Karst Wetlands Biodiversity Ani) Continuity Through Major Climatic Change: an Example from Arid Tropical Western Australia
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KARST WETLANDS BIODIVERSITY ANI) CONTINUITY THROUGH MAJOR CLIMATIC CHANGE: AN EXAMPLE FROM ARID TROPICAL WESTERN AUSTRALIA W.F.Humphreys krrestrial InvertebrateZoology, Museum o.f l,{atural Science, WesternAustralian Museum, Francis Street,Perth, WA6000, Australia; E-mail: [email protected]. gov. au Abstract Subterraneankarst wetlands,recently included as a wetland type under the Ramsarclassification, are introducedhere to the wetlandsbiodiversity literature.The generalcharacteristics of karst land- scapesexplain the connectionswith non-karsticwetlands, the often diffuse limits to karst wetlands and their vulnerability to exploitation and pollution. Less than l0oA of the Earth shows distinctive karst landforms but on a global basiskarst wetlandssupport the greaterpart of subterraneanbiodi- versity.This introduction is amplified by a detailed account of a single karst wetland Cape Range peninsula,Western Australia. Cape Range,the only orogenicTertiary limestone in Australia, occurs on a continent relatively devoid of karst. It lies in the arid tropics where surface water flows are mostly episodic and fre- quently saline. The Cape Range karst wetlands contain both freshwater and anchialine wetlands whose natural accessis through a few karst windows and caves.The anchialine system shows marked stratification of their physico-chemicaland biological profiles. The lack of surfacewaters and the particular geographical context means that much of the groundwater fauna represents ancient geographical or phyletic relicts. A general case is made supporting the persistenceof groundwaterfauna in karstic systemsthrough geologicaleras. Introduction The 6th Conferenceof ContractingParties to the RamsarConventionl (Brisbane 1996) voted to include subterraneankarst wetlandsas a wetland type under the Ramsarclassification system. This inclusionhas far reachingimplications, not only for karst regions,but also for a wide rangeof wetlands,and for groundwatercon- servationgenerally. Karst hydrologicalsystems are extensive,occur in many guis- es, are vulnerable,and frequentlysupport unique phyletic and distributionalrelict- ual faunas(see Humphreys. in pressa). In addition,they havedrainage characteris- tics (Ford and Williams 1989) that serveto influencethe hydrologicalchanges experiencedby wetlandsremote from the karst systemsthemselves. I Conventionon Wetlandsof InternationalImportance especially as Waterfowl Habitat (RamsaqIran 197r). BiodiversiQ in wetlands: assessment,function and conservation, volume l edited by B. Gopal, W.J.Junk and J.A. Davis, pp. 227-258 @ 2000 Backht:s Publishers. Leiden. The Netherlands 228 WE Httmphrevs The biota of karstlandscomprise a wide rangeof epigeantaxa, including calci- phytesand calciphile lineages such as molluscs - which canbe both highly diverse andlocally endemic (Solem 1981a, 1981b, 1984,1985, 1997, Woodruff and Solem 1990)- and,in karstwetlands, phreatophytic vegetation that makesdirect use of the groundwaterand which canhave a major influenceon the local hydrology(Tromble 1977).But it also includesthe specialisedfauna restrictedto subterraneanaquatic systems- thesewill be referredto generallyas stygobitesor stygofauna(Gibert et al. 1994)in contradistinctionto troglobiteswhich are essentiallyterrestrial fauna restrictedto subterraneanair-filled voids (the term troglobitesis sometimesused sensulqto to encompassall hypogeanenvironments). Stygofauna can alsobe both highlydiverse and locally endemic (e.g. Holsinger 1978, Longley 1981, Holsinger and Culver. 1988,Wilson and Ponder1992, Stoch 1995,Bradbury and Williams 1996a, 1996b, 1997a, 1997b, Knott and Jasinska 1998, Humphreys 1999a, Humphreys,in pressb, Wilson andKeable 1999). What is Karst The unusualgeomorphic features of the Kras - known as Karst in the period of the Austro-Hungarianempire - region on the Italo-Slovenianborder have become known as 'karst phenomena'.The term karst denotesthe distinctivestyle of terrain resulting predominantlyfrom the dissolutionof rock by natural waters,hence the term 'solublerock landscape'is sometimesused. Owing to their solubilitykarst is most fully developedin carbonaterocks suchas limestoneand dolomite,and evap- oraterocks suchas gypsum.Such areas are characterrzedby sinking streams,caves, encloseddepressions, fluted rock outcropsand large springs(Fig. 1, Ford and Williams 1989).The integrity of such landscapesis dependentupon the mainte- nanceof the natural hydrologicalsystem and they are potentiallyhighly sensitive,, comparablein this respectto desertsor coastalmargins. Whether the coincidence of karst,desert and coastalsystems, as in the CapeRange karst discussedbelow, makesthe areaextraordinarily vulnerable is a moot point. The Limits to Karst Wetlands The dissolutionof carbonaterocks occursmost rapidly with running water and at the interfacesbetween waters of differing chemicalcomposition, and especially betweenfreshwater and saltwater(Ford and Williams 1989).In mostregions water tables,at somestage, have been lower thanat presentowing, inter alia, to previously drier climateor lower sealevel, hence, for this reasonalone, karst formationsare found commonly well below the current water table. Even in arid zonesthe lower partsof karstare usually below the watertable (phreatic zone) and may containboth lentic(groundwater) and lotic (pressuretubes) wetland systems. Even in thoseparts of the karst abovethe local water table (vadosezone) there may be lotic wetlands, as found for example,in undergroundstream passages. Wheresurface exposures of wateroccur in karstlands,the normalrange of wet- land featuresis producedbut,, in addition,a numberof distinctivefeatures - some- times with specializedfaunas - may also occur associatedwith karst.Amongst theseare anchialine pools (Maciolek 1986, Brock et al. 1987,Thomas et al. 1991, Karst wetlands biodiversity and continuiQ through ntajor climatic chctnge 229 Fig. l. The comprehensivekarst system.(Fig 1.1 of Ford and Williams 1989;with permission). 1992,I1iffe1992, in press,Sket 1996), springs and spring brooks (Botosaneanu 1998), mound springs(Williams 1965,Harris 1992,Ponder et al. 1995,Knott and Jasinska 1998),tufa (travertine)dams (Julia 1983,Marias 1990,Pentecost 1992, Drysdale and Head 1994,Humphreys, Awramik andJebb 1995), calcretes (Humphreys 1999a) and blue holes(Stock 1986b,Ilitre 1992),details of which will be presentedonly where they are relevantto the examplepresented later in this chapter. Furthermore,open conduit flow permits large-scalemovements within karst which may facilitatedynamical processes - which may themselvesbe extrinsicto the karst system- supportingthe fauna associatedwith the karst. For example,in the EdwardsAquifer (artesian),Texas (Kuehn and Koehn 1988,Longley 1981, 1992),and in Movile Cave (karstic) in Romania(Sarbu and Popa 1992,Sarbu, in press,Sarbu et al. 1996),sulphides, respectively of petroleumand magmatic origin, supportchemoautotrophic ecosystems (Poulson and Lavoie,,in press).Analagous systemsare found associatedwith both hydrothermal(Childress and Fisher 1992) and cold deepsea vents (Arp and Fisher 1995,Scott and Fisher1995), with fauna in the latter also utilising methaneharvesting bacteria (MacDonald et al. 1989). Chemoautotropyhas also been demonstratedin FrasassiCave (Sarbu et al., in press),and strongly indicatedin anchialinesystems (Pohlman et a1.,in press, Humphreys1999b). Karst wetlandsmerge imperceptiblyinto groundwater- water which doesnot interact in the short term with surface waters - and with flow paths beneath (hyporheic)and alongside(epirheic zone))surface water courseswith which short- term interactionstake place through upwellings and downwellings(Stanford and Ward 1988,Gibert et al. 1994,Ward et al.,in press)which haveprofound effects on 230 WF Htmphreys stygalecology (e.g. Dole-Olivier et. al. 1994,Danielopol et al., in press).The fauna typical of karstwetlands may extendinto lavatubes (Wilkens et al.1986),riverine gravels(Boulton 1993),and into immediatelysub-littoral systems such as subma- rine springs (Danielopol and Bonaduce 1990). In addition, through Gyben- Hetzbergsystems - in which freshwaterlens overlie seawater intrusions(Ford and Williams 1989) they mergewith the marinesystems especially in coastsexhibit- ing anchialine(var. anchihaline) waters which may supportextremely diverse, often relictualfaunas (Thomas et al. l99l,Iliffe 1992,in press,Thomas et al. 1992,Sket 1996,Yager and Humphreys 1996).Anchialine systemsare near coastal subter- raneanmixohaline waters that are undertidal influencebut haveonly subterranean connectionswith the sea;they typically occur in volcanicor limestonebedrock and have limited surfaceexposure (Stock et al. 1986).Such waterstypically exhibit markedchanges in the depth profiles of numerousphysico-chemical parameters (Iliffe, in press,Yager and Humphreys1996, Humphreys 1999b). Karst wetlandsinclude two distinctprocesses that integrateto producethe char- acteristickarst landscapes. For, in additionto the dissolutionprocesses defining karst, there are also constructionalkarst featuresinvolving the accumulationof new car- bonatemasses that rangein scalefrom minor speleothemdeposition to massivetufa depositson hillsides.Such depositions may involveboth biogenicand physicochem- ical processes(Humphreys, Awrwnik and Jebb 1995).Associated coral reef com- plexesmay be consideredan integralparl of the karst system(Hamilton-Smith et al. 1998),as, for example,the largeNingaloo reef fringing the CapeRange peninsula. Calcretesare a form of carbonaterock generallyomitted from discussionsof