Archived 2018 topic: Giant Weaver (Ploceus grandis): revise global status?

Giant Weaver (Ploceus grandis) is endemic to the island of São Tomé, São Tomé and Príncipe, occurring throughout a range of habitat types from forest and plantations to degraded habitats, although seldom in savanna (Craig 2018). Recent surveys even resulted in more records in both non-forest habitats and plantations than primary and secondary forest combined (de Oliveira Soares 2017). As such, habitat loss and degradation as a result of the growth of small farms on the island has not been thought to having a significant effect on the species and the population is currently considered to be stable. Thus the species is currently listed as Least Concern (see BirdLife International 2018b).

The species has been described as common (Fry and Keith 2004), but recent survey work only found the species at a limited number of sites (268 out of 3,056 point counts) (de Oliveira Soares 2017). Additionally, it has been suggested that the species’s tolerance for degraded habitats may actually bring it more into contact with direct anthropogenic pressures and habitat alterations (R. F. de Lima in litt. 2018). Therefore, the species has been reassessed here against all criteria.

 

Criterion A – It has been suggested that if the species were declining it could warrant listing under this criterion (R. F. de Lima in litt. 2018). However, we would need to more accurately assess the species against threshold population reductions to see whether this is the case.

The main threat to the species appears to be habitat loss, yet it seems to be very tolerant of degraded habitats. Its tolerance of degraded habitats may bring it more into contact with introduced predators such as House Rats (Rattus rattus), but it is highly uncertain whether such predators are having an impact on the species at all. Therefore, even if it were to be considered to be in decline, there is insufficient evidence to suggest that it would even approach the threshold for Vulnerable under this criterion (reduction of 30% over 3 generations [12 years]). Thus, the species does not warrant listing under this criterion based on current information.

 

Criterion B – The species has a restricted range (Extent of Occurrence = 970km2), and forest habitats are being degraded within its range. However, it is not reliant on primary forest, frequently occurring in degraded habitat types (Craig 2018) and in fact a greater proportion of sightings were made by de Oliveira Soares (2017) in both plantations and non-forest habitats than primary and secondary forest combined. Therefore, it is uncertain whether such impacts on forest are significantly impacting the species. The species could be being affected by introduced predators, but at the moment there is insufficient evidence to say they are having any impact, and as such the impact of this potential threat is essentially unknown. Additionally, the species is not thought to undergo extreme fluctuations, is not severely fragmented, and likely occurs at considerably more than 10 locations*. Thus, it would not warrant listing under this criterion.

 

Criterion C – Survey work by de Oliveira Soares (2017) found the species at only 268 out of 3,056 point counts, which is similar to species with a population size estimated in the range 250-999 mature individuals, such as Sao Tome Scops-owl (Otus hartlaubi) (see BirdLife International 2018a). However, the species has been described as common across the island (Fry and Keith 2004, Craig 2018), and so it could be that the species is more abundant than this. Looking at the population size range bands employed by IUCN, taking into account descriptions of abundance and using population densities estimates for congeners (admittedly all continental species), then it could be appropriate to place the population size in the range 1,000-2,499 mature individuals. We do request further information and comment regarding this though.

This would then meet the threshold population size for Endangered under criterion C. To be listed under this criterion does still require further conditions to be met, in addition to being at least inferred as undergoing a continuing decline. We cannot assess the species against criterion C1 as we have no direct estimate of any potential population trends, and the species is not thought to undergo extreme fluctuations so does not warrant listing under criterion C2b.

The species likely occurs in only one subpopulation so it could then warrant listing under C2a(ii) as long as the species is inferred to be in decline. However, the evidence available is not particularly strong regarding this. The species is tolerant of habitat degradation, and was seen more often in plantations and non-forest habitat by de Oliveira Soares (2017). Therefore, we cannot infer a population decline based on loss of forest habitat. R. F. de Lima (in litt. 2018) suggests that its tolerance of degraded habitats may bring it into greater proximity to anthropogenic impacts, but again this is more circumstantial evidence and in effect the impact of other threats is unknown. Thus, it likely does not warrant listing as threatened under this criterion, and in the absence of evidence of any ongoing declines the population trend may warrant listing as stable. We could though still potentially list the species as Near Threatened under criterion C2a(ii) as a precautionary measure given the uncertainty over population trends, such that if clearer evidence becomes available to support the idea of a declining population then the species would warrant listing under a threatened category.

 

Criterion D – Its range is not sufficiently restricted to warrant listing under criterion D2, but with a population estimate of 1,000-2,499 mature individuals, the species would approach the threshold for Vulnerable under criterion D1. Therefore, it would warrant listing as Near Threatened under this criterion.

 

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 Giant Weaver be listed as Near Threatened under at least criterion D1, and potentially also C2a(ii). Any insights on the population size or thoughts on whether the species is declining would be valuable, as are comments in support of the proposed change in status as these allow the timely acceptance of a proposal. However, 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 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).

 

References

BirdLife International. 2018a. Species factsheet: Otus hartlaubi. Downloaded from http://www.birdlife.org on 02/03/2018.

BirdLife International. 2018b. Species factsheet: Ploceus grandis. Downloaded from http://www.birdlife.org on 02/03/2018.

Craig, A. 2018. Giant Weaver (Ploceus grandis). 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/61019 on 1 March 2018).

de Oliveira Soares, F. M. C. 2017. Modelling the distribution of São Tomé bird species: Ecological determinants and conservation prioritization. Masters Thesis, Universidade de Lisboa.

Fry, C. H.; Keith, S. 2004. The birds of Africa vol. VII. London: Christopher Helm.

IUCN. 2001. IUCN Red List Categories and Criteria: Version 3.1. Gland, Switzerland and Cambridge, UK: IUCN Species Survival Commission.

IUCN. 2012. Guidelines for Application of IUCN Red List Criteria at Regional and National Levels: Version 4.0. Gland, Switzerland and Cambridge, UK: IUCN.

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2 Responses to Archived 2018 topic: Giant Weaver (Ploceus grandis): revise global status?

  1. Ricardo Faustino de Lima says:

    I agree with listing the Giant Weaver as NT, but have some doubts concerning which criteria should be used.

    Unlike most endemics, this species is associated to areas with intermediate disturbance levels, avoiding both native forest and more intensive land-uses. On one hand, this might mean that it is not as affected by (or might even benefit from) deforestation and forest degradation, but on the other hand, it might mean that the core of its population is reliant in anthropogenic environments, which are clearly more prone to the impact of human activities than more natural environments. Ongoing agricultural intensification is therefore likely to impact this species more than most of the other endemics. These would be more prone to agricultural expansion, which in ST is strongly limited by the rugged terrain. As a precautionary measure, and until further evidence is available, I suggest that it is considered that this species might be under decline.

    The restricted range (777 sqkm – based on de Oliveira Soares 2017) and the ongoing agricultural intensification across the island means that it might be appropriate to consider that the species as having a single population, and close to triggering distribution-based criteria for being classified as EN, namely B1ab(iii,v)+2ab(iii,v).

    Even though there is no evidence-based population estimate, the species seems to be less frequent (de Oliveira Soares 2017) than what had been previously assumed (Fry and Keith 2004, Craig 2018). Most likely because it can be locally abundant and because previous authors did not recognize its preference for ecosystems with intermediate disturbance levels (Atkinson et al. 1991). Abundance comparisons based on frequency with species such as the Sao Tome Scops-owl might not be appropriate, since the Giant Weaver is gregarious and often multiple individuals are found near a single point count. Also, the owl has a call that can be detected at much greater distances. Comparisons with continental species might also not be appropriate, since it is well known that island species often occur at higher densities. Taking this into consideration, I feel like the species might fit into criteria C2a(ii), but find it difficult to believe that it fits criteria D1. It is far too abundant to have less than 1,000 mature individuals.

    Atkinson, P.; Peet, N.; Alexander, J. 1991. The status and conservation of the endemic bird species of Sao Tomé and Príncipe, West Africa. Bird Conservation International 1: 255-282.

  2. James Westrip (BirdLife) says:

    Preliminary proposals

    Based on available information, our preliminary proposal for the 2018 Red List would be to list this species as Near Threatened under criterion C2a(ii). We highlight though that the use of criterion B in this case is very dependent on the number of locations the species occurs in. As explained in the topic a location is not a locality, and instead depends on the area covered by the major threatening event. Given current deforestation data, it is likely that the area of impact of separate deforestation events is sufficiently low that the number of locations will far exceed the threshold for listing as threatened. This combined with the fact that declines are only suspected rather than inferred means that given current evidence the species would not approach the required thresholds for listing under criterion B.

    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 2018 Red List categories will be published on the BirdLife and IUCN websites in November, following further checking of information relevant to the assessments by both BirdLife and IUCN.

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