Prehistory

The Riverina bioregion straddles the Murray and Murrumbidgee Rivers, its central point being approximately 35^oS and 145^oE. The climate grades from temperate cool-season wet in the south, to Mediterranean and semi-arid in the north (Hutchinson et al. Reference Hutchinson, McIntyre, Hobbs, Stein, Garnett and Kinloch2005). The region is dominated by a semi-arid outwash plain of alluvial fans, with sediments built up from a system of prior streams (Butler Reference Butler1950). The present river systems have been cutting down through these sediments and are now at a lower level than the prior streams. Despite the relatively flat topography, a variety of plant communities occur in this bioregion, many having evolved in direct response to zones of flooding frequency (Beadle Reference Beadle1948). Historically, wetlands on the plain filled via flooding of the major rivers and streams, driven by upstream precipitation, or in some cases via local rainfall events. Floods would usually have occurred in late winter or spring every year, driven by snowmelt and rainfall in the headwaters. Aboriginal people lived in the region at least 40 000 years prior to the arrival of Europeans in the 19th century (Hope Reference Hope1995). Their life centred on riverine and wetland sources of water, with occasional excursions into drier country.

European settlement and agricultural development

The first major impact of European settlement in the area was the establishment of pastoralism in the mid-1800s, which resulted in clearing and overgrazing throughout the region (Beadle Reference Beadle1948). The second major phase was the establishment of extensive irrigation schemes, with the creation of the Murrumbidgee Irrigation Area starting in 1914, followed by the Murray Valley Irrigation Districts in the 1940s, and the Coleambally Irrigation Area in the 1960s. Other less intensive developments occur to the west of the Murrumbidgee Irrigation Area. These schemes have resulted in the conversion of 456 000 ha of the Riverina to intensively managed irrigation land with considerable expansion of European settlement and influence (Leigh & Noble Reference Leigh and Noble1972). Floodplain agricultural development and upstream water resource development have resulted in much of the floodplain area being replaced by agriculture or isolated from natural inundation (Kingsford Reference Kingsford2000). For example, in the western Riverina, the maximum period between Murrumbidgee River floods has more than doubled (from 4 to 10.5 years) while the average annual flooding volume has more than halved (CSIRO 2008).

Establishment of irrigation infrastructure involved the creation of supply and drainage channels, storage dams, and associated roads and settlements. Horticultural crops and broad-acre row crops (such as maize) have been generally watered by flooding furrows, while rice, wheat and pasture have been grown in levelled bays that are completely flooded. These activities involve significant and ongoing earthworks (contour banks, levees and ditches) and altered flow and drainage patterns across the landscape. Channels and flooded rice bays have provided large areas of free water continuously over the warmer months, in a landscape that would have been mainly dry in summer.

Changes to native vegetation communities

We reviewed major vegetation communities in the Riverina bioregion, and both past impacts of agricultural development on vegetation extent and ongoing threats (summarized in Table 1). Together, these communities supported the full range of the area's native fauna and flora. Virtually all community types have been seriously affected by agricultural development, either directly by clearing, or indirectly through changed flooding regimes, weed invasions and grazing by livestock. The woodlands of Eucalyptus, Acacia and Callitris that once dominated the bioregion have been subject to the greatest amount of clearing. Wetlands in the Riverina were most commonly found interspersed with woodlands and forests dominated by either river red gum (Eucalyptus camaldulensis) or black box (E. largiflorens). Understorey species composition and responses vary according to flooding regime (Williams Reference Williams1955, Reference Williams1956; Paijmans Reference Paijmans1978; McIntyre & Barrett Reference McIntyre and Barrett1985; McIntyre et al. Reference McIntyre, Ladiges and Adams1988). Wetlands lacking a tree layer may take the form of reed beds, rushlands and grasslands. Lignum (Muehlenbeckia spp.) shrubs dominate wetlands with or without a tree layer.

Table 1 Major vegetation formations and plant communities of the Riverina bioregion in New South Wales. Formations, community ID and status are from Benson et al. (Reference Benson, Allen, Togher and Lemmon2006); additional notes from Cunningham et al. (Reference Cunningham, Mulham, Milthorpe and Leigh1981).

Most of the wetlands are impermanent, are fed by local rainfall or channel flooding, and are variable in size, depth and flood regime. Since river regulation, they have suffered from insufficient flooding, particularly those distant from the main river channels. Many are so altered that they are no longer recognized as wetlands. Others have been artificially flooded for too long or at inappropriate times, their altered hydrology being indicated by dead and unhealthy trees, and invasion of species tolerant to long-term inundation (Roberts Reference Roberts2005).

While the conservation management of all remaining native vegetation is critical for the region, we focus here on: (1) river red gum forests, woodlands and associated wetlands; and (2) black box woodlands and associated wetlands. These communities have been subject to the greatest impact of irrigation development and activities, and have a strong dependence on flooding. They form most of the small remnants of vegetation in the intensive irrigation areas and are therefore directly affected by changes in management.

Constructed habitats resulting from irrigation development

Two major habitats for native biota have been artificially created by irrigation development, namely irrigation channels and flooded bays in which broad acre crops are grown. Other habitats associated with farming in general are dams and impoundments that are generally permanently flooded but occupy a relatively small area.

Large open earth supply channels (7–30 m wide and up to 3 m deep) distribute water from the Murrumbidgee and Murray Rivers to the irrigation areas. These channels are nearly permanently inundated. Similar open earth drainage channels have been created to manage used and excess water. These include modified pre-existing creek lines, and many eventually empty into dams, rivers or low-lying land. Smaller shallow open earth channels (usually <5 m wide and <1 m deep) distribute water across each farm. Most irrigation canals are drained periodically for dredging and vegetation control by direct removal or use of herbicides. This dumping of sediment and mechanical disturbance results in exotic species dominating the adjoining terrestrial vegetation. Native vegetation is highly susceptible to this combination of soil disturbance and nutrient enrichment (McIntyre & Lavorel Reference McIntyre and Lavorel2007).

Rice and terrestrial crops are grown in levelled bays separated by earth contour banks with a fall of approximately 7 cm between them. For rice production, bays are continuously flooded from spring to early autumn. Terrestrial crops such as wheat and pasture are often grown in the contoured paddocks during winter generally in rotation with rice. Rice bays have rapidly changing moisture conditions favourable to mobile and opportunistic organisms that are able to exploit temporary resources, such as some aquatic invertebrate species (Bambaradeniya et al. Reference Bambaradeniya, Edirisinghe, De Silva, Gunatilleke, Ranawana and Wijekoon2004; Wilson et al. Reference Wilson, Watts and Stevens2008), some frog species (Wassens et al. Reference Wassens, Sass, Swan and Thompson2004; Doody et al. Reference Doody, Osborne, Bourne, Rennie and Sims2006) and plants with large seed banks (McIntyre Reference McIntyre1985).

Predicted changes to water regimes and irrigation practices

Changes in irrigation practices stem from reduced availability of water, resulting in the development of water conservation strategies and fewer areas under irrigated broad acre crops. Although Riverina irrigators have a history of increasing water-use efficiency (Humphreys et al. Reference Humphreys, Lewin, Khan, Beecher, Lacy, Thompson, Batten, Brown, Russell, Christen and Dunn2006), current and forecast circumstances will require even greater levels of ingenuity. Projected changes have been identified by the authors through information synthesis and consultation with the industry (see Gaydon et al. Reference Gaydon, Beecher, Reinke, Crimp, Howden, Stokes and Howden2010) and include:

1. (1) A further general reduction of flows in rivers. Climate change is expected to result in reductions in Murray-Darling stream-flows of 16–25% by 2050 and 16–48% by 2100 (Christensen et al. Reference Christensen, Hewitson, Busuioc, Chen, Gao, Held, Jones, Kolli, Kwon, Laprise, Magana Rueda, Mearns, Menendez, Räisänen, Rinke, Sarr, Whetton, Arritt, Benestad, Beniston, Bromwich, Caya, Comiso, de Elia, Dethloff, Solomon, Qin, Manning, Chen, Marquis, Averyt, Tignor and Miller2007).

2. (2) Changes in water regimes for some natural habitats, associated with changes in management of drainage and surplus irrigation water (i.e. water rejected due to natural rainfall events). For example, surplus water may be stored for later use in crops rather than being discharged onto wetlands.

3. (3) Reductions in area under irrigated agriculture in a higher proportion of seasons, including reductions in the area of paddy rice.

4. (4) Methods of water application to reduce water use, such as increased adoption of efficient lateral move, centre pivot and drip irrigation technology on lighter soils, and reductions and increased efficiency in the use of flood irrigation techniques.

5. (5) Changes in the methods of delivering water to reduce leakage and evaporative losses. This could include sealed lining of earthen irrigation channels and the piping of water which has previously been carried in open channels. Under current water prices, economics dictate that these changes are likely to be limited to locations where channels or drains cross highly permeable soils. Increasing costs of water could result in more widespread piping and lining.

6. (6) Increases in herbicide use may occur, because the use of ponded water has been the primary method for controlling weeds in rice crops.

7. (7) Adjustments to farming practices and associated changes to farm layout are likely. For example recycling of water will involve increasing on-farm water storages and associated engineering works.

If correct, these predictions will create further challenges to a biota that has already been dramatically perturbed by pastoralism and irrigation development. In the following sections, we draw on available information on species and community response to consider how the predicted changes may further affect flora and fauna conservation in the Riverina.

Wetland flora

The creation of constructed habitats such as rice bays, channels and roadside ditches has provided habitat permitting native herbaceous wetland plants to persist long after their associated trees and shrubs have disappeared from these habitats (McIntyre et al. Reference McIntyre, Ladiges and Adams1988). Unlike their terrestrial native counterparts, wetland species appear to have been pre-adapted to productive highly disturbed situations (McIntyre & Barrett Reference McIntyre and Barrett1985; McIntyre et al. Reference McIntyre, Ladiges and Adams1988). The largest threat to this assemblage in constructed habitats will be lining and piping of channels, and reduced areas of paddy rice production. This will certainly reduce population sizes of some species. In the case of channels, where diversity is limited by deep water and the effects of dredging and herbicides, a reduction in area or impermeable lining will pose little threat. Rice bays are potentially of more concern, as a survey comparing flooded bays with natural swamps found 11 native species to be found only in rice bays compared with 29 species restricted to swamps (McIntyre et al. Reference McIntyre, Ladiges and Adams1988). This situation would need to be reassessed to identify the current threats more precisely, as the diversity status of both habitats may have changed over the 30-odd years since this survey. The fate of wetland species is linked with that of their associated woodlands, as discussed below.

Woodlands

Notwithstanding any effects of climate change on rainfall and natural water flows, the major issues for flood-dependent woodlands are those resulting from local management of irrigation water. Irrigation infrastructure has interfered with natural drainage patterns, and where ‘waste’ water may have previously been applied to woodland remnants, there is a trend toward more careful recycling and storage of water on-farm in dams and channels. This could have positive effects on flood-dependent woodlands by avoiding prolonged waterlogging, or could have negative effects, due to induced drought compounding the effects of loss of natural flooding. More broadly, irrigation and tree clearing in the Riverina have caused water tables to rise, with associated increases in soil and water salinity. The recent drought, together with changes in infrastructure and management, has lowered the water table, and these factors have combined to reduce the urgency of this problem, at least in the short term. Changes in irrigation practices in the future may further reduce the amount of water reaching the water table; this issue requires ongoing monitoring and management.

Removal of paddock trees is a conservation issue with potential to escalate under changing irrigation techniques, as installation of lateral move and centre pivot irrigation systems requires large treeless areas to operate. Isolated paddock trees in intensively farmed landscapes are increasingly recognized as irreplaceable habitat elements for native fauna (Manning et al. Reference Manning, Fischer and Lindenmayer2006). Retained paddock trees are typically mature and bear hollows upon which native fauna rely for breeding and shelter (Gibbons & Boak Reference Gibbons and Boak2002). Isolated trees may also act as ‘stepping stones’ or provide some form of connectivity across the agricultural landscape. They can also provide a feeding resource for fauna such as bats, birds and mammals (Gibbons & Boak Reference Gibbons and Boak2002; Lumsden & Bennett Reference Lumsden and Bennett2005).

Amphibians (frogs)

Of the 14 species of frog that have been historically recorded in the Riverina, 12 have been recorded in irrigation areas in recent years (Table 2). The two unaccounted for, namely the green tree frog (Litoria caerulea) and eastern banjo frog (Limnodynastes dumerilii), are more likely to be detected in periods of several successive wet years. There appear to be no species restricted to rice bays or irrigation channels, though one species, the endangered southern bell frog (Litoria raniformis) may now rely on permanently flooded channels or dams for over-wintering and dry-season persistence (Wassens et al. Reference Wassens, Roshier, Watts and Robertson2007, Reference Wassens, Watts, Jansen and Roshier2008).

Table 2 Frogs recorded in the Riverina. Summary of historic records and recent habitat records in the irrigation areas as collated and reported by: 1 = Ehmann (Reference Ehmann1996); 2 = Wassens et al. (Reference Wassens, Sass, Swan and Thompson2004); 3 = Doody et al. (Reference Doody, Osborne, Bourne, Rennie and Sims2006); and 4 = Wassens et al. (Reference Wassens, Watts, Jansen and Roshier2008).

In general, black box depressions in the Riverina have slightly greater frog species richness (eight spp.) than river red gum wetlands (six spp.), rice bays (six spp.) and channels (five spp; Wassens et al. Reference Wassens, Sass, Swan and Thompson2004). One species (the broad-palmed frog, Litoria latopalmata) appears to be restricted to river red gum billabongs. Dams with abundant vegetation support the highest number of species (nine spp.). We interpret this to be because of the density and diversity of fringing vegetation and number of microhabitats, which are elements of favourable frog habitat elsewhere in Australia (Hazell et al. Reference Hazell, Hero, Lindenmayer and Cunningham2004).

The changes in irrigation practice that are most likely to affect amphibian diversity in the Riverina are those relevant to dams, and those affecting the condition of black box depressions. While there is some chance that unpredictability in water supply may lead to construction of more dams to increase water security over time, the value of both new and old dams will depend on maintaining a variety of vegetation in and around dams (Hazell et al. Reference Hazell, Cunnningham, Lindenmayer, Mackey and Osborne2001). Amphibian habitat quality of both dams and black box remnants may also be affected by agro-chemical usage. In the Riverina, organically grown rice was found to have more diverse macro-invertebrate communities than rice bays treated with agrochemicals (Wilson et al. Reference Wilson, Watts and Stevens2008), which may affect the quality, if not the quantity of food supply for frogs in channel, dams and rice bays.

Reptiles

Reptile abundance and diversity are low in the Riverina compared with other sites in south-eastern Australia (Wassens et al. Reference Wassens, Sass and Swan2005b; Brown et al. Reference Brown, Bennett and Potts2008). Although 29 species have been recorded in vegetation remnants of the Murrumbidgee Irrigation Area (Wassens et al. Reference Wassens, Sass and Swan2005b), other studies in the Riverina have located far fewer species (AMBS [Australian Museum Business Services] 2005; Doody et al. Reference Doody, Osborne, Bourne, Rennie and Sims2006; Brown et al. Reference Brown, Bennett and Potts2008). Only four species were considered both abundant and widespread in the southern Riverina (Herring et al. Reference Herring, Webb and Pisasale2006a, Reference Herring, Webb and Pisasaleb, Reference Herring, Webb and Pisasalec, Reference Herring, Webb and Pisasaled): Boulenger's skink (Morethia boulengeri), Carnaby's wall skink (Cryptoblepharus carnabyi), the southern marbled gecko (Christinus marmoratus) and the eastern brown snake (Pseudonaja textilis).

In irrigation areas, reptiles are more abundant in black box remnants than in other rice farm habitats such as rice bays, dams, dry crops or river red gum woodland (Doody et al. Reference Doody, Osborne, Bourne, Rennie and Sims2006; Brown et al. Reference Brown, Bennett and Potts2008). Ten species have been found in black box remnants of the Murrumbidgee Irrigation Area, compared to six species in river red gum (Wassens et al. Reference Wassens, Sass and Swan2005b). Numbers of reptiles in river red gum communities are thought to be limited by long periods of flooding, though the habitat is important for skinks, geckos and carpet pythons. Resident species are commonly large mobile generalists, or arboreal in habit. The highest richness, abundance and frequency of reptiles have been recorded in roadside remnants of black box, possibly reflecting greater structural complexity due to protection from grazing (Brown et al. Reference Brown, Bennett and Potts2008). Overall, species richness varies with grazing pressure, fallen timber and connectivity between patches of vegetation (Sass et al. Reference Sass, Wassens, Swan and Thompson2004; Wassens et al. Reference Wassens, Sass and Swan2005b).

Most reptile species in the Riverina are restricted to terrestrial habitats or the margins of wet areas, though they may be attracted to and benefit from the higher abundances of frogs and insects associated with irrigation waters. However, Doody et al. (Reference Doody, Osborne, Bourne, Rennie and Sims2006) found no difference in diversity or abundance between rice bays and dry crops, except for tortoises. The eastern long-necked tortoise (Chelodina longicollis) uses large irrigation channels, with feeding forays into rice bays during the irrigation season (Doody et al. Reference Doody, Osborne, Bourne, Rennie and Sims2006). Loss of these habitats would negatively affect tortoise populations in localized areas, but being an abundant and widespread species, such changes would not greatly reduce reptile diversity in the Riverina. Improving the condition of remnant vegetation would provide significantly greater long-term benefit.

Birds

The Riverina provides internationally significant habitat for waterbirds (Kingsford & Thomas Reference Kingsford and Thomas2004) and supports a range of rare and threatened terrestrial birds (Jansen & Robertson Reference Jansen and Robertson2005). Many Riverina bird species have suffered regional and national population declines in the last 25 years (Kingsford et al. Reference Kingsford, Wong, Braithwaite and Maher1999; Ford et al. Reference Ford, Barrett, Saunders and Recher2001; Porter et al. Reference Porter, Kingsford and Hunter2006). Successful breeding by waterbirds in the region has been linked to the water regime required to produce suitable habitat (Briggs et al. Reference Briggs, Lawler and Thornton1997; Briggs & Thornton Reference Briggs and Thornton1999) and in many sites water regimes have been changed by irrigation practices. Clearing and grazing have also affected species composition and abundance in the region (Jansen & Robertson Reference Jansen and Robertson2001). In recent years, surveys have found that black box communities generally have had higher terrestrial bird diversity and abundance than several other major vegetation types in the region (Antos & Bennett Reference Antos and Bennett2005; Herring et al. Reference Herring, Webb and Pisasale2006a, Reference Herring, Webb and Pisasaleb, Reference Herring, Webb and Pisasalec, Reference Herring, Webb and Pisasaled).

Constructed habitats associated with irrigation have the potential to increase resources for birds (for example herons and egrets in southern Europe), but also create hazards, for example through pesticide use (Czech & Parsons Reference Czech and Parsons2002). A wide range of Australian birds use rice bays, irrigation channels and water storages for foraging. These habitats can partially substitute for lost or altered habitat; in the rice growing regions of Italy, Spain and California, irrigation channels (canals and ditches) and their margins provide nesting and foraging habitat for waterbirds (Czech & Parsons Reference Czech and Parsons2002; Taft & Elphick Reference Taft and Elphick2007). In general, terrestrial bird and waterbird diversity associated with irrigation channels is greatest when channels are large and have extensive complex vegetation, both inside and outside the channel itself (Herzon & Helenius Reference Herzon and Helenius2008). Irrigation channels in the Riverina rarely have these characteristics. Lining channels with concrete further reduces habitat value (Lane & Fujioka Reference Lane and Fujioka1998; Maeda Reference Maeda2001). In Australia, there is evidence of ducks and egrets foraging in channels (Frith Reference Frith1957a a, Reference Frithb; Richardson & Taylor Reference Richardson and Taylor2003) but there are no records of associated breeding.

Waterbirds exploit rice crops for their food resources worldwide (Frith Reference Frith1957b; Richardson et al. Reference Richardson, Taylor and Growns2001; Czech & Parsons Reference Czech and Parsons2002; Taft & Elphick Reference Taft and Elphick2007). Ducks have had minor economic impacts in Australian crops through feeding on grain and young plants in the establishment phase. This was found by Frith (Reference Frith1957b) to be offset to some extent by their consumption of seed from the major grass weed Echinochloa, and the damage was usually confined to those areas in crops where growth was unsatisfactory for other reasons. Australian egrets (Ardea alba and Egretta intermedia) have been recorded foraging in rice during their breeding season but shifted to natural wetlands as the crops matured and chick rearing took place. In contrast the introduced cattle egret (Bubulcus ibis) foraged in rice fields until after their chicks had fledged, leading to speculation that this invasive species may have some advantage over the native egrets in the irrigated agricultural landscape (Richardson & Taylor Reference Richardson and Taylor2003).

Targeted management practices can be important for improvement of food availability for birds in irrigation areas that have replaced natural wetland habitats. For example, in the USA, stubble management, shallow winter flooding, reduced use of pesticides, fallow and secondary crop rotation practices that encourage seeding plants are beneficial (Taft & Elphick Reference Taft and Elphick2007). In the Riverina, reports indicate that terrestrial birds and waterbirds increase around rice bays following flooding and decrease after draining (Doody et al. Reference Doody, Osborne, Bourne, Rennie and Sims2006).

Although constructed habitats in irrigation areas may provide some resources for native birds, large regional declines in the populations of many waterbird species have coincided with irrigation development (Kingsford & Thomas Reference Kingsford and Thomas2004). So while we do not understand the net population effects of particular constructed habitats on birds, it would appear that irrigation development overall has not been able to do more than partially substitute for the alterations and losses of the natural habitats and resources that have ensued. Consequently the impacts of further change to water availability in the Riverina via changes in irrigation practice are difficult to estimate.

Mammals

Many mammal species present in the Riverina in the past are now rare. The only abundant and widespread native mammals are bats, the eastern grey kangaroo (Macropus giganteus) and the brush-tailed possum (Trichosurus vulpecula). Currently the greatest diversity of mammals appears to be in river red gum forests (Herring et al. Reference Herring, Webb and Pisasale2006a, Reference Herring, Webb and Pisasaleb, Reference Herring, Webb and Pisasalec, Reference Herring, Webb and Pisasaled), which contain several species of bats, as well as yellow-footed antechinus (Antechinus flavipes), water rats (Hydromys chrysogaster), black wallaby (Wallabia bicolour), sugar glider (Petaurus breviceps) and platypus (Ornithorhynchus anatinus). Black box woodland also supports bats, eastern grey kangaroos and brush-tailed possums, and the presence of these species is dependent on specific landscape and woodland characteristics, for example landscape complexity, shrub and log cover, presence of hollow-bearing trees and woodland patch size (Lewis Reference Lewis2006).

The use of constructed irrigation habitats by mammals is poorly known, both in Australia and overseas. There are potential benefits of habitat in close proximity to a water source such as an irrigation channel. However in the Riverina this seems to be offset by the loss of adjacent native vegetation due to channel maintenance activities. The introduced house mouse (Mus domesticus) was the only mammal recorded in a large survey (45 000 trap-nights) adjacent to irrigation channels and in fields in the Riverina (Brown et al. Reference Brown, Davies, Singleton and Croft2004). House mice are usually the most abundant mammals in rice bays (Brown et al. Reference Brown, Davies, Singleton and Croft2004; Doody et al. Reference Doody, Osborne, Bourne, Rennie and Sims2006). In tropical rice systems overseas, these may attract carnivores such as mongoose, wild cats, otter and civet cats (Bambaradeniya et al. Reference Bambaradeniya, Edirisinghe, De Silva, Gunatilleke, Ranawana and Wijekoon2004); in the Riverina they may attract introduced predators such as cats and foxes as well as provide food resources to some native raptors (Sinclair et al. Reference Sinclair, Olsen and Redhead1990). One native mammal that might be expected to occur is the water rat; however a recent survey in the Murrumbidgee Irrigation Area failed to find water rats in or near irrigation channels (Lewis Reference Lewis2006) even though they occasionally use rice bays (Scott & Grant Reference Scott and Grant1997). Overall, constructed habitats offer poor habitat for native mammals, which appear to be more dependent on native vegetation than the other fauna groups. Therefore changes in the availability of constructed habitats and water sources are unlikely to significantly change abundance or diversity of mammals in the Riverina.

A further general reduction of flows in rivers

A further general reduction of flows in rivers will reduce the frequency and magnitude of flooding events, which will affect natural water regimes of river red gum and, to a lesser extent, black box communities. All components of biodiversity will be affected by these changes. The Murray-Darling system is highly controlled and, except in exceptionally wet years and in circumstances of local flooding, flooding events will be explicitly controlled. There is an acute awareness of the trade-off between environmental uses of water (such as to flood wetlands) and its use for human purposes, and the issue is currently being debated at the Australian national level. It is likely that flooding regimes approximating pre-European conditions will be restored for a limited number of wetlands in the Murray-Darling system.

Changes in water regimes for some natural habitats

On-farm management of irrigation waters is important for the remnant flood-dependent woodlands that exist within the rice agro-ecosystem. For this reason, all components of biodiversity are subjected to the vagaries of individual irrigation decisions and managed flooding will become even more important in the face of more generally dry conditions due to climate change. There is great scope for refining management in ways that can benefit remnant native vegetation. The New South Wales Murray Wetlands Working Group has successfully inundated many wetlands in southern NSW, and such cooperation between irrigation companies, landholders, government departments and catchment management groups can be effective (Nias et al. Reference Nias, Alexander and Herring2003). Even partial rehabilitation of a wetland can provide multiple benefits to both irrigators and the environment. For example, Barren Box Swamp, an intermittent wetland in the Murrumbidgee Irrigation Area, was permanently inundated by irrigation drainage for about 50 years, killing most of the black box trees. Restoration consisted of splitting the area into three cells, including a deep storage cell, resulting in more efficient storage (less evaporation) and restoration of 1650 ha of the wetland to an intermittent regime (Murrumbidgee Irrigation 2009). The extent to which these habitats can be maintained, when overall water availability is likely to decline, will be driven by political and economic imperatives.

Reductions in area under irrigated agriculture

As well as reducing the average area of flooded rice bays, this may reduce the total average amount of free water in supply and irrigation channels. These are discussed under the following two headings.

Irrigation techniques for reduced water use

Future reduction in area and duration of flooding in irrigation bays is highly likely, and this will reduce a previously extensive habitat for a range of species in the warm season. As for channels, the local abundance of some frogs and waterbirds may be reduced, but these changes will not necessarily be deleterious in terms of total biodiversity in the region, providing natural habitats are maintained. The situation is less clear for wetland plants, which appear to have some dependence on flooded rice bays, but it could be argued that the improved management of natural wetlands would compensate for reduced areas of flooded rice.

Reducing leakage and evaporative losses in water delivery

In Japan, concrete lining and piping of irrigation channels has resulted in loss of habitat for aquatic invertebrates, amphibians and some waterbirds (Fujioka & Lane Reference Fujioka and Lane1997; Lane & Fujioka Reference Lane and Fujioka1998). If such engineering modifications are widely implemented within Australia, the likely result will be reductions in abundance of species that exploit channels (such as frogs and turtles) and possibly sedentary terrestrial species from the surrounding landscape that obtain food resources from them. However, many species associated with channels are relatively common, and while channels provide additional habitat, refuge during unfavourable seasons and connectivity of aquatic habitat across the landscape, there is no evidence that they provide habitat that cannot be maintained with appropriate management of remaining native vegetation, natural watercourses and wetlands.

Increases in herbicide use

Herbicides and fertilizer are important in rice cultivation, but there are significant contributions from flooding in suppressing some major weeds (McIntyre et al. Reference McIntyre, Finlayson, Ladiges and Mitchell1991). Dependence on herbicides is likely to increase with reduced flooding depth or duration. While discharging drainage water on native vegetation could provide beneficial watering, residual chemicals and nutrients could be detrimental. Strategies to control these, such as retention and recycling, need to be integrated into any recommendations for changed practices to reduce water use.

Adjustments to farm layout

While most constructed aquatic habitats are likely to decline in the future, the number of dams will most likely increase to conserve and recycle water on-farm. This has the potential to benefit a range of common species, most notably frogs and turtles. There will be options to improve habitat for fauna within irrigation areas by creating physical variability within and among dams, excluding livestock and restoring vegetation (Hazell et al. Reference Hazell, Cunnningham, Lindenmayer, Mackey and Osborne2001; Jansen & Healey Reference Jansen and Healey2003; Hazell et al. Reference Hazell, Hero, Lindenmayer and Cunningham2004; Jansen & Robertson Reference Jansen and Robertson2005).

It is likely that changed farm layout will further threaten remnant vegetation with earthworks during the construction process. Modified irrigation techniques may also threaten remnant vegetation. For example, efficiency practices such as laser levelling and centre pivot irrigation may affect critical landscape elements such as isolated mature native trees, which may have otherwise persisted in paddocks. There is scope for strategies that mitigate for these effects, as well as more proactive approaches to farming that could result in better prospects for biodiversity.