Conservation of old-growth dependent mallee fauna
Prepared by David Baker-Gabb for the Black-eared Miner Recovery Team, February 2001
(Revised February 2003)
The Black-eared Miner has been listed as Endangered (ANZECC 1991; Garnett 1992), and Critically Endangered (criteria C2a, D1) (Collar et al 1994; IUCN 1996). The species is classified as Endangered under the Commonwealth Environment Protection and Biodiversity Conservation Act 1999, the South Australian National Parks and Wildlife Act 1972 (amended May 1991) and the New South Wales Threatened Species Conservation Act 1995, and is listed as a threatened taxon under the Victorian Flora and Fauna Guarantee Act 1988. The discovery of large numbers of Black-eared Miners in the Bookmark Biosphere Reserve and recovery actions such as land purchase for reserves has led to the status of the bird being changed from Critically Endangered to Endangered (criteria B1+ 2bde, C2a, D) (Garnett and Crowley 2000).
4. Decline and Threats
Most data on the species' decline come from Victoria. Black-eared Miners were once considered either common or locally common within their mallee habitat prior to 1940 (Wilson 1912; Howe and Tregellas 1914; Favaloro 1966; Starks 1987; McLaughlin 1990). However, there have been few recent records. In Victoria, the decline of the Black-eared Miner has been in the number of colonies, the numbers of birds within colonies and the quality of birds (Considine 1986; Starks 1987; McLaughlin 1990, 1994). This decline has continued despite the retention of a considerable area of apparently suitable habitat within conservation reserves (LCC 1989).
Joseph (1986) summarised the decline of the Black-eared Miner in South Australia and considered the species very nearly, if not already, extinct. However, following sightings of hybrid miners in the extensive mallee habitat of the Bookmark Biosphere Reserve north-west of Renmark in the early 1990s, surveys were conducted in this region in 1996, which resulted in over 80 sightings of miners (McLaughlin 1996; Backhouse et al 1997), and over 200 colonies are now known from this area (Clarke and Clarke 1999b). Although containing many hybrids, over a third of colonies contain mainly phenotypically pure Black-eared Miners.
In New South Wales the Black-eared Miner was less well-known, with only eight likely records up until 1985 (Franklin 1996). However, hybrid birds were observed in 1997 and 1999 in two areas of the Scotia Mallee region adjacent to the border with South Australia (Boulton and Clarke 2000a).
Four major causes of decline of the Black-eared Miner have been postulated.
A major factor implicated in the decline of the Black-eared Miner is the loss and modification of suitable habitat (Favaloro 1966; Schodde 1981, 1990; Joseph 1986; Starks 1987; McLaughlin 1990, 1992). In Victoria, Johnson (1989) and McLaughlin (1990, 1992) identified the most fertile dunefield soils as being important to Black-eared Miners. Historically, these soil types have been selectively cleared for agricultural use, primarily wheat production (LCC 1987; Blakers and MacMillan 1988), and conservation reserves in the Murray Mallee substantially under-represent the vegetation of fertile soils.
Clearance and modification of vegetation has also favoured a range expansion of the Yellow-throated Miner which prefers open habitats (Schodde 1981, 1990; Joseph 1986; Starks 1987). Prior to widespread clearing in the Murray Mallee, the Yellow-throated Miner occurred infrequently in open woodlands (Chandler 1937; Ashby 1922; Joseph 1986; Emison et al 1987; Starks 1987). However, it is now abundant and commonly recorded occupying shelter belts and roadside vegetation adjacent to cleared farmland (Emison et al 1987; Starks 1987; McLaughlin 1990, 1992).
Immediately post-clearing, Black-eared Miners were known to occur in remnant patches of mallee scrub, such as shelter belts adjacent to roads and fences (McGilp and Parsons 1937; Rix 1937; McGilp 1943). They occupied these areas prior to expansion into this vegetation by Yellow-throated Miners. Observers who recorded Black-eared Miners in these habitats did not record Yellow-throated Miners (Starks 1987). Black-eared Miners were apparently rapidly eliminated from these remnants, due possibly to a combination of competition, introgressive hybridisation or reduced population viability (McLaughlin 1994).
Clearing of habitat still remains a threat in some parts of the Black-eared Miner's range, though nowadays it has much less impact than in past decades. There are controls on the clearing of mallee on private land in Victoria, although some small-scale clearing still occurs. Further extensive loss of habitat through land clearing is also possible in New South Wales. In South Australia, while there are clearing controls for both public and private land under the Native Vegetation Act 1991, exemptions in the Act mean that clearing for mineral exploration and extraction, and public utilities such as power lines can still occur. In addition, joint proclamation of conservation reserves under both the National Parks and Wildlife Act 1972 and the Mining Act 1972 makes it possible for mineral exploration and mining to proceed within conservation reserves. Therefore, much of the habitat of the Black-eared Miner in South Australia is potentially still under threat from clearing.
Total grazing pressure from domestic stock and feral and native herbivores is sufficiently high on most reserves and pastoral properties that it limits the regeneration of many mallee plants and encourages the growth of woody shrubs (Forward and Robinson 1996). Strategic closure of artificial water points is a key means of reducing total grazing pressure and enhancing biodiversity conservation (Landsberg et al 1997). Dams and their associated clearings and degradation attract Yellow-throated Miners and so they are a threat to Black-eared Miners which do not need permanent water (Clarke and Clarke 1999b). A program to decommission all artificial water points or make them unavailable to herbivores is under way in the core of the Black-eared Miner's range in the Bookmark Biosphere Reserve. Most dams in Victorian and New South Wales mallee reserves where Black-eared Miners occur have been decommissioned. An exception is the private reserve, Scotia Sanctuary, in western New South Wales which contains small numbers of Black-eared Miners and numerous artificial water points.
One of the major causes of decline in this species is introgressive hybridisation or 'genetic swamping' by the conspecific Yellow-throated Miner (Schodde 1981; Starks 1987; McLaughlin 1990). Black-eared and Yellow-throated Miners were clearly separable on phenotypic characters before extensive clearing occurred after 1950 (Clarke et al in press). Starks (1987) proposed that miners exhibiting intermediate plumages resulted from hybridisation between the two species, not because Yellow-throated Miners moved into uncleared areas, but because Yellow-throated Miners were able to colonise habitats newly created by land clearing, and come into contact with populations of Black-eared Miners then occupying remnant stands of mallee. Starks (1987) further proposed that hybrid birds created in these situations were physically and behaviourally intermediate, and as such were able to move into areas of uncleared mallee and become incorporated into colonies of phenotypically pure Black-eared Miners.
McLaughlin (1992) demonstrated that habitat occupied by breeding Black-eared Miners is significantly structurally dissimilar from Yellow-throated Miner habitat, and that the two species are predominantly allopatric. However, colonies of distinctly intermediate-plumaged miners were found to occupy a range of habitat types that in structure overlapped both Black-eared and Yellow-throated Miner habitat. This suggests that although Yellow-throated Miners and Black-eared Miners would not normally come into contact (as would have been the case when Black-eared Miners were occupying remnant habitat), the flow of genetic material between the two species is maintained by the presence of hybrid miner colonies (these colonies would not have been present prior to extensive land clearing). In this situation, the hybrid birds in these colonies are able to act as a 'genetic bridge' (Sibly 1961).
The range of the Yellow-throated Miner now encompasses the distribution of the Black-eared Miner. In most areas Yellow-throated Miners and hybrids are more numerous than Black-eared Miners, and the Black-eared Miner now represents an insular population. Under these conditions, uncontrolled genetic introgression will eventually result in the loss of the biological and genetic diversity contributed by the Black-eared Miner (eg Cade 1983).
Hybridisation is occurring in all Black-eared Miner populations, but it has been particularly severe in small (eg 20,000 ha) reserves in Victoria such as Annuello and Bronzewing. Habitat fragmentation by extensive wildfires has probably promoted the more extensive hybridisation exhibited in Murray-Sunset National Park than is seen in the relatively intact populations in Bookmark Biosphere Reserve. Hybridisation has been extensive in the Scotia mallee in western New South Wales.
Mallee habitats are some of the most flammable habitat types in the semi-arid zone, and rates of litter accumulation in these habitats may be sufficient to support large sustainable fires every 10-20 years (Noble 1984). Black-eared Miners prefer mallee vegetation that has not been burnt for at least 40 years (Starks 1987; McLaughlin 1990, 1992), and habitats of this age possessing suitable structural characteristics are now uncommon throughout the historical distribution of the Black-eared Miner (eg LCC 1987), except in the Bookmark Biosphere Reserve. In New South Wales, occasional large-scale intentional burning of leasehold land has occurred, ostensibly to increase productivity for pastoral activities (Hodgkinson et al 1984; Noble 1984; Choate 1989; MacLeod 1990; Muir 1992).
Although conservation and other reserves that recently supported or still support Black-eared Miner and hybrid populations are large (several hundred thousand hectares), the potential scale of wildfire in mallee habitats suggests that even the largest reserves may be consumed by fire (Benshemesh 1990,1999, Clarke and Clarke 1999b). Single wildfires have burnt many hundreds of thousands of hectares of Murray Mallee vegetation in most decades (LCC 1987; Noble et al 1980; Noble 1984; Blakers and MacMillan 1988), and large wildfires remain one of the most serious threats to the Black-eared Miner (McLaughlin 1990, 1992).
Habitat fragmentation from both large wildfires and extensive clearing accelerates the decline of small, isolated colonies of Black-eared Miners by impeding the dispersal of young independent females from colonies. Even in the relatively intact Bookmark Biosphere Reserve where female nestlings outnumber males, adults in colonies are male-biased as a consequence of more dispersing females being lost from the population. The loss of just a single breeding female resulting in the disintegration of small, isolated colonies has been recorded in Victoria (Boulton and Clarke 2000b).
Fire management planning to address the threat from large wildfires is more advanced for sites with Black-eared Miners in Victoria and New South Wales than it is for the Bookmark Biosphere Reserve in South Australia. The lack of a regional fire management plan for Bookmark, and a full complement of appropriate on-ground works, is one of the gravest threats now confronting the Black-eared Miner.