Four desert waters: Setting arid zone wetland conservation priorities through understanding patterns of endemism

Biological Conservation 144 (2011) 2459–2467

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Four desert waters: Setting arid zone wetland conservation priorities through understanding patterns of endemism R.J. Fensham a,b,⇑, J.L. Silcock b, A. Kerezsy c,d, W. Ponder e a

Queensland Herbarium, Department of Environment and Resource Management, Mt. Coot-tha Road, Toowong, Qld 4066, Australia The Ecology Centre, School of Biological Sciences, University of Queensland, St. Lucia, Qld 4072, Australia c Australian Rivers Institute, Griffith University, Qld 4111, Australia d Bush Heritage Australia, Melbourne, Victoria 3000, Australia e Australian Museum, 6 College Street, Sydney, NSW 2010, Australia b

a r t i c l e

i n f o

Article history: Received 11 February 2011 Received in revised form 16 June 2011 Accepted 27 June 2011 Available online 6 August 2011 Keywords: Arid zone Conservation Endemism Wetlands

a b s t r a c t Long-lasting surface water in arid-lands provide oases for aquatic biota, but their values as biological refugia have rarely been assessed. This study identified and mapped permanent natural wetlands across the Eastern Lake Eyre Basin in Australia and classified them into four types: riverine waterholes, rockholes, discharge springs and outcrop springs. Waterholes are the most widespread and numerous source of lasting water, while springs and rockholes are confined to relatively discrete clusters. The characteristics of each wetland type are summarised, and their biological values compared by examining various scales of endemism for vascular plant, fish and mollusc species. Discharge springs contain an exceptional concentration of endemic species across all three lifeforms at a range of scales. Waterholes are critical drought refugia for native fish species that also utilise a vast network of ephemeral streams during and after floods. Rockholes and outcrop springs do not contain any known specialised endemics, although the latter have disjunct populations of some plants and fish. The existing knowledge of antiquity, connectivity and habitat differentiation of the wetland types is compiled and their role in determining biological endemism is discussed. Exotic fish are a major conservation issue, the recovery of the discharge springs should be paramount, and the intact network of permanent waterholes should be preserved. A focus on endemism, combined with an understanding of the biogeographical processes underlying the observed patterns provides an effective and systematic approach to setting priorities for regional biodiversity conservation. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction The sharp contrast of permanent surface waters with the surrounding environment in arid-lands engenders assumptions about their biological importance. Desert wetlands encompass a range of hydrochemical, hydrophysical and morphological characteristics (Williams, 1999) that create many different habitat types for aquatic organisms. Biological diversity and endemism of wetlands has been assessed on a global scale (Junk, 2006; Abell et al., 2008; Balian et al., 2008) and regional studies have focused on single wetland types (Sorrie, 1994; Pinder et al., 2000; Fensham and Fairfax, 2003; Timms, 2007; Strong et al., 2008) or taxonomic groups (Simovich, 1998; Kingsford et al., 1999; Ponder and Walker, 2003; Arthington et al., 2005; Balcombe et al., 2006; Brock et al., 2006; Ponder and Slatyer, 2007). These studies reveal that some desert wetlands exhibit local

⇑ Corresponding author at: Queensland Herbarium, Department of Environment and Resource Management, Mt. Coot-tha Road, Toowong, Qld 4066, Australia. Tel.: +61 (0)7 38969547; fax: +61 (0)7 38969624. E-mail address: [email protected] (R.J. Fensham). 0006-3207/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocon.2011.06.024

endemism, but more typically they support species with broad ranges, including fish across entire catchments and many plants with cosmopolitan distributions. With good quality survey data, biological diversity can be analysed in relation to habitat specialisation and endemism can be circumscribed, allowing for a rigorous assessment of the conservation significance of the various wetland habitats (Keddy and Sharp, 1994; Fensham and Price, 2004; Horwitz et al., 2009). Focussing conservation efforts on hotspots of biological endemism is judicious because these species often have small populations, are vulnerable to extinction (Gaston, 1998) and represents the efficient accumulation of species for reserve planning (Lamoreux et al., 2006). Conservation priorities will be better informed by an understanding of ecosystem processes and biogeographic history. In general, wetlands tend to have low levels of endemism, because severe fluctuations in water levels during the Pleistocene and Holocene favour plants and animals that are mobile or readily dispersed (Junk, 2006; Horwitz et al., 2009). In particular, species occupying ephemeral wetlands must have dispersal capabilities and generally exhibit little genetic structure (Marten et al., 2006; Abellan et al., 2009). However,

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wetlands that have persisted through glacial periods of aridity provide habitat for relictual species that were more widespread in wetter times (De Deckker, 1986; Davis et al., 1993; Horwitz et al., 2009; Murphy et al., 2010). For example, artesian springs fed by the Great Artesian Basin have many endemics (e.g., Ponder, 2003; Fensham et al., 2010) and wetlands within sandstone formations in northern Australia harbour a relatively high number of endemic amphibians and reptiles that live in ancient, permanent wetlands (Finlayson et al., 2006). The physical and chemical conditions presented by some wetland habitats are also important for promoting differentiation of organisms and ultimately speciation (King et al., 1996; Hopper et al., 1997; Simovich, 1998; Ponder and Colgan, 2002). For example the endemic flora of vernal pools in California is associated with the physical and chemical nature of different pool types (Holland and Jain, 1981). Isolation is another factor that begets genetic divergence and allopatric speciation (Moritz et al., 2000; Krosch et al., 2009). Rivers act as conduits allowing for the dispersal of aquatic species between wetlands within catchments and isolation between catchments. As such, freshwater fish assemblages are typically characterised by low levels of endemism within catchments (Burridge et al., 2008; Nogueira et al., 2010). The eastern Lake Eyre Basin of Australia is a vast area with renowned wetland values, a range of wetland types and difficult conservation challenges (Morton et al., 1995; Kingsford et al., 1999; Fensham and Fairfax, 2003; Ponder, 2003; Reid, 2010). This paper describes a classification scheme and compiles comprehensive mapping of all permanent natural surface waters for that region. The biological endemism of each wetland type is then explored and compared, using the distribution of vascular plants, fish and molluscs. These groups were chosen because they exhibit endemism (unlike other vertebrates) and there taxonomy is adequate (unlike other invertebrates). Patterns of endemism are discussed in relation to the antiquity, habitat differentiation and isolation of the wetland habitats. The implications for conservation and wetland management in the Australian arid zone are presented. Given limited conservation resources, a focus on endemism provides an effective and systematic approach to setting priorities for regional biodiversity conservation.

Agency, 2005), historical sources (Fensham and Fairfax, 2003) and interviews with 170 long-term land managers using annotated maps to guide discussions (Silcock, 2009). All wetlands containing water for more than 70% of the time were included in a spatial database, but the permanent wetlands are the focus of this paper. Permanence is defined as never having been without free water during European pastoral settlement, as ascertained through the oral and written record. Information about the permanence of large waterbodies has usually been passed down through successive land managers, so records typically begin around 1870– 1880. The classification of wetland types is regionally specific, based on geomorphology and hydrology (Semeniuk and Semeniuk, 1995; Kingsford et al., 2004). 2.3. Assessing biological values The most complete surveys for the permanent wetlands in the study area are for vascular plants, fish and molluscs. For some wetland types surveys of these groups have been comprehensive and there is also substantial information in Herbaria and Museum databases (Table 1). Only native species that are dependent on surface water for all or nearly all of their life-cycle within the study area were included in the analysis. Waterbirds have been well studied, but were excluded because they move large distances between wetlands so are not intimately dependent upon permanent waters (Kingsford and Norman, 2002; Roshier et al., 2002). Plants such as Eremochloa bimaculata, which are restricted to permanent water in the study area but occur in other habitat outside the arid zone were included in the analysis. Undescribed species were included where a taxonomic expert regards the taxa as a species, but subspecies were excluded. Endemism only has meaning in a geographic context (Anderson, 1994) and the term is applied here at a range of scales for each wetland type. Each species was given a ‘geographic endemism’ score, ranging from 1 to 5 depending on meaningful scales for the particular wetland type (Table 2). Sites were ranked according to their concentration of narrow endemics (category 5, Table 2). 3. Results

2. Methods

3.1. Overview of classification

2.1. Study area

Permanent natural wetlands in the ELEB are classified into four types: riverine waterholes, rockholes, outcrop springs and discharge springs (Fig. 2). None of the lakes in the study area are permanent. There are 260 permanent waterholes across the ELEB (200 in the Cooper catchment, 38 in the Diamantina and 22 in the Georgina), 18 rockholes, 52 outcrop spring complexes and 24 active discharge spring complexes. Many of the wetlands have been subject to modification. Most of the wetlands are subject to disturbance by domestic or feral grazing animals. Some of the springs (both outcrop and discharge) have been excavated to the extent that the extinction of local populations has probably occurred. Many of the discharge springs have been deactivated with reduced groundwater pressure (see below). There are also artificial (human constructed) permanent water sources including aboriginal wells, bore-drains and large dams. Some of these have conservation values (Noble et al., 1998), including occasionally providing habitat for endemic species (Appendix A) associated with discharge springs, but they are not considered further in this paper.

The Eastern Lake Eyre Basin covers an area of 679,000 km2, or approximately one-seventh of the Australian continent, and comprises the catchments of the Georgina and Diamantina Rivers and Cooper Creek (Fig. 1). The climate is semi-arid to arid, with average annual rainfall ranging from 500 mm in the northern and eastern headwater regions to 120 mm in the southwest (McMahon et al., 2008). These rivers are characterised by low gradients, wide floodplains, large transmission losses, limited base-flows and are probably the most variable in the world (Puckridge et al., 1998). In their mid and lower catchments, they spread out to form extensive braided river channels and floodplains. The surrounding matrix of grassland, stony plains, low ranges and open shrublands (Tyler et al., 1990; Sattler and Williams, 1999) are used for extensive grazing (predominantly cattle), with relatively small areas of mining leases and conservation reserves. 2.2. Mapping and classifying permanent natural wetlands

3.2. Waterholes Wetlands were mapped using a combination of satellite imagery, which identified long-lasting waters from a time-series of Landsat scenes (Wainwright et al., 2002; Environmental Protection

Waterholes are enlarged segments of an ephemeral or seasonal watercourse which hold water after stream-flow has ceased

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Fig. 1. The distribution of the four types of natural permanent wetlands in the ELEB and placenames from the text. Pelican Creek Springs, Coreena Springs and Smokey Springs are in the Barcaldine Supergroup; and Elizabeth Springs and Reedy Springs are in the Springvale supergroup. Preservation of these spring complexes would also encompass category 4, 3 and 2 endemics from discharge springs.

Table 1 Sources and sampling effort for assessing biological values of wetlands. Wetland type

Species group

Source

Waterholes

Plants Fish Molluscs

Queensland Herbarium records 5% Surveyed, Museum records Museum records

Rockholes

Plants Fish Molluscs

39% Surveyed 39% Surveyed 39% Surveyed, Museum records

Outcrop springs

Plants Fish Molluscs

60% Surveyed 60% Surveyed 60% Surveyed, Museum records

Discharge springs

Plants Fish Molluscs

Comprehensive survey Comprehensive survey 90% Surveyed

(Knighton and Nanson, 2000). They are connected to the entire stream system during flows which can scour their muddy base

and refresh local aquifers in underlying sand deposits (Cendon et al., 2010). Between flow events they become isolated and diminish in size and depth due to evaporation, although some are augmented by groundwater. Waterholes are the most widespread, numerous and conspicuous source of permanent water in the ELEB (Fig. 1) and can range in length from 50 m to over 20 km. Most permanent waterholes occur on main river channels, often forming at points where flow becomes concentrated or constricted (Knighton and Nanson, 1994a). Waterhole permanence is determined by four interrelated factors: depth, frequency of inflow, rate of water loss and groundwater interactions. In general, a depth of about 4 m is required for a waterhole in the ELEB to persist through extended droughts (Costelloe et al., 2007). Frequency of inflow depends upon the position of a waterhole on the river channel (Hamilton et al., 2005) and the nature of surrounding country, which determines local run-off. Water is lost primarily through evaporation, rates of which are higher in wide, flat waterholes. In many ELEB waterholes, suspended clays settle out after flow events to form

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Table 2 Definition of geographic endemism in order of decreasing scale and score as applied to the aquatic species of the ELEB. Endemism category (geographic range) 5 (<50 km2) 2

4 (20,000–300,000 km ) 3 (680,000–1.7 M km2) 2 1

Waterhole

Rockhole

Outcrop spring

Discharge spring

Confined to an individual site or local area Single catchment (i.e. Georgina, Diamantina, Cooper) Eastern Lake Eyre Basin Australian arid-zone (<500 mm mean annual rainfall) Elsewhere including coastal and higher rainfall areas

Confined to an individual site or local area Single mountain range system

Confined to an individual site or local area Single mountain range system

Eastern Lake Eyre Basin Australian arid-zone (<500 mm mean annual rainfall) Elsewhere including coastal and higher rainfall areas

Eastern Lake Eyre Basin Australian arid-zone (<500 mm mean annual rainfall) Elsewhere including coastal and higher rainfall areas

Confined to an individual site or local area Spring super-group (Fensham and Fairfax, 2003) Great Artesian Basin Australian arid-zone (<500 mm mean annual rainfall) Elsewhere including coastal and higher rainfall areas

Fig. 2. The four desert waters from the ELEB study area clockwise from top left: waterhole; rockhole; outcrop spring; discharge spring.

a bottom ‘seal’, minimising seepage losses (Knighton and Nanson, 1994a). In contrast, sandy river beds allow water percolation, so generally do not harbour long-lasting surface water. Although groundwater inflow is not a major factor influencing permanence in the Cooper and Diamantina systems (Hamilton et al., 2005; Costelloe et al., 2007), 17 of the 22 permanent waterholes in the Georgina catchment are enhanced by groundwater. Permanent waterholes occur in clusters throughout the catchments and their major tributaries, but do not occur in the lower reaches towards Lake Eyre where flow transmission is greatly diminished (Knighton and Nanson, 1994b; Costelloe et al., 2003).

their persistence. Unlike waterholes, rockholes only 2 m deep can be permanent, as they are replenished during small rainfall events in the surrounding rocky landscape. Rockholes in the ELEB are poorly known and they are not mentioned in any land resource assessments or vegetation mapping of western Queensland (Perry, 1964; Dawson, 1974; Mills, 1980). In some areas, long-term landholders have detailed knowledge of the location and permanence of rockholes. Often, however, only the approximate location of rockholes is known and information on their permanence is sketchy. 3.4. Outcrop springs

3.3. Rockholes Rockholes are natural hollows in rocky landscapes, formed from fracturing and weathering, which store water from local run-off. Rockholes are dotted throughout the granite and sandstone ranges of inland Australia, particularly in sheltered gorges and in the heads of gullies (Bayly, 2001). Most are ephemeral or semi-permanent, and the 18 recorded permanent rockholes in the ELEB occur in three clusters in Tertiary sandstone ranges on catchment watersheds in the northern and eastern higher-rainfall areas (Fig. 1). They often occur in gully heads at the base of small cliffs. Rockhole permanence is determined by their size and depth, while shade from rock overhangs and fringing vegetation may also enhance

The defining character of springs is that they are entirely dependent on groundwater. Outcrop springs occur where sediments forming the aquifer are outcropping. They include springs in the recharge areas of the Great Artesian Basin emanating from Lower Cretaceous-Jurassic sediments on the eastern margin of the study area and also springs emanating from Tertiary sandstone widespread in the central part of the study area (Fig. 1). Because the groundwater in these local aquifers can have relatively short residence times, some of these springs dwindle to seepages or disappear completely in dry times. The water is generally slightly acidic (comparable to rainwater) reflecting low concentrations of dissolved solids (Fensham et al., 2004a).

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Accurately counting both outcrop and discharge springs is difficult because they tend to be clustered at a range of scales, from individual vents to ‘super-groups’ at regional scales (Fig. 1). For the purposes of this study we have mapped at the ‘spring complex’ scale, defined as a group of springs where no adjacent pair of springs is more than 6 km apart and all springs within the complex are in a similar geomorphic setting (Fensham and Fairfax, 2003).

3.5. Discharge springs Discharge springs are distinguished from outcrop springs because they emanate through confining beds (aquitards), including alluvial soil, in areas remote from where the aquifer receives its input. Because the groundwater has a very long residence time in the GAB aquifer (Torgersen et al., 1991) discharge springs show minimal fluctuations in flow rates, and the water is alkaline with high concentrations of dissolved solids (Habermehl, 2001; Fensham and Fairfax, 2003; Fensham et al., 2004a). The discharge springs typically occur through fault structures, where the aquifer adjoins protrusions of basement rock, or where the confining beds are sufficiently thin to allow discharge. In the study area all discharge springs emanate from the vast aquifer of the Great Artesian Basin (GAB), and sustain small permanent wetlands. Of the 49 GAB spring complexes in the study area, only 25 remain active, and nine of these are diminished due to declines in flow (Fensham and Fairfax, 2003) (Table 3). The discharge spring complexes comprise three ‘super-groups’: the Mulligan River super-group on the western edge of the GAB, the Springvale super-group on the Diamantina-Georgina watershed east of Boulia, and the Barcaldine super-group along the eastern margin of the Basin (Fig. 1). All spring complexes in the Mulligan River super-group still have at least some active springs, however only one-third of spring complexes in the Barcaldine and Springvale supergroups remain active (Table 3).

Table 3 Discharge spring complexes in the study area. Supergroup

Inactive

Some springs inactive

Barcaldine Springvale Mulligan River Grand total

14 9 0

3 4 3

23

10

All springs active

Grand total

Proportion active (%)

5 2 9

22 15 12

36 40 100

15

49

49

3.6. Biological values and geographic endemism A full list of the species and their associated habitats are included in Appendix A. Although waterholes in the ELEB contain many fish, mollusc and plant species, most have very large geographic ranges with the vast majority also known from other drainage divisions (Fig. 3). No fish from the waterholes are narrow-range endemics (category 5), although the Cooper Creek catfish (Neosiluroides cooperensis) is restricted to the Cooper catchment and both Barcoo grunter (Scortum barcoo) and Welch’s grunter (Bidyanus welchi) are known only from catchments within the Lake Eyre Basin. Only one plant, Nymphaea georginae, is apparently restricted to the ELEB, having been collected only from the Georgina and Thomson Rivers. Similarly, only one mollusc, Larina lirata, is restricted to waterholes in the ELEB. The biota of rockholes is the most poorly known because of the general inaccessibility of these wetlands (Table 2). The five permanent (plus numerous semi-permanent) rockholes sampled contain low diversity of all species and revealed no evidence of endemism at any scales. Outcrop springs have a higher number of species, however all species are widespread. Geographically isolated populations of plants (Cyclocorus interruptus, Scleria rugosa, Stylidium velleioides and Spirodela punctata), a mollusc (Sermyla sp.) and the Flinders Ranges mogurnda (Mogurnda clivicola) are known from some outcrop springs despite not occurring elsewhere in the ELEB. GAB discharge springs have the highest concentrations of endemic species and contain the only narrow-range endemics found across the four wetland types, with 18 species of molluscs, four plants and three fish restricted to single meta-populations spanning <50 km2 (Table 2 and Fig. 3). The diversity of the molluscs in the genus Jardinella is notable, with 12 locally endemic species. Almost half of all species recorded from discharge springs are endemic to the GAB aquifer (scores 3–5), including 12 plant species with populations widely separated in disjunct populations (Appendix A). Endemic fish are the red-finned blue-eye (Scaturiginichthys vermeilipinnis), known only from the Pelican Creek spring complex and two species of goby, Edgbaston goby (Chlamydogobius squamigenus) and Elizabeth Springs goby (Chlamydogobius micropterus), also restricted to single but widely separated spring complexes. The sites with narrow endemics can be ranked and include five sites (Fig. 1): Pelican Creek Springs (2 fish, 14 snails, 3 plants), Elizabeth Springs (1 fish, 1 snail), Reedy Springs (1 snail), Coreena Springs (1 snail), Smokey Springs (1 snail). Preservation of these spring complexes would also encompass category 4, 3 and 2 endemics from discharge springs.

40 35

Number of species

30 25

5 4 3 2 1

20 15 10 5 0 F

M Waterholes

P

F

M Rockholes

P

F

M

P

Outcrop springs

F

M

P

Discharge springs

Fig. 3. Geographic endemism by water-type and lifeform (F, fish; M, molluscs; P, plants), Eastern Lake Eyre Basin.

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4. Discussion 4.1. Patterns of endemism There is very little endemism at any scale for plants, fish and molluscs known to inhabit waterholes, rockholes and outcrop springs. This is similar to wetland endemism in other areas (Ritter and Crow, 2005; Junk, 2006; Horwitz et al., 2009). Most species of aquatic plants are readily dispersed by water or birds, so are generally widespread and common, although often restricted to wetland habitats, including floodplains and ephemeral swamps. Similarly, the fauna species found in rockholes, outcrop springs and waterholes seem to be opportunistic dispersers, are widespread and inhabit a variety of wetland types (Pinder et al., 2000; Ponder and Walker, 2003; Arthington et al., 2005). The extraordinary endemism of the GAB discharge springs in the study area is discussed in relation to the antiquity, isolation and habitat differentiation of the wetland types in the ELEB (Table 4). 4.2. Antiquity There is substantial evidence of more powerful stream flows than those that occur at present through most of the Quaternary (Maroulis et al., 2007). The clays underlying channels in Cooper Creek have been dated at 80,000 y BP. Before this time the stream was significantly more active with perennial flow hydrology and large meandering channels resulting in extensive sand units being laid down with limited clay at depth (Nanson et al., 2008). It seems likely that the ancestors of extant aquatic organisms (such as the red-finned blue-eyes, gobies, molluscs and presumably the other endemic invertebrates) currently confined to the discharge springs originated in these extensive and very different streams. After about 80,000 y BP narrower, sinuous river channels became the norm as fluvial mud was transported and deposited. The development of source bordering dunes contributed to the concentrated flows, resulting in the scouring that created waterholes (Maroulis et al., 2007). During the height of the last glacial period (since 55 K BP) there were arid phases (Nanson et al., 2008; Magee et al., 2009) that may have resulted in a reduction in the network of permanent waterholes compared to the present. Despite short dry phases, there is substantial evidence of more powerful streams than at present through most of the Quaternary (Maroulis et al., 2007). In contrast to the much larger riverine waterholes, and given their local catchment area and shallow depth, it is likely that rockholes would not have been permanent during the most arid phases of the Quaternary. The throughflow structure of the Great Artesian Basin (GAB) was initiated by a sequence of uplift events, the most important of which formed the Great Dividing Range some time after 20 Ma (Ollier, 1982). However, individual springs are almost certainly much youn-

Table 4 Ascribing the significance of antiquity, isolation and habitat differentiation on a threepoint scale (0, +, ++) to the four wetland types discussed in this paper with supporting references.

Waterholes Rockholes Outcrop springs Discharge springs a b c d e f

Antiquity

Isolation

Habitat differentiation

++a +b +b ++e

0b +b +b +b

+c 0b +d ++f

Nanson et al. (1992) and Magee et al. (2009). This paper. Marshall et al. (2006) and Kerezsy (2010). Fensham et al. (2004a). Prescott and Habermehl (2008). Fensham and Fairfax (2003) and Fensham et al. (2004a).

ger than the age of the Basin itself. Fossilised spring deposits at Beresford Hill in South Australia date to 128 ka and are elevated on a residual land surface 40 m above the surrounding surface that supports current active discharge springs (Prescott and Habermehl, 2008). In addition to erosion and deposition in the surrounding landscapes, the disruption of fault structures is also likely to have profound effects on spring activity over longer time-scales. The accumulation of material where sub-soil has been transported upwards by artesian water, the accretion of calcium carbonate as cemented travertine, the accumulation of aeolian sand in wetland vegetation and the development of peat from spring wetland vegetation can allow for the development of mounds. Lateral and vertical dynamics in wetland vegetation and substrates can disrupt the surface flow of groundwater at individual vents (Fatchen, 2000; Fensham et al., 2004b). Given the protracted residence in the aquifer, it is also possible that groundwater discharge may be affected by climate regimes from previous eras (Prescott and Habermehl, 2008). While discharge from individual spring vents and across spring clusters and super-groups has been variable, some springs within the network must have remained active over extremely long time periods, maintaining populations of spring-dependent biota and the persistence of endemic species. The age of the outcrop springs is more difficult to define, but they are certainly more dynamic than the discharge springs on decadal timescales. The water feeding these springs has a relatively short residence in the aquifer and hence their flow regimes will reflect recent rainfall history. Furthermore there seems to be some natural dynamism in the outcrop springs as some springs in Tertiary sandstone have become inactive in elevated landscape positions where the absence of bores makes it difficult to invoke groundwater extraction as the cause. 4.3. Isolation The apparent failure of certain fish species to colonise from the Cooper west to the Georgina/Diamantina (Australian smelt, carp gudgeon, Cooper Creek catfish) and vice versa (golden goby, banded grunter) could be related to the highly temporary and saline migration pathway of Lake Eyre. With the exception of Cooper Creek catfish, all of these species are distributed in other Australian drainage divides and are widespread. This suggests that the current distributional range of these species may be constrained by migration barriers rather than habitat and dietary requirements or life history characteristics. Most of the other wetland types have some connectivity with stream systems during rainfall and/or flood events, although some outcrop springs occupy elevated positions in the landscape and are completely isolated from current watercourses. All of the discharge springs with endemic fish occur on floodplains and are periodically connected to river systems. In theory, this provides colonisation opportunities between springs and waterholes, but to-date there is no evidence of endemic spring species occurring in adjacent riverine areas. The exotic fish species gambusia (Gambusia holbrooki) is capable of colonising discharge springs following overland flow events (Fairfax et al., 2007) and its small size allows it to persist in the shallow pools. Larger fish such as spangled perch (Leiopotherapon unicolour) and desert rainbowfish (Melanotaenia splendida tatei) are also able to colonise springs, rockholes and other isolated water points such as bore drains and pools but are generally unable to persist except in sufficiently deep areas. 4.4. Habitat differentiation In contrast to waterholes and rockholes, both types of springs often form shallow pools <0.5 m deep. Most of the discharge springs exhibit comparatively high conductivity (1000 lS/cm)

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and alkalinity (150–1400; typically pH 7.5–9.5) (Ransley, 2003) with high loads of carbonate that can be precipitated as travertine around spring vents. The water chemistry of the discharge springs most obviously sets them apart from the other three wetland types in the region, and the endemic biota is associated with the springs of alkaline and calcareous water chemistry. The eight springs in the ELEB with sulphate levels greater than 50 ppm represented in a water chemistry data-set of 49 springs taken from the study area support no endemics at all (Ransley, 2003). Turbidity is generally low in discharge springs and outcrop springs and moderate in rockholes. In contrast, waterholes are typically extremely turbid (Secchi depth 2–20 cm), generally non-saline (mostly <300 lS/ cm) and pH values tend more towards neutral (A. Kerezsy, unpublished data). Water chemistry and in particular salinity has been identified as an important control on wetland assemblages (Holland and Jain, 1981; Kirkpatrick and Harwood, 1983) and aqueous calcium carbonate is an essential component of mollusc shells. The discharge springs are also distinct because the perennial flows to the springs support perennial mat- and clump-forming plants (Fig. 1). These create micro-habitats including shallow pools and mini-stream channels with well-oxygenated clear water. Such habitats seem to be particularly well-suited to small-bodied invertebrates, including snails, leeches, flatworms and crustaceans, all of which are represented by local endemics (Ponder, 2003; Fensham et al., 2010).

4.5. Interacting factors While the waterholes are probably sufficiently stable and ancient to support endemism, because they are well-connected this is not expressed at local scales. However, isolation alone does not appear to generate endemism, as the most isolated wetlands are some of the outcrop springs. Although these wetlands can support highly disjunct populations of widespread plants and fish, they are either too impermanent or not sufficiently distinct from nearby transient aquatic habitats to support endemics. While many of the discharge springs are connected by overland flow during flooding, these shallow surface waters may be too unreliable for long distance dispersal between the tiny isolated spring habitats (Ponder and Colgan, 2002; Worthington-Wilmer et al., 2008). There is some evidence that protracted inundation during flood can cause local extinctions of hydrobiid snails (Ponder et al., 1989). Given the likely dynamism of the springs, the persistence of endemic populations in clusters of spring wetlands requires that local extinctions are balanced by local colonisation events, and the genetic structure of snail populations in spring clusters at local scales suggests that they act as meta-populations (Worthington-Wilmer et al., 2008). The persistence of endemic snails in the discharge springs seems to be influenced by two apparently opposing processes – isolation over long time scales allowing for evolution of endemic taxa (Ponder and Slatyer, 2007) and meta-population dynamics, that allow species to survive in clusters of small aquatic habitats where individual wetlands can become permanently or temporarily unsuitable. The same processes have undoubtedly shaped the evolution and persistence of the endemic fish within the discharge springs. Thus the combination of isolation, antiquity and their water chemistry and morphology appears to be essential to the development of endemism in the springs, with capacity for dispersal and life history differences influencing the genetic structure in the aquatic species (Murphy et al., 2010). The dispersal capacities of the taxa partially relate to the patterns of endemism. Most of the endemic snails have little capacity for dispersal (Ponder and Colgan, 2002), and narrow ranges, the endemic fish have capacity for dispersal but also have narrow ranges, while the plants

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have unknown dispersal capacities and exhibit endemism at variable scales (Appendix A). The assemblage of ecological communities is governed by complex processes such as dispersal, biogeographic history, evolution and environmental filtering. The development of analytical tools to evaluate the relative influence of these factors has been recently advanced, but is limited by adequate data in complex ecosystems. There is increasing recognition of the importance of evolution in the assembly process (Cavender-Bares et al., 2009). Arid-zone wetlands are very discrete and relatively simple ecosystems with clear evidence for divergent controls on their biological assemblages. The opportunity to advance our understanding of community assembly processes in the wetlands of the ELEB requires further development of the phylogenetic relationships between taxa both within, and beyond the wetland network; more comprehensive attribution of habitat characteristics (such as water chemistry), and dispersal syndromes (such as seed viability). Biogeographic history has been inferred from the extent to which these factors fail to explain community assembly (Leibold et al., 2010). However, in the case of springs at least there are opportunities to provide independent corroboration of antiquity through the ageing of spring deposits from the d18O signature (Asmerom et al., 2010). 4.6. Conservation issues and implications The analysis presented here is only one perspective on the natural values of arid-zone ecosystems and conservation priorities. Other perspectives include the preservation of the unique flow regimes in the streams (Puckridge et al., 1998) and the mass migration and nesting of birds (Kingsford et al., 1999). However, the development of endemism reflects the patterns and processes that drive biological diversity within wetlands and sharpens the focus of conservation priorities. The GAB discharge springs provide habitat for an astounding assemblage of endemic plants, fish and invertebrates. These springs must therefore be the primary focus for biological conservation. Many discharge springs in the GAB have become extinct due to aquifer drawdown, resulting in loss of unique biotic assemblages (Ponder, 1986; Fensham et al., 2004a, 2010). In addition, numerous springs have been excavated in an attempt to make their water supply larger and more accessible to stock, irrevocably damaging their habitat and biodiversity values (Fairfax and Fensham, 2002). The original network of springs may be important for the long term survival of the biota, because individual springs act as repositories for recolonisation when other springs falter. Recovery of vegetation of reactivated springs should be possible with restoration of aquifer pressure after a concerted effort to cap flowing bores through the Great Artesian Basin Sustainability Initiative (GABSI). The restoration of habitat and assisted colonisation of organisms to reactivated springs should be informed by a more sophisticated understanding of the processes that control community assemblage (see Section 4.5 above). Invasion of discharge springs by alien species is one of the most urgent conservation issues. The exotic gambusia G. holbrooki appears to displace the endangered endemic fish, (red-finned blueeye) when the two species co-occur following gambusia colonisation (Fairfax et al., 2007). Alien species such as Gambusia, goldfish, and cane toads Bufo marinus, and translocated species such as redclaw crayfish Cherax quadricarinatus and sleepy cod are generally rare or patchily distributed in the waterholes of the ELEB. A dedicated public education campaign is recommended in order to prevent further liberation events of non-native species. Despite being characterised by low levels of endemism, waterholes provide vital drought refugia for riverine aquatic biota (Sheldon et al., 2010). The rivers and landscapes of the Lake Eyre Basin remain comparatively unaffected by the suite of factors that have

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been implicated in the ecological decline of the Murray-Darling, a similar sized drainage basin situated to the east (Fig. 1). These factors include flow regulation (Arthington and Pusey, 2003), barriers such as dams and weirs (Reynolds, 1983), cold-water pollution resulting from deep discharge from reservoirs (Astles et al., 2003), the impact of commercial fishing (Reid et al., 1997) and alien species (Koehn and MacKenzie, 2004). It seems highly likely that the comparative absence of these stress factors has contributed to the generally healthy condition of waterways. The streams of ELEB are among the worlds’ last unregulated river systems (Costelloe et al., 2004) and every effort should be made to preserve the natural hydrological regimes (Puckridge et al., 1998). Although rockholes and outcrop springs generally do not contain restricted or endemic species, they represent important sources of permanent water in an arid region, and the latter contain geographically isolated populations of plant species possibly representing distinct ecotypes. While rockholes generally occur in remote and inaccessible areas and are in good condition, many outcrop springs have been excavated and some even dynamited in an attempt to enhance the amount of available water. This has usually had the opposite effect, and most of these springs no longer flow. The approach developed here emphasises the importance of endemism for the assessment of conservation values across a region. Conservation hotspots can be simply identified, and when considered alongside a review of threatening processes conservation activities can be directed to where they are most required (Prendergast et al., 1993; Dobson et al., 1997). The method depends upon accurate field survey data, but potential actions can be prioritised as surveys progress. Understanding how biogeography and history have shaped patterns of endemism strengthens the pursuit of meaningful conservation actions beyond typologies based on wetland classification alone. Acknowledgements This study would not have been possible without the assistance of the many pastoralists and land managers, past and present, who shared their knowledge. The project was funded by the South Australian Arid-lands NRM Board, through Henry Mancini. Paul Wainwright (South Australian Department of Environment and Heritage) and Nick Cuff (Queensland Herbarium) provided wetland mapping data and discussed methods and findings. Russell Fairfax, Maree Rich, Alicia Whittington and Rosie Kerr assisted in various ways. Jerry Maroulis provided valuable advice on waterhole history. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.biocon.2011.06.024. References Abell, R., Thieme, M.L., Revenga, C., Bryer, M., Kottelat, M., Bogutskaya, N., Coad, B., Mandrak, N., Balderas, S.C., Bussing, W., Stiassny, M.L.J., Skelton, P., Allen, G.R., Unmack, P., Naseka, A., Ng, R., Sindorf, N., Robertson, J., Armijo, E., Higgins, J.V., Heibel, T.J., Wikramanayake, E., Olson, D., Lopez, H.L., Reis, R.E., Lundberg, J.G., Perez, M.H.S., Petry, P., 2008. Freshwater ecoregions of the world: a new map of biogeographic units for freshwater biodiversity conservation. Bioscience 58, 403–414. Abellan, P., Millan, A., Ribera, I., 2009. Parallel habitat-driven differences in the phylogeographical structure of two independent lineages of Mediterranean saline water beetles. Mol. Ecol. 18, 3885–3902. Anderson, S., 1994. Area and endemism. Quart. Rev. Biol. 69, 451–471. Arthington, A.H., Pusey, B.J., 2003. Flow restoration and protection in Australian rivers. River Res. Appl. 19, 377–395. Arthington, A.H., Balcombe, S.R., Wilson, G.A., Thoms, M.C., Marshall, J., 2005. Spatial and temporal variation in fish-assemblage structure in isolated waterholes

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