Land Birds on Barrow Island: Status, Population Estimates, and Responses to an Oil-Field Development
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Journal of the Royal Society of Western Australia, 87:101–108, 2004 Land Birds on Barrow Island: Status, Population Estimates, and Responses to an Oil-field Development S Pruett-Jones1 and E O’Donnell2 1 Department of Ecology and Evolution, University of Chicago, 1101 East 57th Street, Chicago, Illinois 60637. USA. 2 Department of Evironmental Biology, Adelaide University, Adelaide, South Australia 5005. Australia. [email protected] (Manuscript received October 2003; accepted June 2004) Abstract A census of land birds on Barrow Island, Western Australia was carried out during September and October 2001. A total of 178 transects was conducted in six major vegetation zones, with 777 individuals of 16 species of birds recorded. The six most common species, in order of their abundance, were Spinifexbird (Eremiornis carteri), White-winged Fairy-wren (Malurus leucopterus), Singing Honeyeater (Lichenostomus virescens), White-breasted Woodswallow (Artamus leucorynchus), Zebra Finch (Taeniopygia guttata), and Welcome Swallow (Hirundo neoxena). Our total population estimates for these species were generally comparable to those of a survey of Barrow Island in 1976 (Sedgwick 1978), but some differences existed for these and other species. In an analysis of habitat preferences, more complex vegetation zones supported more species. The density of both Spinifexbirds and Singing Honeyeaters was significantly correlated with the number of emergent shrubs. Also, across vegetation zones, the number of different bird species was correlated with the maximum height of vegetation in a habitat. Species richness and numbers of individuals were compared between natural areas, areas surrounding oil pads, and along roadsides. There was no significant difference in species richness across these three areas, but there was a significant increase in number of individuals along road sides. This latter result occurred because disturbed areas along roadsides were colonized by a large species of spinifex, Triodia angusta, which supported greater numbers of birds than other species of spinifex away from roads. The oil-field development on Barrow Island occupies approximately 4% of the land area on the island and appears, at present, to have had a relatively minor impact on the populations of land birds. This conclusion is qualified in that several species of insectivorous birds have declined in numbers over the last 25 years, and the reasons for these declines are not yet known. Keywords: birds, Barrow Island, oil-field development, species diversity Introduction beginning in 1964 and continuing to the present. During this collective work, a total of 113 bird species has been Barrow Island is the second largest island (after Dirk recorded (110 species listed by Sedgwick (1978) plus Hartog Island) in Western Australia. Barrow Island three additional species recorded by Butler (pers. comm.), experiences a tropical arid climate, and its vegetation is and 32 species have been recorded breeding there. dominated by spinifex (Triodia sp.). The ecosystem on Additional work on the avifauna of Barrow Island Barrow Island is interesting because it has been relatively consisted of an analysis of the diet of insectivorous birds unimpacted by the negative consequences of human (Wooller and Calver 1981) and a study of breeding activity and development, a situation unique in Australia chronology (Ambrose and Murphy 1994). and elsewhere. Barrow Island was designated as a Class A Reserve in 1910, which greatly limited human activity. Given the approximate 25 years since the survey by In 1964 an oil-field development began on the island (by Sedgwick (1978), we sought to conduct a current survey Western Australian Petroleum (WAPET), and continues of land birds on Barrow Island, with three objectives: 1. with Chevron Inc. Despite this, however, there are no to quantify the occurrence and abundance of species; 2. introduced animal species on the island, the fauna has to document any changes in the avifauna since the survey never been persecuted or hunted, and the terrestrial of Sedgwick (1978); and 3. to quantify the effects of the vertebrates are extremely common. oil-field development on the island’s land birds. Our focus on land birds differs from the complete avifaunal The avifauna on Barrow Island was surveyed three survey of Sedgwick (1978), and this earlier research times during the 20th century: by F. Lawson Whitlock in remains the primary source for a list of all bird species 1917 and 1918 (Whitlock 1918, 1919), by D. L. Serventy recorded on the island. We focused on land birds because and A. J. Marshall in 1958 (Serventy and Marshall 1964), our time on the island was limited and we wanted to and by E. H. Sedgwick in 1976 (Sedgwick 1978). obtain an accurate estimate of population densities. We Additionally, W. H. Butler has made extensive and long- acknowledge that oil-field development can have both term observations of the avifauna on Barrow Island direct and indirect consequences for water birds and sea birds as well as land birds but our focus here is only with © Royal Society of Western Australia 2004 the island’s land birds. 101 Journal of the Royal Society of Western Australia, 87(3), September 2004 Study area and methods disturbances associated with the oil-field development were located (excluding the airport and camp). The total Study area area of disturbed areas (again excluding the airport and Barrow Island (20°43'S, 115°28'E) is a continental camp) is 915.7 ha or 3.93% of the total area. This includes island, 56 km from the Western Australian coast between the area occupied by 821 oil wells (as of 1997), and Onslow and Port Hedland, and approximately 1300 km approximately 550 km of roads. In correcting the areas of north of Perth. It has been isolated from the mainland for each vegetation zone, we reduced the area by 3.93% to 7500 to 8000 years. It was formed from Tertiary account for the disturbance in that zone. We recognize limestone, and consists of high coastal cliffs on the west, that this correction makes the assumption that sloping down through central uplands to coastal plains disturbances are distributed randomly across vegetation on the east. The climate is classified as tropical arid, or as zones, which is not correct. It would be logistically a northern monsoonal climate (Gentilli 1972), with impossible to precisely calculate how much of each seasonally extremely high temperatures (> 45°C), and vegetation zone is disturbed, however we believe that sporadic and highly variable rainfall. The rainfall our correction provides as accurate a calculation of the averages 330 mm, but in four out of five years, the available habitat area as is possible. We also fully rainfall averages just 20% of this value. Most of the recognize that land birds do use roads and other man- annual rainfall occurs during cyclones between February made structures (although no birds were seen to use oil and June. Creeks and claypans will hold fresh water after wells as ‘habitat’). Nevertheless, our correction of the a cyclone, but otherwise freshwater is extremely limited. area of each vegetation zone by the amount of disturbed Heavy dews, however, occur throughout the year area within that zone seemed to us as the most (Buckley 1983) and provide a source of fresh water for conservative solution in order to calculate overall birds, as do shoreline seeps. Additionally, facilities population estimates. associated with the oil-field development now contain standing fresh water. Vegetation on Barrow Island has been studied by Methods several investigators. Butler (1970) identified six major Fieldwork was conducted from 14 September 2000 to vegetative zones, and Buckley (1983) identified nine 12 October 2001. A census was carried out on birds via major zones, with 29 subtypes. On contract from transects in quadrats in each of the 31 vegetation zones. WAPET, Matiske Consulting Group produced a Our initial goal was to carry out a census in six quadrats detailed vegetation map of the island, identifying six within each vegetation zone, but in some cases this was major terrestrial vegetation types and 31 subtypes not possible because some habitats were limited in area. (excluding marine and two tidal vegetation zones). Our Although the quadrats were located in specific vegetation work and analyses were based on this latter map. A zones, we made a concerted effort to census birds in all digital version of this map was available from National areas of the island. A total of 178 quadrats were Geographic Information Systems (NGIS, Perth, Western censused. Australia), and under contract, NGIS produced a field map of the vegetation zones for our use and calculated The transect counts were conducted each morning, the area occupied by each zone, as well as areas weather permitting, from 0545 to 1100 and again from occupied by various types of development (roads, oil 1500 to 1700. The quadrats were 2 ha in size, and in most wells, etc.). cases were 200 m long X 100 m wide. The length of the quadrat was determined using a portable GPS unit Briefly, the six land forms/vegetation units were: (Garmin 45; accuracy = 10 m). The width of the quadrat coastal complexes and dune systems (type C in Table 1, was estimated visually. In order to maintain accuracy, creeks or seasonal drainage lines (D), flats (F), limestone both observers practiced estimating 50 m lengths and ridges (L), clay pans (S), and valley and escarpment then checked these distances with a tape measure. We slopes (V). Triodia hummock grassland is the dominant surveyed each quadrat together. One of us walked slowly vegetation type and Triodia sp. occurs over 93% of the in a straight line down the middle of the quadrat, and island. A complete list and description of each vegetation the other person zigzagged through the quadrat, type, and subtype, is available on request from the intersecting the path of the person walking straight every authors.