White-fronted Tern (Sterna striata): revise global status?

White-fronted Tern (Sterna striata) is found in south-western Australasia, and is split into three subspecies. The nominate S. s. striata breeds on North, South and Stewart Islands of New Zealand; S. s. aucklandorum breeds on the Chatham Islands and Auckland Islands; and S. s. incerta breeds on Flinders Island and Cape Barren Island off the coast of Tasmania, Australia (see Gochfeld et al. 2017). While many individuals remain on their breeding grounds, some will move from New Zealand to overwinter along the coast of Australia from southern Queensland south to Tasmania and west to South Australia.

A key threat to the species appears to be from invasive mammals, although native species such as Kelp Gull (also known as Southern Black-backed Gull) (Larus dominicanus) can predate chicks and eggs, with smaller colonies being unable to drive off predators (Frost 2017). The spread of the weed Sea Spurge (Euphorbia paralias) has also been put forward as a potential future threat in Australia as it could reduce the amount of habitat for the species (Garnett et al. 2011). The species can also suffer from damaging weather/environmental events such as flooding, given its choice of breeding habitat (Frost 2017), and possibly changes in food availability (Frost 2017). Additionally it may suffer from deliberate and accidental anthropogenic disturbance (Frost 2017). Thus this species has been assessed as declining, but it is currently listed as Least Concern (see BirdLife International 2017).

In Robertson et al. (2017) the conservation status of the two New Zealand subspecies were assessed separately, while the breeding population in Australia was assessed in Garnett et al. (2011). In New Zealand, S. s. striata has been considered to have a population size of 5,000-20,000 mature individuals and undergoing a decline in the range 10-30% over 10 years or 3 generations (whichever is longer); and as such was assessed as nationally ‘At Risk’ (Robertson et al. 2017; see also Townsend et al. 2008). S. s. aucklandorum on the other hand was considered to be stable, but with a population size of 1,000-5,000 mature individuals, and was assessed as nationally ‘Vulnerable’ (Robertson et al. 2017). The breeding population size and trend in Australia (S. s. incerta) is considered to be 120 mature individuals and stable (Garnett et al. 2011). Therefore, these data can be combined to allow for a global assessment against IUCN Red List categories and criteria (see IUCN 2001, 2012).

 

Criterion A – Using the subpopulation trends and subpopulation sizes provided in both Garnett et al. (2011) and Robertson et al. (2017) the species may overall be expected to be in decline. The overall rate of decline depends highly on the population size, but would fall in the range 5.2-29.1% over 3 generations (30.3 years). This would not meet the threshold required for listing as Vulnerable (30% over 3 generations), but as it could be potentially approaching the threshold it may warrant listing as Near Threatened under criterion A2ce+3ce+4ce.

 

Criterion B – The species’ Extent of Occurrence is 3,060,000km2 and so is far too large to qualify this species under criterion B1. The Area of Occupancy has not been directly calculated, but given the species is widespread around coastal New Zealand it is unlikely to approach the threshold for listing under criterion B2 (2,000km2).

 

Criterion C – Given the population estimates provided in Robertson et al. (2017) and Garnett et al. (2011) the overall population size falls in the range 6,120-25,120 mature individuals. The lower end of this range meets the threshold population size for Vulnerable, and the population is assessed as declining. However, even though the rate of decline is suspected to approach the threshold for Vulnerable under criterion A, it has not been clearly estimated or observed as per IUCN guidelines (IUCN Standards and Petitions Subcommittee 2017) so the species wouldn’t warrant listing under criterion C1.

While there is some movement of individuals in winter, many do just remain in New Zealand, and the species is separated into three subspecies. Therefore, it appears unlikely that the species would warrant considering as occurring in only one subpopulation. The size of each subpopulation is uncertain though, and so further information is required to better assess the species against criterion C2a(i). If there were any data to show that individual subpopulations were all ≤1,000 mature individuals (or approaching this figure) then it could warrant listing as Vulnerable. However, in the absence of this information we cannot clearly estimate this and if we use the very crude measure that each subspecies is a separate subpopulation then the species would not even approach the threshold for Vulnerable.

While the species is thought to undergo some local fluctuations (Frost 2017), but it is not thought that these would be fluctuations of an order of magnitude, as required to meet the definition of ‘extreme fluctuations’ per IUCN Standards and Petitions Subcommittee (2017) for listing under criterion C2b. Therefore, it is difficult to assess this species against criterion C. The only conditions it may meet may be for C2a(i), but further information is required before the species can be said to warrant listing under this criterion.

 

Criterion D – The population size and range is too large for the species to warrant listing 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, in the absence of further information this species would warrant uplisting to Near Threatened under criterion A2ce+3ce+4ce. However, we do request any further information on subpopulation sizes, as it may warrant even further uplisting to Vulnerable.

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 information that is sought, or about the species’ Red List status.

 

References

BirdLife International. 2017. Species factsheet: Sterna striata. Downloaded from http://www.birdlife.org on 08/12/2017.

Frost, P. G. H. 2017. Population status and trends of selected seabirds in northern New Zealand. Report prepared for the Department of Conservation in partial fulfilment of the project INT 2016-04.

Garnett, S. T.; Szabo, J. K.; Dutson, G. 2011. The Action Plan for Australian Birds 2010. CSIRO Publishing, Collingwood.

Gochfeld, M.; Burger, J.; Garcia, E. F. J. 2017. White-fronted Tern (Sterna striata). 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/54022 on 8 December 2017).

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. Available at: www.iucnredlist.org/technical-documents/categories-and-criteria.

IUCN Standards and Petitions Subcommittee. 2017. Guidelines for Using the IUCN Red List Categories and Criteria. Version 13. Prepared by the Standards and Petitions Subcommittee. Downloadable from http://www.iucnredlist.org/documents/RedListGuidelines.pdf.

Robertson, H. A.; Baird, K.; Dowding, J. E.; Elliott, G. P.; Hitchmough, R. A.; Miskelly, C. M.; McArthur, N.; O’Donnell, C. F. J.; Sagar, P. M.; Scofield, R. P.; Taylor, G. A. 2017. Conservation status of New Zealand birds, 2016. Department of Conservation, Wellington.

Townsend, A. J.; de Lange, P. J.; Duffy, C. A. J.; Miskelly, C. M.; Molloy, J.; Norton, D. A. 2008: New Zealand Threat Classification System manual. Department of Conservation, Wellington

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One Response to White-fronted Tern (Sterna striata): revise global status?

  1. Graeme Taylor says:

    It would be useful to get the validity of the three subspecies tested using modern techniques. Its hard to imagine how there would be gene flow directly between the Chatham Islands and the Auckland Islands to maintain that subspecies and yet no mixing with populations of the other subspecies in southern New Zealand. The 1990s population assessment by Birds New Zealand was the first attempt at a national census for this species. The numbers of breeding pairs was substantially smaller than expected but backed up anecdotal observations of reduced numbers of breeding colonies and fewer birds observed breeding, especially in northern New Zealand. There is a need to repeat this census to determine if the population is continuing to decline across the species range.

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