Responses of waterbirds to flooding in an arid region of Australia and implications for conservation

Biological Conservation 106 (2002) 399–411 www.elsevier.com/locate/biocon

Responses of waterbirds to flooding in an arid region of Australia and implications for conservation D.A. Roshiera,*, A.I. Robertsona, R.T. Kingsfordb a

Johnstone Centre, School of Science and Technology, Charles Sturt University, Locked Bag 588, Wagga Wagga, NSW 2678, Australia b NSW National Parks and Wildlife Service, PO Box 1967, Hurstville, NSW 2220, Australia Received 27 July 2001; received in revised form 20 November 2001; accepted 23 November 2001

Abstract Floods are a frequent but irregular feature of Australia’s dryland river catchments. We investigated changes in abundances of waterbirds in north western New South Wales with changes in wetland distribution at local, catchment and broad scales. The abundance of most functional groups of waterbirds changed in response to broad scale changes in wetland distribution, while local abundance remained highly variable. Patterns of abundance varied among functional groups of waterbirds, with some immediately responding to changes in wetland distribution and area flooded, and others apparently responding to sequences of wetting and drying. In Australia, the main conservation issue for waterbirds is water and its use across the landscape and not the spatial arrangement of any fixed array of reserves established to protect them. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Waterbirds; Wetlands; Arid zone; Australia; Distribution and abundance

1. Introduction Floods are a frequent but irregular feature of Australia’s dryland river catchments (Puckridge et al., 1998; Kingsford et al., 1999a; Roshier et al., 2001b). Flood events range in duration from minutes to months as floodwaters flow quickly down dry drainage lines or traverse many hundreds of kilometres of river channel in the larger drainage basins. Despite the distribution of wetland habitat across inland Australia being highly variable in time and space (Roshier et al., 2001b), the arid interior of Australia has an abundance of habitat for mobile waterbirds that are able to exploit widely dispersed wetland habitats (Roshier et al., 2001a). Wetlands in the arid zone of Australia range in size from a few square metres to thousands of square kilometres and support a diverse biota (Williams, 1998a,b). These wetlands can be terminal water bodies filled by major drainage systems, lakes filled by local drainage, and creeks, overflow lakes and floodplains associated

* Corresponding author. Tel.: +61-2-69332538; fax: +61-269332737. E-mail address: [email protected] (D.A. Roshier).

with major drainage systems filled by flood events (Bowler, 1982, 1986; Kotwicki, 1986; Kotwicki and Allan, 1998; Timms, 1993, 1998a,b; Kingsford and Porter, 1994; Seddon and Briggs, 1998; Walker et al., 1995; Puckridge, 1998; Williams et al., 1998). Most wetlands in the arid zone retain water for short periods relative to the lifespan of individual birds, and waterbirds must move frequently to seek feeding and breeding habitat. Waterbirds might respond to changes in wetland distribution at the local scale, at the scale of the catchment, or to changes that occur at scales that extend beyond that of individual catchments. These responses may be immediate or occur over months or years, depending on the frequency and extent of flooding. The nature of these responses has implications for the management of wetlands for waterbirds and the success of conservation strategies for individual species (Briggs and Lawler, 1991; Maher and Braithwaite, 1992; Robinson and Warnock, 1997; Haig et al., 1998). As rivers in Australia’s arid zone come under increasing development pressure for diversion of water or damming for water storage (Walker et al., 1997; Kingsford et al., 1998; Kingsford, 2000a,b), there is an urgent need for a landscape perspective on responses of waterbirds to wetland dynamics.

0006-3207/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0006-3207(01)00268-3

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In this paper, we analyse the responses of waterbirds in a dryland river catchment in north western New South Wales to changes in wetland distribution at local, catchment and broader scales. We show that the abundance of most functional groups of waterbirds changes in response to broad scale changes in wetland distribution and discuss the implications for conservation of waterbirds in spatially dynamic landscapes.

2. Methods 2.1. Waterbird study area The wetlands of northwestern New South Wales have been identified as important for the conservation of waterbirds in arid Australia (Maher, 1991; Maher and Braithwaite, 1992; Watkins, 1993; Kingsford et al., 1994). The major wetlands are distributed across the Paroo River and Cobham catchments and the southern portion of the Bulloo Overflow (Fig. 1). Both the Bulloo Overflow and the wetlands of the lower portion of Paroo River catchment are fed by waters that may have originated hundreds of kilometres to the north. By contrast, the wetlands of the Cobham catchment fill from local rainfall. Annual rainfall in northwestern NSW ranges from 140 mm in the west to 300 mm in the east, of which 60% falls in the summer (Goodrick, 1984; Maher and Braithwaite, 1992). Rainfall in the region is episodic rather than seasonal and the wetland systems of the region tend to follow a ‘boom’ and ‘bust’ pattern (Kingsford et al., 1999a). Wetland area can change by orders of magnitude in a short period (Fig. 2). There are four broad wetland types within the Paroo River and Cobham catchments, defined by their vegetation, geomorphology and/or water chemistry. Floodplain wetlands such as the Cuttaburra channels and Paroo River channels are extensive areas fringed by eucalypts that are flooded for a few months at a time and define the majority of the floodplain (Kingsford et al., 2001). Waterholes along the course of the Paroo River are the most permanent wetland feature of these floodplains. These wetlands may have extensive cover of lignum (Muehlenbeckia florulenta Meissner) or canegrass (Eragrostis australasica (Steud.) C. E. Hubbard). The second wetland type are small wetlands filled by local rainfall that include claypans, and black box (Eucalyptus largiflorens F. Muell.) and spike rush (Eleocharis spp.) swamps. The other two distinctive wetlands are freshwater lakes and salt lakes. Freshwater lakes are natural open-water areas, that are usually greater than 50 ha, where surface aquatic vegetation is sparse or absent (e.g. Lake Numalla). Salt lakes vary enormously in size and are usually wetlands that are inundated less frequently than freshwater

lakes. Of the 28 lakes surveyed in the waterbird study area (see later) 19 are freshwater and nine saline (Kingsford et al., 1994). 2.2. Waterbird abundance At 3-monthly intervals between March 1987 and December 1990, and again in March 1993, the abundance of individual waterbird species or taxonomic groups was estimated on 28 wetlands in the study area (Fig. 1), giving a total of 476 (timewetland) estimates of abundance of each species (Kingsford et al., 1994). The methods used to estimate waterbird abundance are detailed elsewhere (Kingsford et al., 1994; Kingsford et al., 1999b). Briefly, two observers identified and estimated waterbird numbers from a low-flying aircraft flown parallel to the shore, 150 m from the edge, at a height of 30 m at an airspeed of 167 km h 1 (90 knots). One observer counted waterbirds around the edge of the lake and the other observer counted waterbirds in the middle. Observers’ counts were combined to give a total count for each species. Each wetland was surveyed four times within 3 days and means and standard errors calculated for each survey. Where the whole wetland could not be surveyed, the percentage of lake edge counted was estimated (usually > 50%) and the counts proportionally adjusted. Over large, shallow wetlands with very large perimeters transects were flown across the wetland at a height of 46 m and birds counted within a 200 m wide band on each side of the aircraft. For these wetlands, transect counts were combined to give a total count for each species. These wetlands were surveyed on consecutive days and means and standard errors calculated for each survey. Each survey estimated the abundance of 43 species and four taxonomic groups (Kingsford et al., 1994). Mean precision estimates (SE/mean) of counts varied between abundance classes (range 0.09 for counts > 1000 to 0.70 for counts < 10), underestimating abundance when individual species counts were less than 10 or greater than 5000 (Kingsford et al., 1999b for discussion of survey errors and biases). To determine the relationships between waterbird abundance and wetland area, counts of 43 species and four taxonomic groups were aggregated into seven functional groups based on feeding habits (Appendix, after Kingsford, 1991). For waders, functional grouping was based on size, creating a group of large endemic species and a group of smaller species that included long-distance migrants. These data were collated at two levels, for the individual wetland and for each of the Paroo River and Cobham catchments to investigate possible spatial interactions. The data for Lake Altibouka (wetland No. 28, Fig. 1) were not used at the catchment scale (the lake was in a different catchment to all other wetlands and the data for that catchment were

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Fig. 1. Location of catchments in relation to Australian continent and wetlands surveyed in northwestern NSW. Numbered wetlands are those surveyed for waterbirds in the Paroo River catchment and Cobham basin (Kingsford et al., 1994). Hatched areas are regions subject to inundation.

incomplete) but were included in analyses of waterbirds on individual wetlands. 2.3. Estimation of wetland area In order to determine responses of waterbirds in the waterbird study area to changes in wetland area at the (1) local, (2) catchment and (3) broader scales, we mapped wetlands across arid Australia (about 70% of the continent) using NOAA AVHRR satellite imagery

(Roshier et al., 2001b). NOAA AVHHR satellite imagery has a spatial resolution of 1.1 km near nadir and a path width of  2600 km, giving continental coverage with two or more cloud-free images. During the period March 1987–March 1993, corresponding to 17 periods when aerial surveys were done, composite views of arid Australia were analysed using spectral matching to estimate wetland area in the study area and in the Lake Eyre Basin (see Roshier et al., 2001b for detailed methods).

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Fig. 2. Wetland area from satellite data (solid line) and monthly rainfall totals (grey bars) in Upper and Lower Eyre catchments, Cobham basin and Darling region of Murray–Darling Basin for the period 1986–1997 (after Roshier et al., 2001b). The Paroo, Warrego and Darling catchments lie in the Darling region. Rainfall data were not available after 1995. Solid line is discontinuous because cloud cover occasionally prevented determination of wetland area.

Wetland area was determined for individual wetlands on which birds were counted (local scale), the Cobham and Paroo River catchments (catchment scale), and the adjacent Darling and Warrego River catchments and for the Lake Eyre and Bulloo River basins (broad scale). The Lake Eyre Basin covers an enormous area (1,140,000 km2) with its northern parts extending into the tropics. It is internally draining with shallow gradients (Puckridge, 1998). Most rivers within the Basin inundate large areas of their floodplain after extensive rainfall, despite the Lake Eyre Basin having the lowest mean annual runoff of any major drainage basin in the world (Kotwicki and Allan, 1998). Two major rivers that drain the upper reaches of the Basin, the Diamantina River and Cooper Creek, are among the worlds’ most variable in flow (Puckridge et al., 1998). These rivers have a high ratio of floodplain area to total catchment area (Graetz, 1980), low flow velocities of floodwaters (Kotwicki, 1986) and long flood transmission times (Knighton and Nanson, 1994). The above characters combine with spatial variability in topography and hydrology to produce a diverse range of wetland habitats and biotic assemblages (Puckridge, 1998), including habitats that support all functional groups of waterbirds (see Kingsford and Porter, 1993; Kingsford et al., 1999a). At times the Lake Eyre Basin has vast areas of newly inundated wetland habitat (Roshier et al., 2001b). 2.4. Statistical analyses In order to determine whether waterbird abundance on one wetland was correlated with abundance on

another, count data for individual wetlands were analysed for spatial dependence (Legendre and Fortin, 1989; Legendre, 1993). To quantify large-scale trend an unweighted three-point spatial moving average (Bailey and Gatrell, 1995, p. 156) was applied to transformed [log (x +1)] count data. Residuals were generated by subtracting the smoothed data from observed log abundance and correlograms and variograms of the residuals examined for spatial dependence. To determine whether there was a relationship between abundance and wetland size, counts on individual wetlands were modelled against wetland area. Wetlands where no water and no waterbirds were recorded were removed from the analysis [186 of 476 counts (wetlandtime)]. Of the remaining 290 counts, 233 were for wetlands smaller than the 120 ha resolution of the satellite imagery. Dropping these counts to facilitate generation of a linear model was inappropriate because these counts accounted for much of the variation in waterbird abundance. Instead, the wetland data were divided into three area categories (0–120, 120–1200 and 1200+ha). The first category was the lowest resolution of the satellite imagery from which the wetland area data were derived and the other two roughly divided the remaining data. Estimates of the mean and standard error of each category were generated from the negative binomial distribution and the second and third categories compared with the first using Student’s t-test. Generalised linear models (GLM) with a negative binomial error structure (McCullagh and Nelder, 1989; White and Bennetts, 1996) were used to model changes in waterbird abundance at catchment and broad scales.

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For a negative binomial distribution, the mean is E(Y)= and the variance var(Y)=(1+F/F), where Oˆ is a dispersion parameter. Null deviance and F were estimated from an intercept model and the estimate of F fixed for all comparisons of deviance (Venables and Ripley, 1994, p. 201). Analyses were performed using Splus 3.4 (Mathsoft, 1995). To determine the best-fit model, variables were added to the model using a forward stepwise selection process (Nicholls, 1989), and the variable with the lowest residual deviance retained. All remaining variables were again added singly to the new model. This was continued until no additional variables were significant and the previous model was retained as the final model. Goodness-of-fit of the model was determined using the Chi-squared statistic (Venables and Ripley, 1994, p. 187) with direction of response in abundance to each significant variable determined from the sign of the t-value. Wetland area was treated as a continuous variable and change in wetland area as a categorical variable based on thresholds of change in wetland area (Table 1). Wetland area in all catchments or basins was highly variable and wetland area changed by orders of magnitude in a few weeks (Roshier et al., 2001b). As a result, there was a marked separation between events that resulted in incremental changes in wetland area and events that changed wetland area by several fold (Fig. 3). This separation was used to determine the lower threshold for incorporation of change in wetland area into the models as one of three categories of change; positive, negative or no change (Table 1). These analyses lacked power because of the limited number of data points (max. 17) and large variation in bird numbers. As a result, single data points could exert considerable influence, as was the case in December 1990 when an influx of black-tailed native hen increased counts of shoreline foragers five-fold to 8086 (853  1941; mean  SD). Therefore, the analyses were interpreted conservatively.

403

3. Results 3.1. Relationship between waterbird abundance and local wetland area Waterbird abundance on individual wetlands was independent of abundance on adjacent wetlands in all functional groups. Mean abundance of all functional groups of waterbirds was significantly greater on large (1200+ha) than on small wetlands (< 120 ha; Table 2) but, a general model of increasing waterbird abundance with increasing wetland area was not supported. Peak abundance on small wetlands (< 120 ha) in four functional groups approached or exceeded peak abundance on large wetlands (1200+ha). Furthermore, peak abundance in six of the seven functional groups was lower on mid-sized wetlands (120–1200 ha) than it was on the small wetlands. For deep-water foragers, mean abundance was also lower on mid-sized wetlands than on small wetlands. 3.2. Catchment and broad scale relationships Abundance of all functional groups of waterbirds changed markedly in the three months between surveys in all catchments (Fig. 4). For six of the seven functional groups, the greatest numbers overall occurred on the wetlands of the Paroo River catchment (Fig. 4). The exception was deep-water foragers. Numbers of individual species were highly variable and changed markedly in the three months between surveys. For instance, estimates of abundance of pink-eared duck in the Paroo River catchment occasionally changed by an order of magnitude in the three months between counts, for example from 28,268 to 1154 between March and June 1990. On another occasion, March 1993, numbers of waterbirds on a single lake in the Cobham catchment

Table 1 Thresholds of change in wetland area used as factors in generalised linear models to model changes in waterbird abundance in the survey area Catchment or region

Change in wetland area (ha)

Paroo Cobham Darling Warrego Bulloo Lower Eyre Upper Eyre Lake Eyre Basin

20,000 2000 15,000 10,000 100,000 100,000 100,000 100,000

Fig. 3. Frequency of changes ( ) of different magnitude in wetland area in the Paroo River catchment. Changes in wetland area were placed into classes of 5000 ha. The lower limit of the next to smallest size class was used as the threshold for change in wetland area in the analyses. In the case of the Paroo River catchment, this was 20,000 ha.

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Table 2 Comparisons of mean abundance and range (in parentheses) of waterbirds on wetlands categorised by size: <120 ha (group 1), 120–1 200 ha (group 2) and 1200+ha (group 3)

Mean (range) Group 1 (n=233) Group 2 (n=31) Group 3 (n=26) Tests Ho: m2=m1 Ho: m3=m1 Ho: m2 m3=0

Fish-eaters

Deep-water foragers

Dabbling ducks

Grazing waterfowl

Shoreline foragers

Large waders

Small waders

102 (0–2669) 173 (0–1066) 465 (0–3781)

302 (0–9304) 254 (0–2253) 1346 (0–12,545)

894 (0–28,827) 1299 (0–15,927) 5729 (0–56,597)

31 (0–987) 53 (0–405) 314 (0–5402)

21 (0–674) 225 (0–5,683) 106 (0–919)

50 (0–1,334) 51 (0–401) 430 (0–6594)

152 (0–10,388) 254 (0–4220) 485 (0–6546)

* *** **

n.s. *** ***

n.s. *** **

n.s. *** *

*** ** n.s.

n.s. *** ***

n.s. * n.s.

Means and SEs generated from negative binomial distribution and tested for similarity using two-tailed t-test (n.s., not significant). * P <0.05. ** P <0.01. *** P <0.001.

Fig. 4. Total abundance of waterbirds by functional groups for (a) the Paroo River catchment (22 wetlands), (b) the Cobham catchment (five wetlands) and (c) Lake Altibouka.

exceeded 50,000. Similar patterns of highly variable abundance were evident in other species and functional groups. Wetland area in the Paroo River catchment was a poor predictor of waterbird abundance in that catchment for all functional groups (Table 3). For fish-eaters,

dabbling ducks, grazing waterfowl, shoreline foragers, large waders and small waders in the Paroo River catchment the best predictor of abundance was change in wetland area of  100,000 ha in the Lake Eyre Basin and/or one of its component catchments, accounting for 40–56% of the observed deviance (Table 3). For shoreline foragers and small waders, change in wetland area of the same magnitude in the Bulloo River basin was also a significant predictor of abundance in the Paroo River catchment (Table 3). Changes in wetland area in the adjacent Warrego or Darling catchments did not predict waterbird abundance, except for grazing waterfowl. Only for shoreline foragers and small waders was change in wetland area in the local catchment a significant factor, accounting for 34.4% and 42.5% of the observed deviance in abundance, respectively. The direction of the relationships between waterbird abundance in the Paroo River catchment and changes in wetland area in adjacent catchments were not the same for all functional groups of waterbirds (Table 4). When wetland area in the Lake Eyre or Bulloo basins decreased by 100,000 ha or more the association was positive and waterbird numbers increased in the Paroo River catchment. This was true of fish-eaters, dabbling ducks, grazing waterfowl, shoreline foragers, and large and small waders. When wetland area in an adjacent basin increased, the direction of change in abundance differed among functional groups. There was an decline in numbers of dabbling ducks in the Paroo River catchment in the three months between surveys (Fig. 5). The same was true for small waders. By contrast, the numbers of fish-eating species and grazing waterfowl in the Paroo River catchment were positively related to positive and negative changes in wetland area in the Lake Eyre Basin (Table 4). Changes in waterbird abundance in the Cobham catchment could not be modelled because of numerous zero counts and the strong influence of high bird

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Table 3 Summary of single factor models of abundance of functional groups of waterbirds in the Paroo River catchment in relation to wetland area or change in wetland area, from negative binomial generalised linear models Model

Fish-eaters

Deep-water foragers

Dabbling ducks

Grazing waterfowl

Shoreline foragers

Large waders

Small waders

Wetland area in Paroo Change in Paroo (  20,000 ha)

n.s. n.s.

n.s. n.s.

n.s. n.s.

n.s. n.s.

n.s. * (34.4)

n.s. n.s.

n.s. * (42.5)

Adjacent catchments Wetland area in Darling Wetland area in Warrego Wetland area in Cobham Wetland area in Bulloo Wetland area in Lake Eyre Basin Wetland area in Upper Eyre Wetland area in Lower Eyre

n.s. n.s. n.s. n.s. * (25.3) n.s. * (28.2)

n.s. n.s. n.s. n.s. * (27.1) * (22.0) n.s.

n.s. * (26.2) n.s. n.s. n.s. * (21.1) n.s.

n.s. n.s. n.s. n.s. n.s. n.s. n.s.

* (24.1) n.s. n.s. n.s. n.s. n.s. n.s.

n.s. n.s. n.s. n.s. n.s. n.s. n.s.

n.s. n.s. n.s. n.s. n.s. n.s. n.s.

Adjacent catchments Change in Darling (15,000 ha) Change in Warrego ( 10,000 ha) Change in Cobham ( 2000 ha) Change in Bulloo ( 100 000 ha) Change in Lake Eyre Basin ( 100,000 ha) Change in Upper Eyre ( 100,000 ha) Change in Lower Eyre ( 100,000 ha)

n.s. n.s. n.s. n.s. n.s. * (56.3) n.s.

n.s. n.s. n.s. n.s. n.s. n.s. n.s.

n.s. n.s. n.s. n.s. ** (55.7) n.s. n.s.

n.s. * (34.6) n.s. n.s. n.s. ** (47.7) * (38.2)

n.s. n.s. n.s. * (36.9) * (39.7) * (36.3) * (38.7)

n.s. n.s. n.s. n.s. n.s. **(51.3) n.s.

n.s. n.s. n.s. * (38.9) * (45.7) * (40.6) n.s.

Value in parentheses is % deviance accounted for by model. Note: no multi-factor models were significant (n.s., not significant). * P <0.05. ** P <0.01.

abundance on one wetland. Abundance of waterbirds in this catchment is mostly low (Fig. 4) and the only large concentration of waterbirds in the Cobham catchment occurred in March 1993, following a prolonged period of rainfall that began in December 1992 (Fig. 2). Most birds in the region at this time were concentrated on a single lake, suggesting that that lake temporarily had biophysical characteristics attractive to waterbirds that were not shared by other lakes in the region.

4. Discussion 4.1. Patterns of abundance The dryland river systems of arid Australia tend to have frequent but irregular ‘booms’ and ‘busts’ with orders of magnitude changes in wetland area over a few months (Kingsford et al., 1999a; Roshier et al., 2001b). Our analyses suggest that the wetting and drying of these large temporary wetlands, and the magnitude of the changes, are important factors influencing the abundance of most functional groups of waterbirds in the Paroo River catchment. There are several mechanisms that could account for this relationship. Waterbirds may respond directly to an increase in food resources in adjacent basins following inundation and leave them once the initial pulse of productivity has

declined. Dabbling ducks followed this pattern (Fig. 5), with numbers rising and falling with changes in wetland area at broad scales. The decline in dabbling duck abundance in the Paroo River catchment occurred despite local increases in wetland area at the time. This suggests that the decline in abundance is not the result of local resource limitation. The most likely explanation is that the decline in abundance is the result of breeding dispersal. Rainfall in the region is often widespread and flooding in one catchment often coincides with flooding in adjacent catchments (Roshier et al., 2001b). The extent of wetland area in adjacent catchments is at times vast and food resources are likely to be superabundant (see later). It could be argued that the earlier pattern of abundance could be explained by dispersal away from the surveyed wetlands onto smaller temporary wetlands on the floodplain where birds were not counted in aerial surveys that concentrated on specific large wetlands. Change in wetland area in the Lake Eyre Basin was the best predictor of abundance in six of the seven functional groups of birds. Although the analyses lack power because of the limited data, and some flood events in the different catchments are correlated, this seems an unlikely coincidence. Alternatively, major flood events may draw waterbirds into the region from more mesic areas and numbers increase from recruitment and/or immigration. The

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Table 4 Direction of change (+ or ) in numbers of waterbirds in functional groups in the Paroo River catchment with wetland area and changes in wetland area in other catchments and basins in GLM Factor

Direction of change in abundance

Fish-eaters Wetland area in Lake Eyre Basin Wetland area in Lower Eyre Increase in wetland area in Upper Eyre Decrease in wetland area in Upper Eyre

+ + + +

Deep-water foragers Wetland area in Lake Eyre Basin Wetland area in Upper Eyre

+ +

Dabbling ducks Wetland area in Warrego Wetland area in Upper Eyre Increase in wetland area in Lake Eyre Basin Decrease in wetland area in Lake Eyre Basin

++

Grazing waterfowl Increase in wetland area in Warrego Decrease in wetland area in Warrego Decrease in wetland area in Upper Eyre Increase in wetland area in Lower Eyre Decrease in wetland area in Lower Eyre

+ ++ ++ + ++

Shoreline foragers Decrease in wetland area Wetland area in Darling Decrease in wetland area in Bulloo Decrease in wetland area in Lake Eyre Basin Decrease in wetland area in Upper Eyre Decrease in wetland area in Lower Eyre

+ + ++ ++ ++ ++

Large waders Decrease in wetland area in Upper Eyre

++

Small waders Increase in wetland area Decrease in wetland area Increase in wetland area in Upper Eyre Decrease in wetland area in Upper Eyre Increase in wetland area in Lake Eyre Basin Decrease in wetland area in Lake Eyre Basin

+ + +

Significance of result based on Chi-squared statistic and direction of response on sign of t-value (positive response indicated by +P <0.05, ++ P<0.01 and negative responses indicated by P <0.05, P <0.01).

productivity of dryland rivers in the region can be extraordinary following inundation of the floodplain or a flood pulse (Puckridge et al., 2000). Primary productivity may increase by two orders of magnitude (Peter Davies, Cooperative Research Centre for Freshwater Ecology, personal communication). This pulse of productivity drives food webs across vast areas of floodplain (103 km2) and is accompanied by a proliferation of aquatic invertebrates and fish (Puckridge et al., 2000; Stuart Bunn, Cooperative Research Centre for

Freshwater Ecology, personal communication). Even during the subsequent drying phase resources may remain non-limiting for some time and birds may continue to breed or immigrate into the region. This appears to be the case for fish-eating waterbirds and grazing waterfowl, because numbers are positively related to both increases and decreases in wetland area in the Lake Eyre Basin. These birds may be opportunistic in their use of arid zone wetlands, although breeding opportunities on the temporary wetlands of the arid zone may be important to the maintenance of the greater population. Shoreline foragers exhibit a pattern that combines elements of both the above, increasing only during drying phases in adjacent catchments with peak abundance after a series of wetting and drying events. This group includes black-tailed native hen which is irruptive following floods in inland regions (Marchant and Higgins, 1993) and the same pattern of abundance observed in the Paroo River catchment has been recorded in South Australia (Matheson, 1974, 1976). Given the marked changes in waterbird abundance in the 3 months between surveys and the relationship of abundance to changes in wetland area at broad scales, in the short-term changes in waterbird abundance at local and catchment scales in this environment are a function of movement and not demographic processes. Recruitment in endemic waterbirds is mostly aseasonal and asynchronous across the greater population and dependent on local conditions (Marchant and Higgins, 1990, 1993; Briggs, 1992). Thus, recruitment to populations is a broad scale process not readily quantified from observations at finer scales. The independence of observations on adjacent wetlands and the marked variation in waterbird abundance on wetlands of similar size (Table 2) suggests that birds are responsive to fine scale patterns of resource distribution, for example local prey abundance, as well as the broad scale patterns noted earlier. These patterns of abundance suggest a hierarchy of factors determining waterbird abundance at the catchment scale related to both the biophysical characteristics of individual wetlands and drainage patterns. Regions with extensive floodplains, such as the Paroo River catchment, tend to have wetlands that are more persistent and proximate to each other than elsewhere across the arid zone (Roshier et al., 2001b). As a result, there is a greater range of available habitats through time and waterbird abundance varies less at the catchment scale, even though local abundance may remain highly variable (Maher and Braithwaite, 1992). 4.2. Management implications Conservation of wetland ecosystems for waterbirds tends to concentrate on identifying individual wetlands,

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Fig. 5. Change in numbers of dabbling ducks (hatched bars) in the Paroo River catchment and change in wetland area in the Lake Eyre Basin (open bars). * Indicates minimum estimate; n.a., not available due to cloud cover in satellite imagery.

deemed important based on agreed criteria—the reserve system model. Under the Ramsar Convention for wetlands of international importance, for example, individual wetlands are classified as internationally important based on the regularity with which they support large numbers of waterbirds or portions of populations (Davis, 1994). These criteria do not explicitly recognise that wetlands in many environments are dynamic entities that vary through time in their suitability as wetland habitat for waterbirds (e.g. Kingsford and Porter, 1994; Timms, 1993; Neel and Henry, 1997) and individual wetlands that support significant numbers of breeding birds may remain dry for decades (e.g. Williams et al., 1998; Kingsford et al., 1999a). Thus, the importance of individual wetlands may be transient for waterbirds that interact with temporary wetlands on catchment and broader scales (Maher, 1988). Opinions are mixed on the ability of a reserve system to conserve mobile fauna that exploit ephemeral resources. Some advocate extensive networks of reserves and protected areas (Frederick et al., 1996; Fahse et al., 1998), while others are of the view that nomadic fauna defy adequate conservation using the reserve system model (Woinarski et al., 1992). If the processes that structure waterbird populations operate at broader spatial scales than those at which we usually seek to manage the landscape, then the significance of individual wetlands cannot be ascertained in isolation from the wetland mosaic within which they occur (Maher, 1991; Robinson and Warnock, 1997; Haig et al., 1998).

In such circumstances, resource use decisions that consider only local effects are inappropriate for the conservation of waterbirds. Alternatives to the reserve system are measures that protect conservation values on lands used for other purposes (Woinarski et al., 1992; Briggs, 1994). In Australia, the main conservation issue for waterbirds is water and its use across the landscape (Kingsford 2000a,b) and not the spatial arrangement of any fixed array of wetland reserves. Of more importance for waterbirds in Australia than the tenure of individual wetlands are the frequency and extent of intermittent flows in dryland rivers and how these affect the distribution of breeding habitat and/ or refugia. In Australia, we still lack fundamental knowledge on the broad scale movements of most waterbirds, including an explicit understanding of the scale at which individual species interact with their habitats and the triggers for movement. Wetland distribution at fine and broad scales and local waterbird abundances are so variable that most movements are unlikely to be associated with philopatry or seasonal migration (Anderson et al., 1992), except for the migratory shorebirds that winter on the continent and depart each autumn. Waterfowl command great interest from resource management agencies because they can be agricultural pests and some are hunted. In North America, adaptive management based on extensive population monitoring, the setting of population targets for various species, and manipulation of habitat has been advocated

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for waterfowl populations (Nichols et al., 1995; Williams et al., 1996; Williams, 1997). The North American Waterfowl Management Plan (NAWMP) attempts to manage the sometimes competing interests of hunters, farmers and the broad community. However, environmental variation, partial observability of broad scale processes, partial ability to control management actions and a lack of understanding of biological mechanisms limit the NAWMP’s success (Williams, 1997). In Australia, hunting of waterfowl is restricted and to date there has been little deliberate manipulation of habitat to manage the distribution and abundance of waterfowl. However, if this were to change and the need arose to manage waterfowl populations in Australia more actively the problems associated with environmental variation and partial observability of broad scale processes are likely to be greater than they are in North America. In part, this is due to greater uncertainties in wetland distribution, reproductive success, direction of dispersal and limited capacities to monitor habitat and waterfowl populations. In Australia, changes in total abundance of waterbirds result from changes to the frequency and/or extent of flooding and effect on breeding opportunities, compounded by the effects of water extraction, water storage, sedimentation and/or climate change on the distribution of refugia during dry periods. These changes would be difficult to determine

because of the huge area over which wetland habitat must be monitored and variations in response of individual species to these factors. As a result, the opportunity for management agencies to respond to any significant change in waterfowl numbers would probably have passed by the time the agent(s) of change had been identified. If recruitment to waterfowl populations is primarily driven by processes operating at broad scales, the opportunities to actively manage waterfowl populations in this environment are probably few. Given these uncertainties, the conservation of waterbirds in Australia will take a commitment by the community to conserve wetland resources and ecosystem function over large areas in the face of uncertain benefits and outcomes.

Acknowledgements We thank Rod Rumbachs and Gary McKenzie for advice on technical matters and Bernard Ellem for statistical advice. Comments by Amy Jansen, David Watson and an anonymous referee greatly improved the manuscript. This research was supported by the National Wetlands Program of Environment Australia and Land and Water Australia, AUSLIG and a Charles Sturt University Postgraduate Scholarship to DR.

Appendix Waterbird species counted in the study area classified by functional group. Taxonomic nomenclature follows Christidis and Boles (1994) Functional group

Common name

Scientific name

Fish-eaters

Great crested grebe Small grebes Hoary headed grebe Australasian grebe Australian pelican Darter Great cormorant Little pied cormorant Little black cormorant Pied cormorant Great egret White-necked heron White-faced heron Other egrets Intermediate egret Little egret Nankeen night heron Silver gull Whiskered tern

Podiceps cristatus Poliocephalus poliocephalus Tachybaptus novaehollandiae Pelecanus conspicillatus Anhinga melanogaster Phalacrocorax carbo Phalacrocorax melanoleucos Phalacrocorax sulcirostris Phalacrocorax varius Ardea alba Ardea pacifica Egretta novaehollandiae Ardea intermedia Egretta garzetta Nycticorax caledonicus Larus novaehollandiae Chlidonias hybrida

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Functional group

Common name

Scientific name

White-winged black tern Caspian tern Gull-billed tern

Chlidonias leucoptera Sterna caspia Sterna nilotica

Deep-water foragers

Black swan Hardhead Musk duck Blue-billed duck Eurasian coot

Cygnus atratus Aythya australis Biziura lobata Oxyura australis Fulica atra

Dabbling ducks

Chestnut teal Grey teal Australasian shoveler Pacific black duck Pink-eared duck Freckled duck

Anas castanea Anas gracilis Anas rhynchotis Anas superciliosa Malcorhynchus membranaceus Stictonetta naevosa

Grazing waterfowl

Australian wood duck Plumed whistling-duck Australian shellduck

Chenonetta jubata Dendrocygna eytoni Tadorna tadornoides

Shoreline foragers

Black tailed native-hen Purple swamphen Masked lapwing Banded lapwing

Gallinula ventralis Porphyrio porphyrio Vanellus miles Vanellus tricolor

Large waders

Yellow-billed spoonbill Royal spoonbill Glossy ibis Australian white ibis Straw-necked ibis Brolga

Platalea flavipes Platalea regia Plegadis falcinellus Threskiornis molucca Threskiornis spinicollis Grus rubicundus

Small waders

Banded stilt Black-winged stilt Red-necked avocet Small waders Sharp-tailed sandpiper Red-capped plover Black fronted plover Red-kneed dotterel Black-tailed godwit Common greenshank Marsh sandpiper

Cladoryhnchus leucocephalus Himantopus himantopus Recurvirostris novaehollandiae

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