The Rennell White-eye (Zosterops rennellianus) is found only on the island of Rennell in the Solomon Islands. It is found in forest, especially forest edge, and is less common in regrowth and scrub (van Balen 2019). Its population size has not been estimated, but it has been reported to be common and widespread within its range, although it is also said to be one of the less common passerines on Rennell (van Balen 2019). It 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. An analysis of forest loss from 2000-2012 found that forest was lost within the species’s range at a rate equivalent to 2% over three generations (Tracewski et al. 2016). Assuming that any population reduction occurring is the result of forest loss, it is unlikely that the population has undergone a reduction approaching 30% over the last three generations, and we do not have evidence to indicate that it will do so over the next three generations (10.5 years). The species is therefore assessed as Least Concern under this criterion.
Criterion B – The species’s Extent of Occurrence (EOO) is estimated at 1,200 km2. This meets the threshold for Endangered under Criterion B1. 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 1,476 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.
The species is not severely fragmented. The main known potential threat to the species is considered to be forest loss, which has been estimated to be occurring at a rate of 2% across three generations (10.5 years; Tracewski et al. 2016). The species is therefore 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 having a greater impact on the species, the number of locations could be revised. Forest loss data suggests a low rate of forest loss within the species’s range (Tracewski et al. 2016). Ongoing mining is thought to be contributing to forest loss within the species’s range (Hughes and Tuhanuku 2015, M. O’Brien in litt. 2018). However, this species occurs in forest edge habitats, so its population size may not be negatively affected by forest loss. If there is information that indicates that habitat loss, including that caused by mining, is causing a continuing decline in the area or quality of the species’s habitat, then condition b would be met. There is no evidence that the species’s population or range size are undergoing extreme fluctuations. Condition c is not met.
The species’s EOO falls beneath the threshold for listing the species as Endangered, and its AOO falls beneath the threshold for Vulnerable. 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 or Endangered under Criterion B. 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 common and widespread within its range, although one of the less common passerines on Rennell (van Balen 2019). Based on the area of the species’s mapped range (973 km2), the first quartile and median recorded population densities of other Zosterops species in Asia and the Pacific, and assuming that 13-45% of the range is occupied, the species’s population size is estimated to fall within the broad range 8,800 – 75,800 mature individuals. A recent analysis based on land cover data and a population density model estimated the population size at c.11,700 mature individuals (Santini et al. 2019). This wide 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 (Tracewski et al. 2016). Additionally, this species occurs in forest edge habitats, so its population size may not be negatively affected by forest loss. 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.
There is no evidence that the species has multiple subpopulations, so the species’s population is not composed of separate subpopulations of up to 1,000 mature individuals, meaning that the species would not meet condition 2a(i). The species therefore has a single subpopulation, meaning that it meets condition 2a(ii). There is no evidence that the species’s population size is undergoing extreme fluctuations so the species doesn’t meet condition b.
Based on the information stated above, the species could qualify for listing as Vulnerable, 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 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 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.
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.
van Balen, B. (2019) Rennell White-eye (Zosterops rennellianus). 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/60199 on 19 February 2019.