Rote Leaf-warbler (Phylloscopus rotiensis) has been discovered in 2004 on the Tapuafu Peninsula in the northwest of Rote Island in the Lesser Sundas, Indonesia. Based on genetic and morphological analysis of a specimen taken in 2015, the species was described in 2018 (Ng et al. 2018).
Rote Leaf-warbler is endemic to Rote, where it occurs in the highest strata of intact primary and secondary forests (Ng et al. 2018). Its behaviour and ecological requirements are not well known (Ng et al. 2018).
There is no information on the population trend (Ng et al. 2018). However, forests on Rote are severely fragmented and patchily distributed due to agriculture; it is estimated that only 19% of the island offer suitable habitat for Rote Leaf-warbler (Ng et al. 2018). As the human population grows rapidly, it is expected that the forest will be further depleted for conversion into agricultural land and road developments.
The species’s restricted range and specific habitat requirements make it vulnerable to habitat degradation and loss, so on a precautionary basis we can suspect that Rote Leaf-warbler may be in decline. Below we present our assessment against all criteria for this newly described species.
Criterion A – The population trend of Rote Leaf-warbler has not been quantified, and therefore the species cannot be accurately assessed against Criterion A. However, given the high degree of forest loss and fragmentation on Rote, we can tentatively suspect that the species is undergoing a slow decline.
Criterion B – Using a Minimum Convex Polygon, the Extent of Occurrence (EOO) of this species has been calculated at 1,918 km2. This meets the threshold for qualifying as Endangered under Criterion B1 (EOO < 5,000 km2). The extent of suitable habitat, which we can use as a measure of the maximum Area of Occupancy (AOO) for the species, is c. 244 km2 (Ng et al. 2018). This is also meets the threshold for listing as Endangered under Criterion B2 (AOO < 500 km2). However, in order to be listed under Criterion B, other conditions have to be met.
The subpopulation structure has not been assessed, so it is unclear whether the species meets condition (a). Forests on Rote are severely fragmented, and the species has been recorded patchily throughout a large part of the island (eBird 2019). The majority of records comes from the Tapuafu Peninsula, where large areas of forest persist (Ng et al. 2018, eBird 2019). This suggests that the species cannot be considered severely fragmented sensu IUCN (most individuals are found in small and isolated subpopulations; IUCN 2012). However, the species seems to occur locally, and the most serious plausible threat to the species is deforestation and road development (Ng et al. 2018); as these threats are highly localised, we can suspect that the species occurs at >10 locations*. It does not meet condition (a). Moreover, we can precautionarily suspect that the species is undergoing a slow decline in AOO, habitat quality, number of locations* and population size due to loss and fragmentation of its habitat. As a suspected decline does not fully meet the threshold for condition b, the species may at most be listed as Near Threatened. Overall, while the EOO and AOO are restricted, Rote Leaf-warbler does not qualify for listing as threatened under Criterion B. However, it may be considered Near Threatened, approaching the threshold for listing as threatened under Criteria B1ab(ii,iii,iv,v)+2ab(ii,iii,iv,v).
Criterion C – The population size of this species has not been quantified. However, we can derive a preliminary estimate from a congener, the Island Leaf-warbler (Phylloscopus poliocephalus), which occurs from the Indonesian Archipelago through Papua New Guinea to the Solomon Islands. This species occurs at a density of c. 180 individuals/km2 in intact forest (Marsden 1998). Only an area of 244 km2 offer suitable forested habitat for Rote Leaf-warbler throughout its range (Ng et al. 2018). Assuming that Rote Leaf-warbler occurs at a similar density throughout suitable habitat, and that only about 20% of suitable habitat is occupied, the population may number c. 8,800 individuals, which roughly equates to 5,900 mature individuals. As this estimate is highly preliminary and required confirmation, the species is tentatively placed in the band 2,500-9,999 mature individuals. This meets the threshold for listing as threatened under Criterion C (< 10,000 mature individuals). However, in order to be listed under this criterion, further conditions have to be met.
The population is suspected to decline slowly due to due to habitat fragmentation, degradation and loss. A suspected decline qualifies the species at most for listing as Near Threatened under Criterion C: if there was direct evidence of population declines or of an ongoing rate of habitat loss and decline, it could be assigned a higher confidence, e.g. ‘inferred’ (IUCN 2012). The rate of decline is not known, and so the species cannot be assessed against Criterion C1. It therefore depends on the subpopulation structure whether the species meets condition 2a. Records on eBird show an evenly scattered wide spread of observations (eBird 2019). This supports the assumption that the species forms one single subpopulation sensu IUCN (demographic or genetic exchange of at least one successful migrant or gamete per year; IUCN 2001, 2012). As such, the species meets condition 2a(ii), but not 2a(i). The species is not known to undergo extreme fluctuations, and therefore does not meet condition 2b. Overall, using a preliminary and very precautionary estimate of the population size and structure, Rote Leaf-warbler may be listed as Near Threatened, approach the threshold for listing as threatened under Criterion C2a(ii).
Criterion D – The population size and range are too large to warrant listing as threatened under this criterion, and therefore Rote Leaf-warbler may be considered Least Concern under Criterion D.
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, it is proposed that Rote Leaf-warbler (Phylloscopus rotiensis) be listed as Near Threatened, approaching the threshold for threatened under Criteria B1ab(ii,iii,iv,v)+2ab(ii,iii,iv,v); C2a(ii). We welcome any comments on this proposed listing and specifically request information on the population size and trend of the species.
Please note that this topic is not designed to be a general discussion about the ecology of the species, rather a discussion of its Red List status. Therefore, please make sure your comments are relevant to the discussion outlined in the topic.
*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).
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.
eBird. 2019. eBird: An online database of bird distribution and abundance [web application]. eBird, Ithaca, N.Y., U.S.A. http://www.ebird.org (Accessed on 14/05/2019).
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
Marsden, S. J. 1998 Changes in bird abundance following selective logging on Seram, Indonesia. Conservation Biology 12: 605-611. Ng, N. S. R.; Prawiradilaga, D. M.; Ng, E. Y. X.; Suparno; Ashari, H.; Trainor, C.; Verbelen, P.; Rheindt, F. E. 2018. A striking new species of leaf warbler from the Lesser Sundas as uncovered through morphology and genomics. Scientific Reports 8: 15646.