The 10 Australian ecosystems most vulnerable to tipping points

Biological Conservation 144 (2011) 1472–1480

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Biological Conservation

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The 10 Australian ecosystems most vulnerable to tipping points

William F. Laurance a,⇑, Bernard Dell b, Stephen M. Turton c, Michael J. Lawes d, Lindsay B. Hutley d,Hamish McCallum e, Patricia Dale e, Michael Bird c, Giles Hardy b, Gavin Prideaux f, Ben Gawne g,Clive R. McMahon d, Richard Yu h, Jean-Marc Hero i, Lin Schwarzkopf j, Andrew Krockenberger a,Samantha A. Setterfield d, Michael Douglas d, Ewen Silvester k, Michael Mahony l, Karen Vella m,Udoy Saikia h, Carl-Henrik Wahren n, Zhihong Xu e, Bradley Smith o, Chris Cocklin o

a School of Marine and Tropical Biology, James Cook University, Cairns, Queensland 4870, Australiab School of Biological Sciences and Biotechnology, Murdoch University, Murdoch, Western Australia 6150, Australiac School of Earth and Environmental Sciences, James Cook University, Cairns, Queensland 4870, Australiad Research Institute for the Environment and Livelihoods, Charles Darwin University, Darwin, Northern Territory 0909, Australiae Environmental Futures Centre, School of Environment, Griffith University, Nathan, Queensland 4111, Australiaf School of Biological Sciences, Flinders University, Bedford Park, South Australia 5042, Australiag Murray-Darling Freshwater Research Centre, LaTrobe University, Bundoora, Victoria 3086, Australiah School of the Environment, Flinders University, Bedford Park, South Australia 5042, Australiai Environmental Futures Centre, School of Environment, Griffith University, Gold Coast Campus, Queensland 4222, Australiaj School of Marine and Tropical Biology, James Cook University, Townsville, Queensland 4811, Australiak Department of Environmental Management and Ecology, LaTrobe University, Bundoora, Victoria 3086, Australial School of Environmental and Life Sciences, University of Newcastle, Newcastle, New South Wales 2300, Australiam Griffith School of Environment, Griffith University, Gold Coast Campus, Queensland 4222, Australian Centre for Applied Alpine Ecology, LaTrobe University, Melbourne, Victoria 3086, Australiao Research and Innovation, James Cook University, Townsville, Queensland 4811, Australia

a r t i c l e i n f o

Article history:Received 26 November 2010Received in revised form 16 January 2011Accepted 22 January 2011Available online 21 February 2011

Keywords:CatastrophesClimatic changeEcological resilienceEcological thresholdsExotic pests and pathogensFeral animalsFire regimesGlobal warmingHabitat fragmentationInvasive speciesSalinizationSea-level riseSpecies extinctions

⇑ Corresponding author. Tel.: +61 7 4042 1819; faxE-mail address: [email protected] (W.F. Lau

a b s t r a c t

We identify the 10 major terrestrial and marine ecosystems in Australia most vulnerable to tippingpoints, in which modest environmental changes can cause disproportionately large changes in ecosystemproperties. To accomplish this we independently surveyed the coauthors of this paper to produce a list ofcandidate ecosystems, and then refined this list during a 2-day workshop. The list includes (1) elevation-ally restricted mountain ecosystems, (2) tropical savannas, (3) coastal floodplains and wetlands, (4) coralreefs, (5) drier rainforests, (6) wetlands and floodplains in the Murray-Darling Basin, (7) the Mediterra-nean ecosystems of southwestern Australia, (8) offshore islands, (9) temperate eucalypt forests, and(10) salt marshes and mangroves. Some of these ecosystems are vulnerable to widespread phase-changesthat could fundamentally alter ecosystem properties such as habitat structure, species composition, fireregimes, or carbon storage. Others appear susceptible to major changes across only part of their geo-graphic range, whereas yet others are susceptible to a large-scale decline of key biotic components, suchas small mammals or stream-dwelling amphibians. For each ecosystem we consider the intrinsic featuresand external drivers that render it susceptible to tipping points, and identify subtypes of the ecosystemthat we deem to be especially vulnerable.

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1. Introduction

Various vulnerability assessments have been carried out forAustralian terrestrial and marine ecosystems. Some have focusedon identifying vulnerable ecological communities (e.g. EPBC,1999) or species (e.g. Watson et al., 2010), whereas others have as-sessed particular environmental threats, such as climatic change

: +61 7 4042 1213.rance).

and its potential impacts on biodiversity (Hennessy et al., 2007;Johnson and Marshall, 2007; Steffen et al., 2009) and ecosystemfunction (Hughes, 2003; Murphy et al., 2010).

To date, however, no assessment of Australian ecosystems hasfocused explicitly on their potential vulnerability to tipping points.Such an exercise is important because these ecosystems will faceimportant environmental challenges in the future (Beeton et al.,2006). Current projections of climate change, for instance, suggestthat minimum and maximum temperatures will continue toincrease whereas precipitation will become more seasonal and

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sporadic across large swaths of the Australian continent (CSIRO-Australian Bureau of Meteorology, 2007). By the end of thiscentury, much of southern Australia could become drier (Hennessyet al., 2007), whereas arid and semi-arid zones of northern Austra-lia could experience more heat waves (Tebaldi et al., 2006). Largeexpanses of the Australian continent are likely experiencing fire re-gimes for which their ecosystems are poorly adapted (Ward et al.,2001; Mooney et al., 2010; Setterfield et al., 2010). In the surround-ing oceans, sea levels are rising while sea-surface temperaturesand acidity are both increasing (De’ath et al., 2009; Hughes et al.,2010). Habitat loss and degradation continue apace in parts ofthe continent, and many ecosystems are suffering seriously frominvasions of non-native plants and animals (Rea and Storrs,1999; Rossiter-Rachor et al., 2009; Setterfield et al., 2010) or fromemerging pests and pathogens (Laurance et al., 1996; Garkakliset al., 2004a; Cahill et al., 2008). Key components of the native bio-ta have been lost, and continue to be lost, from many Australianecosystems (Hero et al., 2006; Jones et al., 2007; AWC, 2010; Bur-bidge et al., 2009; Woinarski et al., 2010).

In this paper, we define a tipping point rather loosely as a cir-cumstance by which a relatively modest change in an environmen-tal driver or perturbation can cause a major shift in key ecosystemproperties (Fig. 1), such as habitat structure, species composition,community dynamics, fire regimes, carbon storage, or other impor-tant functions. The tipping point is an ecological threshold beyondwhich major change becomes inevitable and is often very difficultto reverse. Because of ecological feedbacks, many ecosystems seemrelatively stable as they approach a tipping point, but then shiftabruptly to an alternative state once they reach it (see Washing-ton-Allen et al. (2009), Hughes et al. (2010), and referencestherein).

In conducting our analysis we found it useful to distinguishamong three broad categories of ecosystems that vary in their geo-graphic extent and severity of their tipping points. ‘Tipping’ eco-systems are likely to experience profound regime changes acrossmost or all of their geographic range, whereas ‘dipping’ ecosystemsexperience similarly profound changes, but these are restrictedgeographically, affecting only a portion of the entire ecosystem. Fi-nally, ‘stripping’ ecosystems are being stripped of important eco-system components, such as their small mammal, amphibian, orlarge predator fauna, but such changes are more insidious and lessvisually apparent than major regime changes, at least at present.

We present here our ‘top 10’ list of vulnerable Australian terres-trial and near-coastal marine ecosystems. For each we outlinesome of the intrinsic features and external drivers that render itsusceptible to tipping points, and identify subtypes of the ecosys-tem that we consider especially vulnerable. Our emphasis here isprimarily on the physical and biological sciences, and we concedethat a social-science perspective might yield a different list—one

Fig. 1. Striking contrast between a natural tropical savanna-woodland near Bachelor, NorGamba grass (Andropogon gayanus), an exotic species. The grass promotes high-intensit

that considers a range of socioeconomic factors that also affect eco-system vulnerability. We also emphasize that we regard this exer-cise as exploratory and thought-provoking, not definitive. Our goalis to stimulate critical thinking about tipping points while high-lighting Australian ecosystems that we believe could—in the ab-sence of effective conservation or management interventions—change dramatically in the future.

2. Methods

We conducted our assessment in two phases. In early October2010, the 26 coauthors of this paper were invited to submit inde-pendent lists of major terrestrial and marine ecosystem types thathe or she considered vulnerable to tipping points, along with po-tential intrinsic characteristics or external threats that werethought to render each nominated ecosystem vulnerable. Manyof these coauthors have long-term research experience in Australiaand the universities with which they are affiliated span all Austra-lian states except Tasmania (and several coauthors have active re-search programs in Tasmania). These initial data were compiledinto a preliminary list by the lead author, and the nominated eco-systems ranked by the number of investigators that consideredthem vulnerable.

In late October 2010, the authors met in Cairns, Queensland foran intensive 2-day workshop in which we discussed and refinedthe initial list. We had five goals: (1) to identify the ‘top 10’ majorAustralian ecosystems vulnerable to tipping points, (2) to highlightkey subtypes of each ecosystem type currently at critical risk, (3) toidentify the intrinsic features of each ecosystem that predisposed itto tipping points, (4) to identify major external threats to each eco-system, and (5) to cross-tabulate the intrinsic features and externalthreats across all 10 vulnerable ecosystems to identify any generalattributes that render them vulnerable to tipping points. Toachieve aims (3) and (4) we devised general schemes to categorizeintrinsic ecosystem features (Table 1) and external threats (Table2) that predispose ecosystems to tipping points. For all analyses,we reached a final consensus via a combination of discussion, de-bate, and formal voting.

3. Results: vulnerable ecosystems

Among a total of 22 nominated Australian ecosystems, the fol-lowing 10 were judged to be most vulnerable to tipping points.We begin with the ecosystems for which consensus among our pa-nel of experts was strongest.

thern Territory, Australia and similar habitat 300 m away that is heavily invaded byy fires that dramatically transform the ecosystem (photos by S. Setterfield).

Table 1Intrinsic features of 10 Australian ecosystems that can render them vulnerable to tipping points, as perceived by 26 environmental experts. For each ecosystem type, the mostimportant feature is numbered 1 with those of lesser importance numbered subsequently.

Intrinsic feature Mountains Tropicalsavannas

Coastalwetlands

Coralreefs

Drierrainforests

Murray-Darling

SW AustraliaMediterranean

Islands Temperateeucalypt

Estuarinewetlands

Narrow environmental envelope 1 4 1 1 2 1Near threshold 3 3 1Geographically restricted 2 1 2 2 1 2History of fragmentation 2 3 1 1 4Reliance on ecosystem engineers 3 4 4Reliance on framework species 2 2 3 2 6Reliance on predators or keystone

mutualistsPositive feedback 1 4 4 3 5 5Proximity to humans 3 5 5 4 3 3Social vulnerability 2 6 5

Table 2Environmental threats to 10 Australian ecosystems that render them vulnerable to tipping points, as perceived by 26 environmental experts. For each ecosystem type, the mostimportant threat is numbered 1 with those of lesser importance numbered subsequently.

Environmental threat Mountains Tropicalsavannas

Coastalwetlands

Coralreefs

Drierrainforests

Murray-Darling

SW AustraliaMediterranean

Islands Temperateeucalypt

Estuarinewetlands

Increased temperatures 1 1 2 4 2 6 2Changes in water balance and

hydrology2 3 3 2 1 3 3

Extreme weather events 3 3 2 2 8 3 2 1Ocean acidification 3Sea-level rise 1 9 3 2Changed fire regimes 8 2 8 1 4 1Habitat reduction 5 5 5 5 5 8 4 4 4Habitat fragmentation 6 4 6 6 6 6 9 5 5Invasives 4 1 4 4 6 1Pests and pathogens 7 5 7 7Salinization 4 3 7

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3.1. Elevationally restricted mountain ecosystems

Mountain ecosystems in Australia are most predominant in theGreat Dividing Range, which skirts the country’s eastern seaboardfrom western Victoria northward to the Cape York Peninsula innorthern Queensland. Mountains also occur in parts of Tasmania,South Australia, and the southwest of Western Australia. Manyhabitats types in these mountains are elevationally restricted,including alpine ecosystems of Tasmania and southeastern Austra-lia, and montane rainforests at temperate, subtropical, and tropicallatitudes of northern New South Wales and Queensland. In ourview the most vulnerable habitats are those that rely substantiallyon cloud-stripping for moisture inputs during the drier months(Hutley et al., 1997; McJannet et al., 2007), have seasonal snowcover (Pickering et al., 2003), or, like many rainforests, sustain highnumbers of restricted endemic species (Fig. 2) (Williams et al.,1996; Hoskin, 2004).

These ecosystems are considered inherently vulnerable becauseof their often-narrow environmental envelopes, their geographi-cally restricted distribution, and the fact that many appear to benear climatic thresholds (Table 1). We regard global warming (Wil-liams et al., 2003), potential changes in moisture inputs and a ris-ing cloud base (Pounds et al., 1999; Still et al., 1999), and extremeweather events (Tebaldi et al., 2006) as the most serious futurethreats (Table 2). Further perils include invasive plants and fauna,habitat loss and fragmentation (Laurance, 1991), new pests andpathogens (such as the chytrid fungus that has decimated manystream-dwelling amphibian populations; Skerratt et al., 2007),and, in alpine ecosystems, changing fire regimes (Wahren et al.,1999; Fairfax et al., 2009) and a reduction in insulating snow coverin winter (Pickering et al., 2003).

3.2. Tropical savannas

Tropical savanna-woodlands are one of the most extensiveenvironments in Australia, spanning much of the northern thirdof the continent (Mackey et al., 2007). This system is experiencingsevere regime changes in only parts of its geographic range—andhence is a ‘dipping’ ecosystem. Invasive weeds and animals (Setter-field et al., 2010; Woinarski et al., 2010), changing fire regimes(Prior et al., 2010; Midgley et al., 2010), and extreme weatherevents are seen as the major threats, with habitat fragmentationand overgrazing by livestock (Kutt and Woinarski, 2007) being fur-ther perils (Table 2). In addition, this ecosystem is currently expe-riencing an apparently widespread decline of its small mammalfauna—a feature of a ‘stripping’ ecosystem—for reasons that remainuncertain (AWC, 2010; Woinarski et al., 2010).

A key reason for the high vulnerability of tropical savannas is mas-sive weed invasions (Fig. 1) that profoundly alter fire regimes andother fundamental ecosystem attributes such as carbon storage andnitrogen cycling (Rea and Storrs, 1999; Rossiter-Rachor et al., 2009;Setterfield et al., 2010) (Table 1). We believe that sandstone savannasand heaths, which have an endemic flora (Woinarski et al., 2006) andfauna and a highly restricted geographic range, are especially vulner-able habitats, with increasing fire incidence their principal threat(Russell-Smith et al., 2001; Sharp and Bowman, 2004).

3.3. Coastal floodplains and wetlands

Coastal floodplains and wetlands are freshwater (or onlyslightly brackish) ecosystems in coastal areas throughout Australia(Adam, 1992; Kingsford et al., 2004). They are most widespread inthe vast tropical floodplains of the Northern Territory (Cowie et al.,2000), Queensland, and Western Australia. Principal threats to

Fig. 2. Endemic rainforest vertebrates in eastern Australia that are considered exceptionally vulnerable to global warming, and thus could be ‘stripped’ from ecosystems. Allspecies shown have highly restricted geographic ranges and are confined to montane rainforest. From upper left: Bartle Frere barsided skink (Eulamprus frerei), lemuroidringtail possum (Hemibelideus lemuroides), baw baw frog (Philoria pughi), golden bowerbird (Prionodura newtoniana), Daintree River ringtail possum (Pseudochirulus cinereus),buzzing frog (Cophixalus bombiens) (photos by S. Williams, M. Trenerry, G. Webster, G. Guy, G. Calvert, and S. Williams, respectively).

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these systems are rising sea levels caused by global warming, ex-treme weather events (such as storm surges that cause major salt-water incursions inland), and massive plant invasions (Table 2).Hydrological changes, habitat loss and fragmentation, pollution,and changing fire regimes are seen as important localized threats(Table 2).

In general, coastal floodplains and wetlands are vulnerable totipping points because of their restricted and naturally fragmentedgeographic distribution, narrow environmental envelopes, and fre-quently close proximity to land-use pressures in coastal areas (Ta-ble 1). Many sustain sensitive wildlife; for instance, coastal wallumhabitats in eastern Australia contain flora and fauna endemic totheir highly acidic waters (e.g. Meyer et al., 2005). We believethe most susceptible habitats are relatively flat, topographically re-stricted wetlands, especially those trapped between habitat con-version or topography on the inland side and rising sea levels onthe seaward side. Wetlands adjoining coastal areas with high tidalamplitudes (5–13 m), which have more physical energy to driveseawater inland, are also highly vulnerable. They are often con-nected, at least intermittently, to intertidal wetlands, making themvulnerable to saltwater intrusions both at the surface and viagroundwater. Salinity is toxic to amphibians and demonstrably al-ters fish populations (Sheaves and Johnston, 2008; Sheaves, 2009).

3.4. Coral reefs

Coral reefs occur in shallow seas along much of northeastern Aus-tralia with smaller, scattered reefs along the Western Australiancoast. These reefs are considered vulnerable to tipping points be-cause of their narrow thermal and water-quality tolerances, heavyreliance on key ‘framework’ species (reef-building corals), and highsusceptibility to nutrient runoff and eutrophication (Johnson andMarshall, 2007; Hughes et al., 2010). In our view the most vulnerablereefs are those near rivers carrying heavy nutrient loads from nearby

farmlands, and those at near-equatorial latitudes off Cape York Pen-insula and northern Western Australia (Table 1), which are suscepti-ble to coral bleaching associated with global warming. Isolated reefs,such as Ningaloo Reef in Western Australia, are also vulnerable be-cause local species declines are not as easily offset by immigrationas occurs in less-isolated reefs (e.g. Underwood, 2009).

The greatest threat to coral reefs in Australian waters is proba-bly rising sea temperatures, followed by extreme weather events(especially heat waves and destructive storms), ocean acidification,and pollution. Reef destruction and overharvesting of fish, crusta-ceans, gastropods, and other reef species are ancillary threats(Table 2), but are lesser problems in Australia than elsewhere inthe tropics.

3.5. Drier rainforests

Relatively dry rainforest types, including vine thickets, mon-soonal vine-thickets, and semi-deciduous rainforest types such asMabi forest in far north Queensland, occur in moist, comparativelyfire-proof refugia scattered across much of northern Australia(Russell-Smith, 1991; Bowman, 2000). Shifts in fire regime, risingtemperatures, changing rainfall regimes, and extreme weatherevents (especially droughts and heat waves) are considered theirgreatest threats, although many sites are also heavily invaded bylantana (Lantana camara), rubber vine (Cryptostegia grandiflora),and other tropical weeds that can suppress tree recruitment, pro-vide fuel for destructive surface fires (Humphries et al., 1991; Rus-sell-Smith and Bowman, 1992; Fensham, 1994), and render thehabitat unsuitable for some native species (e.g., Valentine et al.,2007). Some are also being degraded by human habitat disruptionand overgrazing by livestock (Table 2).

In broad terms, drier rainforest types are vulnerable to tippingpoints because of their narrow environmental tolerances, theirhighly restricted and patchy distributions (Bowman and Woinar-

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ski, 1994; Price et al., 1999), and the destabilizing positive feed-backs that occur when heavy weed invasions increase fire inci-dence, which in turn opens up the forest and makes it moreprone to further weed invasions and fire (Table 1). We believe thatforest patches that are small, near human settlements, in fre-quently burned areas, and in low-lying areas prone to rising sealevels are especially vulnerable.

3.6. Wetlands and floodplains of the Murray-Darling Basin

Before flowing into the sea near Adelaide, the waters of the vastMurray-Darling Basin must traverse some of the most intensivelyexploited lands in Australia. Wetlands and floodplains in this basinand the linked Coorong estuary are threatened by chronic wateroverharvesting for agriculture and other human uses (Kingsford,2000; Frazier and Page, 2006), salinization (Nielsen et al., 2003),habitat loss (Kingsford and Thomas, 2004), fragmentation (Thomset al., 2005; Wedderburn et al., 2008), sedimentation and associ-ated nutrient changes (Davis and Koop, 2006; Gell et al., 2009),and rising temperatures and sea levels (Table 2).

The Murray-Darling wetlands and floodplains are broadly vul-nerable to tipping points because they are heavily fragmented, relyon vital ‘framework’ species (a limited number of wetland andfloodplain plants) that are approaching environmental thresholds(Colloff and Baldwin, 2010), occur in close proximity to humanpopulations, and are affected by intense inter-jurisdictional de-bates over water rights (Table 1). Southeastern Australia, wherethey occur, is also at high risk of a decline in mean rainfall, accord-ing to future climatic projections (CSIRO-Australian Bureau ofMeteorology, 2007). In our opinion, the most vulnerable habitatsin the Murray-Darling are those that contain mineral sulfide soils(Hall et al., 2006), are susceptible to eutrophication, or are proneto fluctuating water tables. The Coorong estuary is also vulnerable;threshold modeling suggests rapid transitions to different ecosys-tem states are possible in the estuary (Fairweather and Lester,2010).

3.7. Mediterranean ecosystems of southwestern Australia

Recognized as a global biodiversity hotspot because of its mega-diverse plant endemism (Myers et al., 2000), the Mediterraneanhabitats of southwestern Australia sustain a complex mixture ofrelict ancient and modern species. These habitats are intrinsicallyvulnerable for several reasons: they are near important thresholds

Fig. 3. Dieback of native vegetation in Fitzgerald River National Park in Western Ausforeground has suffered dieback whereas that just behind is still unaffected (photo bycomposition.

of temperature and rainfall (Abbott and Le Maitre, 2010), are geo-graphically restricted, rely on vital ‘framework’ species (one ormore locally dominant tree species), have suffered losses of keyfauna (especially mycophagous marsupials; Garkaklis et al.,2004b), and are prone to positive feedbacks between weed inva-sions and destructively intense fires (Table 1). In our opinion, themost vulnerable habitats are the dry sclerophyll forests, wood-lands, and heathlands.

The key threats to these Mediterranean ecosystems are currentand future declines in regional precipitation, especially in winter(Pitman et al., 2004; Yates et al., 2010), rising temperatures, ex-treme weather events (especially droughts and heat-waves butalso frosts), intensifying fire regimes, emerging pathogens andpests (Fig. 3), and salinization. Habitat loss from agriculture andurbanization, fragmentation, timber harvesting, feral animals,and mining operations also pose important threats (Table 2).

3.8. Offshore islands

Excluding Tasmania, Australia has over 8300 offshore islands,ranging in size from <1 ha to nearly 580,000 ha (Ecosure, 2009).In Australia, as elsewhere, islands are considered vulnerable to dra-matic changes because of their restricted size, physical isolation,often-narrow environmental envelopes, and relatively limited(yet often highly endemic) biodiversity that may facilitate speciesinvasions (Table 1) (Burbidge and Manly, 2002; Ecosure, 2009). Webelieve the most vulnerable are small, species-poor islands withmany vacant ecological niches, which are prone to species inva-sions; those with large human populations or visitation; those nearocean-circulation boundaries or with many species that depend onupwelling; and low-lying islands susceptible to rising sea levels.Not all Australian islands have suffered invasions; some have pro-vided important refugia for native wildlife that have been extir-pated elsewhere by introduced predators and competitors(Morton et al., 1995; Burbidge, 1999).

The chief threats to Australia’s islands are myriad invading spe-cies such as rats, mice, rabbits, foxes, pigs, cats, toads, and fire ants(Burbidge and Manly, 2002); extreme weather events such as in-tense storms or droughts that can have disproportionately largeimpacts on insular ecosystems; rising sea levels; habitat loss anddegradation; rising sea-surface temperatures that might affect oce-anic circulation and the upwelling of nutrient-rich waters; andemerging pathogens and pests (Table 2).

tralia caused by the fungal pathogen Phytophthora cinnamomi. Vegetation in theG. Hardy). Dieback causes profound changes in vegetation structure and floristic

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3.9. Estuarine wetlands (salt marshes and mangroves)

Salt marshes and mangroves are estuarine ecosystems that playmany important environmental roles. These include stabilizingcoastal sediments, acting as nutrient and pollution traps, providingprotection from storm surges and tsunamis, sustaining wildlifepopulations, and functioning as vital ‘nurseries’ for breeding fishand crustaceans (Beck et al., 2009). Their narrow environmentaltolerances, geographically restricted nature, proximity to densehuman populations in coastal regions, patchy and fragmented dis-tribution (Duke et al., 2007), and reliance on a few key frameworkspecies generally render them vulnerable (Table 1). We believethat salt marshes and coastal-fringe mangroves (those in narrowstrips along coastlines rather than in estuarine areas) are especiallysusceptible, particularly those in densely populated areas.

In the future, increasing storm intensity could be a seriousthreat to salt marshes and particularly to mangroves at the sea-ward edge (e.g. Cahoon et al., 2003). They also are increasinglylikely to be squeezed between human land-uses or topographyon the landward side and rising sea levels on the seaward side(Eslami-Andargoli et al., 2010). Furthermore, water pollution andsmall changes in salinity and hydrology can cause dramaticchanges in estuarine communities (Table 2).

3.10. Temperate eucalypt forests

In our view, temperate eucalypt forests are ‘dippers’—ecosys-tems that could suffer dramatic future changes but only in partof their geographic range. In general, habitat loss and fragmenta-tion, a reliance on ‘framework’ species (one or a few dominanteucalypt species), close proximity to humans, prior losses of keyfauna (mycophagous and excavating marsupials), and synergismsbetween weed invasions and fire render them especially vulnera-ble (Table 1). We believe that habitats with altered fire regimes(those that deviate from pre-European burning conditions) or suf-fering from heavy habitat loss and fragmentation are most vulner-able (Lindenmayer and Possingham, 1996; McCarthy et al., 1999;Gibbons, 2010).

Among the most important future threats to temperate euca-lypt forests are changes to fire regimes arising from climatechange. Key determinants of fire regime include fuel moistureand weather, factors that will be significantly altered by shifts intemperature, potential evaporation, and the amount and seasonaldistribution of precipitation (Bradstock, 2010). In the future, weteucalypt forests are likely to experience elevated levels of fireactivity. Rising atmospheric CO2 levels and the resulting increasesin plant water-use efficiency might offset drought-induced de-clines in fuel production, although these interactions are complexand uncertain. Habitat loss, fragmentation, overexploitation of tim-ber, and invasive pathogens (especially Phytophthora dieback;Weste and Marks, 1987) are important localized threats (Table 2).

4. Discussion

4.1. A focus on tipping points

We emphasize at the outset that our analysis differs from otherassessments of vulnerable ecosystems in Australia. Our list of the10 ecosystems most vulnerable to tipping points overlaps onlyminimally, for instance, with the Australian government’s list of‘threatened ecological communities’ (EPBC, 1999). The latter iscomposed of finely defined ecosystem types—such as the AquaticRoot-Mat Communities of the Leeuwin Naturaliste Ridge, or East-ern Suburbs Banksia Scrub of the Sydney Region—that often have

very small geographic ranges and are already considered criticallythreatened.

Similarly, in our analysis we considered and rejected a numberof broader ecosystem types, such as the Brigalow Belt, AvonWheatbelt of Western Australia, and Grassy Box Woodlands, be-cause we believe these ecosystems have ‘already tipped’—theyare so drastically diminished or have experienced such profounddegradation and regime changes that their ecology is fundamen-tally altered. Our emphasis, then, is on ecosystems that currentlyretain largely natural characteristics across substantial parts oftheir geographic range but are at risk of changing dramatically inthe near future.

4.2. Predisposing factors

Why are certain Australian ecosystems particularly susceptibleto tipping points? We can draw some tentative conclusions byevaluating the most important features (those ranked 1–3 by ourpanel of experts) across our 10 vulnerable ecosystem types (Table1). The most frequently cited feature of vulnerable ecosystems is arestricted geographic range, which limits their capacity to with-stand anthropogenic pressures simply by persisting in placeswhere such pressures are absent. Elevationally limited mountainecosystems, coastal wetlands, drier rainforests, Mediterraneanhabitats of southwestern Australia, islands, and estuarine ecosys-tems are all considered vulnerable for this reason. The second mostfrequently cited feature, a narrow environmental envelope, is re-lated partially to the first. This feature characterizes mountain eco-systems, coral reefs, drier rainforests, islands, and estuarinehabitats. Such ecosystems appear sensitive to even relatively mod-est changes in environmental conditions.

Four other features were also considered relatively important,being cited among the most important predisposing features for3–4 ecosystems each (Table 1). Ecosystems that have suffered sub-stantial anthropogenic fragmentation, that rely on critical ‘frame-work’ species (such as one or a few species of canopy trees, orcoral-building organisms), that are constrained by close proximityto humans or human activities, or that already live close to an envi-ronmental threshold, also appear particularly vulnerable to tippingpoints. These associations generally seem logical. For instance,fragmented ecosystems are unusually vulnerable to climatic andother environmental vicissitudes (Laurance, 2002). Ecosystemsnear their limits of environmental tolerance, or that rely on oneor a few types of critical framework species, appear similarlyvulnerable.

4.3. Key drivers

We now identify the most pervasive environmental drivers thatpredispose Australian ecosystems to tipping points. Our analysis isbased on ranking the relative importance of 13 environmentaldrivers for each of our 10 vulnerable ecosystems (Table 2). As be-fore, our focus is on the drivers that we regarded as most important(those ranked 1–3 for each ecosystem). Notably, the anthropogenicthreats identified here may well differ from those that have alteredAustralian ecosystems in the past (see Flannery, 1994; Johnson,2006).

The two most important of the top-ranked drivers, extremeweather events and changes in water balance and hydrology, wereeach considered important for seven of the 10 ecosystems. Ex-treme weather events include severe, short-term phenomena suchas heat waves, droughts, and intense storms. We speculate that theAustralian continent, whose precipitation and hydrology arestrongly influenced by the El Niño-Southern Oscillation (Nichollset al., 1997; Chiew et al., 1998), whose ancient, relatively flat landsurface is poor at capturing rainfall, and which is dominated by

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strongly seasonal environments at tropical and subtropical lati-tudes, may be particularly susceptible to such events. Changes inwater balance and hydrology usually arise from water overharvest-ing, such as is occurring in the Murray-Darling Basin, or fromchanges in moisture inputs, a phenomenon that under plausiblescenarios of future climate change could imperil montane ecosys-tems that rely on orographic rainfall and/or cloud-stripping (Stillet al., 1999; Bradley et al., 2006).

Many ecosystems are also vulnerable to rising temperatures orrising sea levels (Table 2), both of which relate directly to globalwarming. Among the myriad ways in which global change phe-nomena could affect Australian ecosystems, one of the potentiallymost important is by altering fire regimes (Bradstock, 2010). Fireregimes are largely determined by weather and fuel loads. Increas-ing atmospheric CO2 could potentially increase fuel loads via en-hanced primary productivity (Donohue et al., 2009; Sun et al.,2010), but this effect could be magnified or diminished by changesin available moisture, depending on the location. In some ecosys-tems, serious weed invasions are profoundly altering fire regimes(Fig. 1). Fire-promoting invaders can dramatically transform eco-systems, usually favoring short-lived annuals and exotics at the ex-pense of long-lived trees.

Although factors relating to climatic change are likely to play akey role in predisposing Australian ecosystems to tipping points,we emphasize that most of our vulnerable ecosystems are beinginfluenced by multiple drivers (Table 2). For us, this reinforces ageneral view that synergisms among different environmental driv-ers can be extremely important, predisposing species and ecosys-tems to serious environmental changes (Laurance and Cochrane,2001; Brook et al., 2008; Laurance and Useche, 2009). In our anal-ysis, examples of such synergisms are pervasive—for example, be-tween weed invasions and fire, between land-use change andclimatic change, between anthropogenic activities and introducedpathogens, and between coastal land-use pressures and rising sealevels. For the Australian environment, as elsewhere, combinationsof environmental perils may be the death knell for manyecosystems.

4.4. Conservation actions to avoid tipping points

The threats facing vulnerable ecosystems in Australia are oftenmulti-faceted and, at least for some perils such as global climatechange, rising ocean acidity, and the continued spread of certaininvasive species and pathogens, largely beyond the control of Aus-tralian resource managers. In practical terms, this limits the toolsthat can be applied to mitigate these pressures. Rather thanpreaching despair, however, we believe much can be done to limitthe further decline of vulnerable Australian ecosystems.

A key priority is to identify likely or imminent changes in vul-nerable ecosystems and taxa (e.g. Abbott and Le Maitre, 2010;Hughes et al., 2010; Woinarski et al., 2010). A full discussion of thisconcept is beyond the scope of this paper, but we note two keypoints. First, the best approach for judging whether an ecosystemis approaching a tipping point may be to examine key ecologicalprocesses involved in proper ecosystem functioning and integrity(Dunning et al., 1992; Didham et al., 1996), rather than biodiversityindicators (such as species richness) that can have delayed re-sponses to disturbance effects (Loehle and Li, 1996; Vellendet al., 2006). Second, a key harbinger of tipping points may be a‘critical slowing’ of ecosystem dynamics. This can include slowerrecovery from disturbances, increased variance in ecosystemdynamics, and increased auto-correlation in ecosystem propertiesas the tipping point is approached (see van Nes and Scheffer(2007), Biggs et al. (2009), Scheffer et al. (2009), Drake and Griffen(2010), Scheffer (2010) for discussion). Further, phenomena suchas an increased variance and spatial auto-correlation might be

detectable from spatial patterns in vegetation (Bailey, in press),potentially allowing ecosystem vulnerability to be evaluated viaremote sensing, rather than requiring detailed field studies. Suchapproaches might provide important insights into the status andvulnerability of particular ecosystems.

In addition, on-the-ground conservation and management ac-tions can often have a profound impact on ecosystem resilience.In broad terms, concrete steps such as increasing the size and num-ber of protected areas, limiting external disturbances such as hab-itat conversion and new roads (Goosem, 2007; Laurance et al.,2009), creating buffer zones and wildlife corridors, restoring keyhabitats and landscape linkages (Shoo et al., 2010), and designingand locating nature reserves to maximize their resilience to cli-mate change (Hannah et al., 2007; Loarie et al., 2009; Shoo et al.,2010) can play vital roles in maintaining ecosystem viability. Keyphenomena such as fire regimes can often be managed via stepssuch as prescriptive burning, silviculture, livestock grazing, firesuppression, and controlling human ignition sources (Yibarbuket al., 2001; Murphy et al., 2009; Russell-Smith et al., 2010).

Managing natural and semi-natural ecosystems in a world thatis continually in flux is a great challenge, but societies are adaptingto these realities. Environmental regulations and policies arechanging profoundly in an effort to address complex and multi-fac-eted environmental challenges (Lockwood et al., 2010). Conserva-tion efforts are increasingly being integrated across institutionsand among public, private, and civil sectors to address uncertaintyand ‘wicked’ environmental problems (Holling, 1978; Robinsonet al., 2009) in an adaptive and flexible manner (Dietz et al.,2003; Armitage et al., 2009). Environmental ‘horizon scanning’ isbeing used to anticipate new threats (Laurance and Peres, 2006;Sutherland and Woodroof, 2009). Great challenges lie ahead forAustralian ecosystems, as elsewhere, but much can still be doneto address them.

Acknowledgements

We thank S.G. Laurance, G.R. Clements, and R.K. Didham forcomments on the manuscript and C. Gemellaro, K. Milena, and P.Byrnes for logistical assistance. The raw data for this study, includ-ing a list of all nominated vulnerable ecosystems, are availableupon request. This paper results from a collaborative investigationamong the Innovative Research Universities of Australia(www.iru.edu.au).

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