2015, M. O’Brien in litt. 2018). Hence, we are undertaking a review of the species’s Red List Category.
Our current information on the species’s conservation status will now be compared to all Red List Criteria.
Criterion A – We have no direct data on population trends. Data on tree cover loss indicates that there is unlikely to have been forest loss approaching the level of 30% over the past ten years (Hansen et al. 2013). In the absence of other known threats, the population is not considered likely to have undergone a reduction at a rate approaching 30% over the past ten years, and we have no evidence to suggest that it will over the next ten years. The species is therefore assessed as Least Concern under this Criterion A.
Criterion B – The species’s Extent of Occurrence (EOO) (which according to IUCN guidelines includes the areas of ocean between occupied islands) is estimated at 27,400 km2. This does not meet the threshold for listing the species as threatened under Criterion B1, although it could qualify the species for listing as Near Threatened under this criterion, depending on whether or not further conditions are met. The species’s Area of Occupancy (AOO) has not been quantified, but based on a 4 km2 grid placed over the area of mapped range, must be smaller than 1,408 km2. This meets the threshold for Vulnerable under Criterion B2. However, to list the species as threatened on the Red List under Criterion B, two of conditions a-c must also be met.
Since a large majority of the species’s range (and, it may be inferred, its population) is found on one island (Rennell), the species does not qualify as severely fragmented. The main known potential threat to the species is forest loss. Since the forest loss data indicates that this is occurring at a slow rate (Hansen et al. 2013), the species is not likely to have 10 or fewer locations, meaning that condition a is not met. If there is information to indicate a greater rate of forest loss, or a different threat (such as invasive species or hunting) having a greater impact on the species, the number of locations could be revised, yet it does indicate that there may be some ongoing loss of habitat. There is no evidence that the species’s population or range size are undergoing extreme fluctuations. Condition c is not met.
The species’s AOO falls beneath the threshold for listing the species as Vulnerable under Criterion B2. However, it is not clear that two of the three conditions are met. If there is evidence to suggest that the species has 10 or fewer locations (according to the IUCN definition) and that there is a continuing decline in habitat area or quality, then the species could qualify as Vulnerable under Criterion B2. If not, the species could qualify as Near Threatened or Least Concern under this criterion.
Criterion C – To our knowledge, no surveys of this species’s population have been undertaken, but it has been described as abundant on Uki in the 1950s and common on Rennell (Baptista et al. 2019). Based on the area of the species’s mapped range (940 km2), the median recorded population density of other Ptilinopus species, and assuming that 13-45% of the range is occupied, the species’s population size is estimated to fall within the range 3,300 – 11,500 mature individuals. A recent analysis based on land cover data and a population density model estimated the population size at 5,931 mature individuals (Santini et al. 2019). This range of population size estimates could qualify the species for listing as Vulnerable, Near Threatened or Least Concern, depending on where the true population size is most likely to fall.However, to list the species as threatened on the Red List under Criterion C further conditions must also be met.
The rate of forest loss indicated by remote-sensed tree cover data is low (Hansen et al. 2013) and it may not be sufficient to infer a continuing decline in the species’s population size. If there is further information to indicate that the species’s habitat is being lost or degraded at a higher rate than the remote-sensed data suggests, or from which we can infer that the species’s population is declining, then the species may be considered to be undergoing a continuing decline. We do not have population data from which to estimate the rate of decline, so the species does not warrant listing as threatened under Criterion C1.
The species’s population is distributed across several islands, some of which have large distances between them, so the species is likely to have between three and eight subpopulations. The largest island on which the species is found is Rennell, which is c. 840 km2 in area. Using the methodology described above and the area of the island, the subpopulation on Rennell is estimated at c. 3,000-10,300 mature individuals. The largest subpopulation is thus likely to be significantly larger than 1,000 individuals, meaning that the species would not meet condition 2a(i), but could be considered Near Threatened under this criterion if being very precautionary. Rennell makes up approximately 90% of the species’s range, although this figure includes the large lake within the island, so the true percentage is likely to be a bit lower than 90%. From this information, we can infer that less than 90% of individuals are likely to be found in any one subpopulation, meaning that the species does not meet condition 2a(ii). There is no evidence that the species’s population size is undergoing extreme fluctuations so the species doesn’t meet condition 2b.
Based on the information stated above, the species could qualify for listing as Near Threatened or Least Concern under Criterion C. The decision on which category to use depends on the best estimate of population size and whether the best information indicates that the population is likely to be undergoing a continuing decline.
Criterion D – Based on the range and population estimates described above, the species’s population size does not meet or approach the threshold for Vulnerable under Criterion D. The species is therefore assessed as Least Concern under Criterion D.
Criterion E – To the best of our knowledge no quantitative assessment of the probability of extinction has been conducted for this species, and so it cannot be assessed against this criterion.
To allow us to achieve a clearer assessment of the species’s status, information is requested on this species’s conservation status. We particularly request information on threats which may be impacting on this species, the rate or extent of habitat loss that is underway within the species’s range and the species’s likely population size.
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’s Red List status. Therefore, please make sure your comments are about the proposed listing.
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.
Baptista, L. F., Trail, P. W., Horblit, H. M. & Boesman, P. (2019) Silver-capped Fruit-dove (Ptilinopus richardsii). 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/54334 on 19 February 2019.
Hansen, M. C., P. V. Potapov, R. Moore, M. Hancher, S. A. Turubanova, A. Tyukavina, D. Thau, S. V. Stehman, S. J. Goetz, T. R. Loveland, A. Kommareddy, A. Egorov, L. Chini, C. O. Justice, and J. R. G. Townshend (2013) High-Resolution Global Maps of 21st-Century Forest Cover Change. Science 342: 850–53. Retrieved from www.globalforestwatch.org on 19/02/2019.
Hughes, T. & Tuhanuku, A. (2015) Report to World Bank and Solomon Islands Government – Logging and Mining in Rennell: Lessons for Solomon Islands. Retrieved from http://www.devpolicy.org/rennell-island-two-halves-20170725/ on 22 February 2019.
Santini, L., Butchart, S. H., Rondinini, C., Benítez‐López, A., Hilbers, J. P., Schipper, A., Cengic, M., Tobias, J. A. and Huijbregts, M. A. (2019) Applying habitat and population‐density models to land‐cover time series to inform IUCN red list assessments. Conservation Biology https://doi.org/10.1111/cobi.13279.
Tracewski, Ł.., Butchart, S. H. M., Di Marco, M., Ficetola, G. F., Rondinini, C., Symes, A., Wheatley, H., Beresford, A. E. & Buchanan, G. M. (2016) Toward quantification of the impact of 21st-century deforestation on the extinction risk of terrestrial vertebrates. Conservation Biology 30: 1070-1079.