Climate change, mean sea levels, wetland decline and the survival of the critically endangered Capricorn Yellow Chat

An isolated population of ~30 Capricorn Yellow Chats (Epthianura crocea macgregori) (Aves: Meliphagidae) occurs on a grassy marine plain (~4000 ha) with complex fresh to hypersaline wetlands bordering unvegetated salt flats and mangroves, the majority less than 2 m above mean sea level (MSL). While there are several species of sedge on the marine plain, only one, a tall salt-tolerant sedge, Schoenoplectus subulatus, regularly provides tall cover and nesting habitat on the marine plain. Comparison of vegetation data between 2002 and 2018 at four breeding sites showed a loss of S. subulatus at two of the sites in conjunction with persistent extreme hypersalinity. In contrast, sites where S. subulatus recovered had elevated salinity levels coinciding with a drought in 2005–2008, but subsequently returning to pre-drought levels. Concomitant with the loss of S. subulatus was a substantial decline in Capricorn Yellow Chat abundance. Possible drivers of persistent hypersalinity were an increase in sea level and consequent greater tidal ingress. The two most severely affected sites were relatively close to tidal influence and low points in the landscape. Evaluation of MSL data showed that habitat decline from 2002 to 2018 occurred over a period in which sea level rose by ~80 mm. While it is recognised that other variables are important, this study indicates that if sea-level rise continues at the 2011–2017 rate of ~8.3 mm per annum, the Curtis Island marine plain will become unsuitable for Capricorn Yellow Chats in the next 60 years or less. Sixty years is likely to be a conservative estimate given that sea-level rise rates are projected to increase further, as are the influence of more extreme weather events such as drought and storm surges on persistence of key vegetation. Ramifications for other Capricorn Yellow Chat sites and marine plain-dependent fauna such as shorebirds also need consideration.