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The Yellow Chat Epthianura crocea is comprised of three disjunct subspecies. Subspecies E. c. macgregori (Capricorn Yellow Chat) is listed as Critically Endangered under the EPBC Act and has a distribution that also appears to be disjunct, with a limited geographic area of less than 7,000 ha. Some populations are threatened by rapid industrial development, and it is important for conservation of the subspecies to determine the extent to which the putative populations are connected. We used 14 microsatellite markers to measure genetic diversity and to determine the extent of gene flow between two disjunct populations at the northern and southern extremes of the subspecies’ range. No significant differences in genetic diversity (number of alleles and heterozygosity) were observed, but clear population structuring was apparent, with obvious differentiation between the northern and southern populations. The most likely explanation for reduced gene flow between the two populations is either the development of a geographic barrier as a consequence of shrinkage of the marine plains associated with the rise in sea levels following the last glacial maxima, or reduced connectivity across the largely unsuitable pasture and forest habitat that now separates the two populations, exacerbated by declining population size and fewer potential emigrants. Regardless of the mechanism, restricted gene flow between these two populations has important consequences for their ongoing conservation. The relative isolation of the smaller southern groups (the Fitzroy River delta and Curtis Island) from the much larger northern group (both sides of the Broad Sound) makes the southern population more vulnerable to local extinction. Conservation efforts should focus on nature refuge agreements with land owners agreeing to maintain favourable grazing management practices in perpetuity, particularly in the northern area where most chats occur. Supplemental exchanges of individuals from northern and southern populations should be explored as a way of increasing genetic diversity and reducing inbreeding.
The average population size of Capricorn Yellow Chat Epthianura crocea macgregori was estimated at 251 +/-31 (SE) by repeated surveys over seven years (2004–2010) using consistent search effort at known occupied sites. Because the survey period coincided with a mixture of dry and wet years (drought from 2004 to 2007 followed by flood rainfall in early 2008 and 2010), it is particularly valuable as a preliminary benchmark upon which to base management decisions. Most of the population (74.5%) was in the Broad Sound area in the north, with lower numbers in the Fitzroy River delta area in the south (22%) and at Curtis Island (3.5%). Sites on Torilla Plain in Broad Sound accounted for two-thirds of the estimated population, making it a priority for conservation efforts. Depending on habitat configuration, some Capricorn Yellow Chats showed a seasonal pattern of habitat use, moving from flooded breeding habitats as they dried to refuge sites such as salt fields or upper marine plains in the dry season; distances moved being < 10 km. Standard surveys from Torilla Plain showed that the chat count during a sequence of above-average rainfall years was almost double that of the average for drought years: 162 +/-28 (2008–2015) compared with 85 +/-15 (2004–2007) respectively. Low population size, large annual fluctuations in population with prior rainfall, rapid declines in low rainfall years, a fragmented distribution and almost half the population concentrated at one site point to a subspecies vulnerable to chance events. Increased climatic extremes predicted by climate change such as higher temperatures, evaporation rates, extended droughts and more intense rainfall events add to its vulnerability.
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