Archived 2017 topics: Cobb’s Wren (Troglodytes cobbi): downlist to Near Threatened or Least Concern?

This discussion was first published as part of the 2016 Red List update. At the time a decision regarding its status was pended, but to enable potential reassessment of this species as part of the 2017 Red List update this post remained open and the date of posting was updated.

T. cobbi is endemic to the Falkland Islands (Malvinas), where it has a scattered distribution. In 2000, the population size of T. cobbi was estimated to be about 6,000 pairs (Woods 2000). In 2008 this estimate had remained broadly unchanged at 9,000-16,000 individuals or 6,000 pairs (12,000 mature individuals) (FC & FIG 2008). There is no evidence of population trends and the population is assumed to be stable. T. cobbi is currently listed as Vulnerable due to its small known range on predator-free islands and the threat of potential invasions of its breeding islands by predatory rats.

Since 2008, more islands have been surveyed for presence of T. cobbi and the total number of confirmed breeding islands now stands at 122 (Poncet 2011, S. Poncet in litt. 2016). The species’ Area of Occupancy has not been calculated, but the total area of the islands where it is known to breed is approximately 256km2, with the species occurring up to 1.6km from coastal tussac grass (Woods 1993). Despite the discovery of additional breeding islands, estimates of population density led to the conclusion that the 2008 population estimate of ca. 9,000-16,000 individuals or ca. 6,000 pairs remains realistic (Poncet 2011).

T. cobbi is threatened by the potential introduction of mammalian predators to its breeding islands and is considered unable to maintain a breeding population on islands with Norway rats (Rattus norvegicus), black rats (Rattus rattus), mice (Mus musculus) or Patagonian foxes (Lycalopex griseus), alone or in combination (FC & FIG 2008). Evidence suggests that extirpation of T. cobbi from an island can occur within 20 years of the arrival of R. norvegicus (FC & FIG 2008). Invasion of one breeding island by rats also could lead to further invasions of nearby breeding islands. Analysis has shown that an island less than 500m from a rat-infested island has a 1 in 3 chance of being invaded (Poncet 2011).

Biosecurity and rat control efforts are underway to reduce the threat posed by invasive rats (FC & FIG 2008) and attempts have been made to eradicate rats from a number of islands since 2002 (FC & FIG 2008; Poncet 2011; Wolfaardt 2011). Although several islands were found to have been subsequently re-invaded, others have been declared rat-free (Poncet 2011; Wolfaardt 2011) and it is likely that T. cobbi has commenced breeding on one such island nine years after rat eradication (Woolfaardt 2011, S. Poncet in litt., 2016).

T. cobbi was listed as Vulnerable under subcriterion D2 as a result of its small range and the future threat of invasions by rats or other invasive species. For the species to be eligible for the Vulnerable category under D2, the plausible future threat from rats would have to be considered capable of rendering the species Critically Endangered or Extinct within around two generations, or seven years. Although rat invasions of breeding islands would be expected to result in declines in the population and Area of Occupancy of the species, it is now known to occur on 122 islands. It seems very unlikely that rat invasions would cause the species to become Critically Endangered or Extinct within seven years meaning T. cobbi should no longer be considered Vulnerable. However, rat invasions could perhaps plausibly drive the species to become Vulnerable or Endangered within a short time, therefore listing as Near Threatened may be appropriate. For example, if the population might plausibly drop below the threshold of 10,000 mature individuals within a short time if rat invasions of breeding islands occurred, then (given that it is unlikely that any subpopulation of T. cobbi exceeds 1,000 individuals) it could in the near future qualify for listing as Vulnerable under criterion C2a(i).

If such a scenario is unlikely to take place, the species should be downlisted to Least Concern.

Additional information and comments on this proposal are welcomed.

References

Falklands Conservation & Falkland Islands Government 2008. A Species Action Plan for Cobb’s Wren 2009-2019.

Poncet, S. 2011. Final report for the Cobb’s wren conservation project 2009-11. Beaver Island LandCare report. BILC. Stanley, Falkland Islands.

Wolfaardt, A. 2011. Final report on Defra funded invasive aliens and climate change work in the UK’s South Atlantic Overseas Territories. Joint Nature Conservation Committee

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3 Responses to Archived 2017 topics: Cobb’s Wren (Troglodytes cobbi): downlist to Near Threatened or Least Concern?

  1. Agree with the assessment that would result in a downlisting from the current status.
    Feel that there are no likely, current scenarios whereby c 10% of the estimated population could be impacted and extirpated either by potential rodent re-invasion/introduction, or by catastrophe. Therefore, with regard to IUCN criteria, would definitely lean towards categorisation as Least Concern. However, would accept if a precautionary approach regarding the likeliness of scenarios was taken and led to ‘Near Threatened’.

  2. James Westrip (BirdLife) says:

    Based on available information, our proposal for the 2016 Red List would be to pend the decision on this species and keep this discussion open until 2017, while leaving the current Red List category unchanged in the 2016 update.

    Final 2016 Red List categories will be published on the BirdLife and IUCN websites in early December, following further checking of information relevant to the assessments by both BirdLife and IUCN.

  3. Andy Symes (BirdLife) says:

    Preliminary proposals

    Based on available information, our preliminary proposal for the 2017 Red List would be to list:

    Cobb’s Wren as Least Concern.

    There is now a period for further comments until the final deadline of 4 August, 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 2017 Red List categories will be published on the BirdLife and IUCN websites in early December, following further checking of information relevant to the assessments by both BirdLife and IUCN.

Comments are closed.