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REVIEWS EcologicalMonographs,84(2),2014,pp.203–244 (cid:2)2014bytheEcologicalSocietyofAmerica The spatial structure of Antarctic biodiversity PETERCONVEY,1,2,19 STEVEN L. CHOWN,3ANDREWCLARKE,1DAVIDK.A. BARNES,1STEFBOKHORST,4 VONDACUMMINGS,5HUGH W.DUCKLOW,6FRANCESCOFRATI,7 T. G.ALLANGREEN,8SHULAMITGORDON,9 HUWJ. GRIFFITHS,1CLIVEHOWARD-WILLIAMS,10 AD H. L. HUISKES,11,20 JOHANNALAYBOURN-PARRY,12 W.BERRY LYONS,13 ANDREWMCMINN,14SIMONA.MORLEY,1LLOYDS. PECK,1ANTONIOQUESADA,15 SHARON A.ROBINSON,16STEFANOSCHIAPARELLI,17ANDDIANAH. WALL18 1BritishAntarcticSurvey,NaturalEnvironmentResearchCouncil,HighCross,MadingleyRoad, CambridgeCB30ETUnitedKingdom 2NationalAntarcticResearchCenter,IPSBuilding,UniversityMalaya,50603KualaLumpur,Malaysia 3SchoolofBiologicalSciences,MonashUniversity,Victoria3800Australia 4DepartmentofForestEcologyandManagement,SwedishUniversityofAgriculturalSciences,Umea˚SE90183Sweden 5NationalInstituteofWaterandAtmosphericResearch,PrivateBag14-901,Wellington6241NewZealand 6Lamont-DohertyEarthObservatory,Palisades,NewYork10964USA 7DepartmentofLifeSciences,UniversityofSiena,viaA.Moro2,53100Siena,Italy 8DepartmentofBiologicalSciences,UniversityofWaikato,PrivateBag3105,Hamilton,NewZealand 9AntarcticaNewZealand,PrivateBag4745,Christchurch8140NewZealand 10NationalInstituteofWaterandAtmosphericResearch,P.O.Box8502,Christchurch8440NewZealand 11NetherlandsInstituteofEcology(NIOO-KNAW),UnitforPolarEcology,P.O.Box140,4400ACYerseke,TheNetherlands 12BristolGlaciologyCentre,SchoolofGeographicalSciences,UniversityofBristol,BristolBS81SSUnitedKingdom 13ByrdPolarResearchCenter,SchoolofEarthSciences,TheOhioStateUniversity,1090CarmackRoad, Columbus,Ohio43210-1002USA 14InstituteforMarineandAntarcticStudies,UniversityofTasmania,Hobart,Tasmania7001Australia 15DepartmentofBiology,UniversidadAutonomadeMadrid,28049Madrid,Spain 16InstituteforConservationBiology,TheUniversityofWollongong,NewSouthWales2522Australia 17DipartimentodiScienzedellaTerra,dell’AmbienteedellaVita(DISTAV),Universita`diGenova,C.soEuropa26, GenovaI-16132Italy 18DepartmentofBiologyandNaturalResourceEcologyLaboratory,ColoradoStateUniversity,FortCollins, Colorado80523-1499USA Abstract. Patterns of environmental spatial structure lie at the heart of the most fundamental and familiar patterns of diversity on Earth. Antarctica contains some of the strongestenvironmentalgradientsontheplanetandthereforeprovidesanidealstudyground totesthypothesesontherelevanceofenvironmentalvariabilityforbiodiversity.Toanswerthe pivotal question, ‘‘How does spatial variation in physical and biological environmental propertiesacrosstheAntarcticdrivebiodiversity?’’wehavesynthesizedcurrentknowledgeon environmental variability across terrestrial, freshwater, and marine Antarctic biomes and related this to the observed biotic patterns. The most important physical driver of Antarctic terrestrial communities is the availability of liquid water, itself driven by solar irradiance intensity. Patterns of biota distribution are further strongly influenced by the historical development of any given location or region, and by geographical barriers. In freshwater ecosystems,freewaterisalsocrucial,withfurtherimportantinfluencesfromsalinity,nutrient availability, oxygenation, and characteristics of ice cover and extent. In the marine biome theredoesnotappeartobeonemajordrivingforce,withtheexceptionoftheoceanographic boundary of the Polar Front. At smaller spatial scales, ice cover, ice scour, and salinity gradients are clearly important determinants of diversity at habitat and community level. Stochastic and extreme events remain an important driving force in all environments, particularly in the context of local extinction and colonization or recolonization, as well as that of temporal environmental variability. Our synthesis demonstrates that the Antarctic Manuscript received 20 December 2012; revised 17 June 2013; accepted 21 June 2013; final version received 20 July 2013. CorrespondingEditor:B.J.Cardinale. 19E-mail:[email protected] 20Presentaddress:RoyalNetherlandsInstituteforSeaResearch,P.O.Box140,4400ACYerseke,TheNetherlands. 203 204 PETERCONVEYETAL. EcologicalMonographs Vol.84,No.2 continent and surrounding oceans provide an ideal study ground to develop new biogeographical models, including life history and physiological traits, and to address questions regardingbiological responses to environmental variability andchange. Keywords: adaptation;biogeography;environmentalgradients;historicalcontingency;marine;spatial scaleandvariation;terrestrialenvironments. INTRODUCTION fied by soils and water bodies in the McMurdo Dry Valleys and the McMurdo Ice Shelf, respectively Spatialstructure [Ferna´ndez-Valiente et al. 2001, Barrett et al. 2007]), is Spatial structure is one of the most fundamental theterminusofmanyglobalgradients(UsherandBooth characteristics of the planet. Early natural historians 1986, Peck et al. 2006), and its biota has formed the and geographers realized that the environment varies focusofmanybiogeographicinvestigations(reviewedin systematically over a range of spatial scales, and Convey et al. 2012a, Fraser et al. 2012, Terauds et al. following the 19th-century voyages of discovery, wide- 2012).ExplicitattentiontospatialvariationinAntarctic spreadrecognitiondevelopedthatthisvariationextends diversity and its mechanistic underpinnings is growing to global scales in the form of a diversity gradient. As apace (e.g., Chown et al. 1998, Adams et al. 2006, the fields of ecology and biogeography developed over Chown and Convey 2007, Peat et al. 2007, Verleyen et the 20th century, much progress was made in under- al.2009,Griffiths2010,Cowanetal.2011,Mortimeret standing the form of spatial variation and the likely al. 2011a,b, Schiaparelli and Hopcroft 2011, Allcock factors underlying it. Several key developments stand and Strugnell 2012, Born et al. 2012). However, few out from a biodiversity perspective. First is the early description and analysis of gradients in the abiotic attempts have been made to draw together the environment,andspeciesresponsestothem(Andrewar- conclusions from this work. Doing so would not only tha and Birch 1954, Whittaker 1967). Such work provide substantial insight into the biodiversity impli- remains important in modern investigations of species cations of spatial variation in a physically extreme distributionandabundance,includingrecognitionofthe system, but would also indicate the extent to which significance of direct, indirect, and resource gradients spatial frameworks developed elsewhere apply to the S W (Austin 1980). Second, geographers realized early on region.Inturn, this wouldtest their validity andreveal E thatcloselylocatedfeaturesaremostlikelytobesimilar, the extent to which the variety encompassed by VI E a feature known as spatial autocorrelation. The signif- Antarctic systems can serve further to establish ecolog- R icanceofautocorrelationforunderstandingvariationin icalgeneralities. biodiversityremainsthesubjectofintenseinterest(e.g., As examples of the insights such an approach can Beale et al. 2010, Hawkins 2012). Third, exploration of deliver, consider two key ecological predictions at variationinfeaturesatdifferentspatialscaleshasledto different spatial scales. First, diversity at high latitudes wide appreciation that different processes are likely to should typically show a decline as a consequence either underlie patterns at each scale (Ricklefs 1987). Scale- ofdecliningenergy(Hawkinsetal.2003,butseeClarke related thinking now permeates almost every area of and Gaston 2006) or the change in the seasonal ecology andevolutionaryphysiology. availability of that energy (Archibald et al. 2010). The Growing integration among these areas has led to extent to which such declines are found varies among recognitionthatenvironmentalspatialstructuremaylie terrestrial and marine systems in the region (Peck et al. at the heart of the most fundamental and familiar 2006)andamongtaxawithinthemarinesystem(Clarke patternsofdiversity(Storchetal.2008).Moreover,itis andJohnston2003).Inconsequence,considerablescope now widely appreciated that to understand how exists for testing unified theories to explain biodiversity biodiversity changes through time, as a consequence of variation (e.g., Price et al. 2012). Second, much dispersal,extinction,speciation,andevolution,andhow ecological theory suggests that predominantly abiotic it may respond to environmental change driven by factorsshouldinfluencethedistributionandabundance humans, requires a spatially explicit approach (Black- oforganismsinlow-diversitysystems(MacArthur1972, burn and Gaston 1998, Sobero´n 2007, Gaston et al. Southwood1988,Ricklefs2011).Ifthisisthecase,then 2008, 2009, McRae et al. 2008, Thuiller et al. 2008, models seeking to explain variation in abundance and Storferetal.2010,Ellneretal.2011,Bellardetal.2012). In consequence, much attention is now being given to distribution, as well as in species richness, should be the collection of spatially explicit data and the descrip- dominated by abiotic terms and by those describing tionofvariationatseveralscalesasthestartingpointfor their spatial structure. In turn, the outcomes of investigating biodiversity. mechanistic and environmental niche models (Kearney Suchaperspectiveisalsobeingbroughttobearonthe andPorter2009)shouldcoincide,soassumptionsabout Antarctic. Antarctica contains some of the strongest the extent to which each of these species distribution environmental gradients on the planet (e.g., from modeling approaches deals with fundamental and extreme hypersalinity to almost zero salinity, exempli- realized nichesshould besubstantiated. May2014 ANTARCTICBIODIVERSITYSPATIALSTRUCTURE 205 Ouraimsherearethereforetoprovideanoverviewof the shelf area on the planet) (Arntz et al. 1994, Clarke spatial variation in the Antarctic terrestrial, limnetic, and Johnston2003, Schiaparelli andHopcroft2011). and marine environments, and to see what lessons for Unlike the Arctic, Antarctic marine and terrestrial broader understanding of biodiversity variation gener- ecosystems are largely isolated, rather than forming a ally have emerged from work in this area. We then continuumfromthoseatlowerlatitudes,aprocessthat provideasynthesisofcurrentknowledgeintheseareas. commenced with the last stages of the breakup of Because biodiversity has an explicitly historical context Gondwana, and was enhanced and then maintained by (Ricklefs 1987, Clarke andCrame 1989), we commence the development of the atmospheric Polar Vortex and with a brief discussion of the history of the region, oceanic Antarctic Polar Front (Clarke et al. 2005, making reference to more comprehensive reviews for Barnes et al. 2006b, Bergstrom et al. 2006). In the deep further information. Throughout, we draw on specific oceanssurroundingAntarctica,theisolatinginfluenceof examples to illustrate our broader understanding, the Antarctic Circumpolar Current extends to about recognizing that as a consequence, some significant 1000 m depth. Below this, connectivity with the other areas of work have to remain less fully covered. The global ocean basins is more significant, exemplified by latterincludetherolesofpolaroceanicfronts,mesoscale the northwards flow of cold Antarctic Bottom Water features, and seabed topography in determining varia- that forms a major driver of the global overturning tioninpelagicbiodiversity(see,e.g.,Murphyetal.2007, circulation or ‘‘ocean conveyor belt.’’ In marine pelagic Tittensoretal.2010,Louzaoetal.2011,Wakefieldetal. orplanktonicecosystems,thedistribution oforganisms 2011, Ainley et al. 2012, Strugnell et al. 2012), the intimeandspaceisdeterminedbycomplexinteractions complexities of diversity–environment interactions in among trophodynamics, population dynamics, physical the many streams that are a feature of the terrestrial mixing, and circulation processes (Cullen et al. 2002, Antarctic in the summer (e.g., Laybourn-Parry and Wakefield et al. 2011). At the mesoscale and larger Pierce 2007), and detailed discussion of the functioning scales, distributions correspond to circulation features of sub-Antarctic terrestrial systems (e.g., Chown and ranging from eddies and rings to basin-scale frontal Froneman2008). boundariesseparatingmajor watermasses (Knox1994, Longhurst1998,Ainleyetal.2012,Rogersetal.2012a). The Antarctic environment Terrestrial ecosystem development is limited to areas R The‘‘Antarctic’’isdefinedhereinitswidestsense.For that are seasonally or permanently snow- and ice-free, E terrestrial and nonmarine aquatic environments, this andhaveappropriateenvironmentalconditions.Ice-free V means the Antarctic continent and Peninsula, the groundiscurrentlylimitedto;0.34%oftheareaofthe IE various archipelagos of the Scotia arc, and the sub- Antarctic continent, equating to ;45000 km2, while WS Antarcticislands.Themarineenvironmentencompasses visible life is largely but not completely restricted to the entire Southern Ocean, formed by southern prov- lower-altitude exposures in coastal regions (Convey et incesoftheAtlantic,Indian,andPacificOceans(Fig.1), al. 2009, Convey 2013). In contrast with much of the and is delineated to the north by the mean position of marine environment, typically island-like terrestrial theAntarcticPolarFront.Antarcticecosystemsvaryon ecosystems are isolated from each other across a range land from polar deserts, including the continent’s ice of scales, from meters to many hundreds of kilometers. itself, freshwater to hypersaline lakes and their ice The ice of Antarctica is also not devoid of life. covers,tolushgrasslandsandeutrophicponds(Thomas Considerable biomass is associated with snow and ice et al. 2008) (Fig. 2). Marine ecosystems range from algalcommunitiesthatdevelopinsummer,especiallyin shallow coastal regions to abyssal depths of the open coastal regions, although these have received relatively ocean, ice-free to permanently ice-covered areas, and littleresearchattention(Bagshawetal.2007,Stibaletal. highlydiverseto verysimple,featureless habitats. 2012; see Hodson et al. 2008 for discussion of Arctic parallels). The existence of subglacial microbial com- The Southern Ocean is dominated by the deep sea munities, increasingly recognized in alpine regions, is (depth .3000 m). However, very little is known about now being examined in Antarctica (Tranter et al. 2005, Antarctic deep-sea biodiversity (Kaiser and Barnes Lanoil et al. 2009), while much attention is focused on 2008), other than that it appears to be rich in some thepotentiallyexceptionalbiotatobefoundinthemany groups and undescribed species (Brandt et al. 2007, lakesnowknowntoliebeneaththecontinent’sicesheets Rogers et al. 2012b), and that this richness is patchy orin itspermafrost (Skidmore 2011). across spatial scales (Kaiser et al. 2007, Griffiths et al. 2009,Conveyetal.2012a).Thecontinentalslope(shelf HISTORY break to 3000 m) is also poorly sampled and known, Cenozoic climate changeandglaciations although it would appear to be central to a cline from species-rich and abundant shelf faunas to generally Understanding current biological processes, particu- poorer abyssal depths. The majority of sampling, larly those relating to diversity, requires sound knowl- recorded diversity, and knowledge of biological struc- edgeoftheregionalhistoryofthebiota.Onthelongest ture concerns the fauna of the continental shelf depths timescales,Antarcticahasshiftedfrombeingwarm,ice- (generallyshallowerthan1000m,accountingfor8%of free, and broadly connected to other land masses, to 206 PETERCONVEYETAL. EcologicalMonographs Vol.84,No.2 S W E VI E R FIG.1. (A,B)OverviewmapsofAntarctica,theSouthernOcean,andadjacentregionsoftheSouthernHemisphere,indicating locationsmentionedthroughoutthetext. cold, glaciated, and isolated (Bertler and Barrett 2010). appear to be enhancements of existing features rather The fossil history of the region is limited and does not thannoveladaptations(Convey1996),whilecontinental permit description of changes in detail, at least not for cooling and extensive glaciation led to more extensive thepurposesrequiredhere.Nonetheless,muchworkhas extinctions thanseenin the marineenvironment. been undertaken on the fossil history of the Antarctic Climatesince the Last Glacial Maximum (seeforinstanceHaywoodetal.2009,StilwellandLong 2011,Barrett2013).ThecoolingofAntarcticwaterswas The last 15–20 kyr have seen considerable climatic keyindrivingchangestothemarinefauna,leading,for change (Steig et al. 2000, Fountain and Lyons 2003, example, to the evolution of antifreeze in teleost fishes Mulvaney et al. 2012). The Last Glacial Maximum (DeVries1988,Chenetal.2008),thelossofaheatshock (LGM) ended at 15 kyr BP with a very rapid warming response (Clark and Peck 2009), and of invertebrate (Bølling-AllerødEvent),whenmeanannualtemperature groups such as many decapods (Clarke and Johnston increased by almost 128C over 1 kyr and the snow 2003), and evolutionary radiations in other groups. On accumulation rate increased. The Younger Dryas land, physiological responses to desiccation and cold cooling event from 13–11 kyr BP then temporarily May2014 ANTARCTICBIODIVERSITYSPATIALSTRUCTURE 207 R E V IE W S FIG.1. Continued. dropped temperatures by ;48C. After 10 kyr BP there 12 kyr BP (Webster et al. 1996). As the Ross Ice Shelf was a gradual cooling of ;58C, with sharp cooling thenretreated,thesedrained,leavingaseriesofsmaller events 9.5 and 6.5 kyr BP. Finally, over the last 1000 lakes. Cooling uptoabout1 kyragowasaccompanied years, there has been a warming of ;28C. A similar by lake evaporation to a minimum volume (Wilson climatic history is evident on the sub-Antarctic islands, 1964, Lyons et al. 1998), and the lakes have since although it varies with their spatial location (e.g., Hall partially refilled as temperatures have warmed. During 2002, Fraseretal. 2012). the high lake stands, previously deposited salts were This historical background has left traces detectable solubilized, now underlying structurally important inmodernterrestrialandaquatichabitats.Forexample, salinity gradients (Vincent et al. 1981), and providing molecular signals are present in populations of Mc- nutrientsforchlorophyllmaximaatdepth(Priscu1995). MurdoDryValleyspringtailsthatindicatetheinfluence The ecological history (or legacy) of a given area is of ancient shorelines (Nolan et al. 2006). Lakes can important in the understanding of inland ice-free preserveclearrecordsofchangeoverlongtimescales.In ecosystemssuchastheMcMurdoDryValleys,Vestfold VictoriaLand,lakesreachedtheirmaximumextent10– Hills (Pickard 1986, Zwart et al. 1998, Moorhead et al. 208 PETERCONVEYETAL. EcologicalMonographs Vol.84,No.2 1999,Lyonsetal.2001,Hodgsonetal.2004),Larseman Refugia andisolation Hills (Burgess et al. 1994), and Schirmacher Oasis Signals of spatial regionalization and temporal (Bormann and Fritzsche 1995). Soils in the McMurdo isolation are apparent in both terrestrial and marine DryValleyregionhavebeenproducedbythesuccessive environments of Antarctica (Griffiths et al. 2009, movementsoftheEastAntarcticandWestAntarcticIce Teraudsetal.2012).Inthesea,theoverwhelmingcause Sheets through the Valleys as climate has fluctuated ofisolationisthatoftheAntarcticCircumpolarCurrent (Hall et al. 2000, Hendy 2000). Thus the soils in the (ACC) and Polar Front (Clarke et al. 2005). This easternendoftheTaylorValleyareyoung(;24kyrand eastwards flowing current, formed between 24 and 41 less), while those in the western part of the valley are mya,createsasteeptemperaturegradientof38–48Cover much older (;75–130 kyr), indicating that even at the a distance of tens of kilometers, and forms a strong last glacial maximum the Antarctic terrestrial environ- biogeographic discontinuity. Subsequent evolution in ment was not completely covered by ice. The younger the cold Antarctic marine environment has selected for soilshavemuchlowerN:Pratios(Barrettetal.2007),as stenothermal (Somero and DeVries 1967, Peck et al. they have more soluble P (low weathering loss), while 2010b)andeurybathictaxa(Breyetal.1996),andledto theoldersoilshavehighertotalnitrogencontentdueto high species-level endemism (50–70%) (Griffiths et al. longerexposuretoatmosphericnitrateinput.Theoldest 2009, Convey et al. 2012a).At glacial maxima, most of soil surfaces have higher conductivities because they the continental shelf was covered by ice, restricting have accumulated atmospheric aerosols, leading to fauna to isolated refugia or forcing them into deeper lower soil invertebrate abundance (Virginia and Wall water(Thatjeetal.2005,Conveyetal.2009).Cyclesof 1999). Some of the soil organic carbon in the Taylor contraction to refugia followed by re-expansion are Valleyisofearlierlacustrineorigin,andisnowamajor likelytohavebeenamajorinfluenceonevolutioninthe energysourcefortheterrestrialinvertebratecommunity Antarctic marine fauna (Clarke and Crame 1989, 1992, (Burkins et al. 2000). The soil stoichiometry gradient is 1997,Fraseretal.2012),andincreasinglyitappearsalso alsoreflectedinthelakeswithinthebasins(Priscu1995). on the Antarctic and sub-Antarctic terrestrial biota, Older soil surfaces also exist in this area (mostly at which also shows substantial endemism (Greve et al. higher elevations) and have been exposed for up to 2005,StevensandHogg2006,Stevensetal.2006,Pugh several million years, having an order of magnitude and Convey 2008, McGaughran et al. 2010, Grobler et S W higherN:Pratioagain(Barrettetal.2007).Barrettetal. al.2011, Mortimer et al.2011a,b). E (2007)hypothesizedthat,ingeneral,theinputelemental VI composition (i.e., C, N, P) not only constrains produc- SPATIALVARIATION INTERRESTRIALSYSTEMS E R tivity within the ecosystem, but is modified by the Terrestrial Antarctica is characterized by limited and biological processes occurring there by changing the insularexposureofice-freegroundaswellaspatchyand stoichiometry ‘‘downstream.’’ As an example, the N:P discontinuous substrates. Thus, the potential to link ratiochangesfrom21:1forglacialsnow/ice,to15:1for biological features with ‘‘continuous’’ physical environ- cryoconite water, to 12:1 for stream water, to 25:1 for mental gradients such as temperature or water avail- lake water. Over longer time periods, as lake levels ability is limited. The sub- and maritime Antarctic fluctuatewithclimatechange,soilandlakenutrientscan islands experience less extreme daily and seasonal be redistributed between the terrestrial and aquatic temperature variations than do locations within the systems, providing legacy subsidies for both systems mainbodyofthecontinent,butshowmorevariationin (Moorhead et al. 1999, Lyons et al. 2000). The other variables such as precipitation (Convey 2013). In drawdownshaveconcentratednutrientsinthehypolim- consequence, investigations of biodiversity at a wider niaofolderlakes(Priscu1995).Thiscryoconcentration range of spatial scales are possible (e.g., Terauds et al. of nutrients in the deeper, saline portions also creates 2011).A spatial approach,basedon latitude,elevation, important nutrient gradients within the lakes them- and distance from the coast (see Table 1), was first selves. The diffusion of nitrate, ammonium, and proposed as a means to investigate variation in the phosphate across the chemoclines drives production in Antarctic terrestrial fauna by Janetschek (1970). In the the deep chlorophyll maxima of the lakes in the lastdecade interest in this approach hasincreased both McMurdo DryValleys. for fundamental theoretical reasons (e.g., le Roux and In the Windmill Islands region the best-developed McGeoch 2008a) and to address complex questions vegetation communities are found in sites where related to climate change impacts in the polar regions nutrientsweredepositedinpenguincoloniesabandoned (Chownet al.2012b). thousandsofyearsago(Goodwin1993).Thevegetation Antarctic terrestrial organisms typically have patchy changes from extensive moss beds in the lower-lying localdistributions(UsherandBooth1984,1986,Caruso regions to lichen-dominated communities toward the and Bargagli 2007, Caruso et al. 2007, 2009, 2012a), as summits (Melick and Seppelt 1997). Opposing resource do the microbiota (Caruso et al. 2011, Chong et al. gradients accompanythe vegetationchange,withwater 2011). Various biotic factors, such as productivity and decreasing and plant nitrogen content increasing with chlorophyll a content (Sinclair and Sjursen 2001), elevation (Wasley et al.2006a, 2012). macroscopic vegetation (Sinclair 2001), and food May2014 ANTARCTICBIODIVERSITYSPATIALSTRUCTURE 209 R E V IE W S FIG.2. TerrestrialandmarineenvironmentsintheAntarcticregion.(A)AmummifiedsealinMcKelveyValley,McMurdoDry Valleys.(B)OnyxRiver,WrightValley,McMurdoDryValleys.(C)ThecushionplantAzorellaselago(Apiaceae)alongsideahigh- altitudelakeonsub-AntarcticMarionIsland.(D)WanderingAlbatross(Diomedeaexulans)nests,lowlandtussockgrassland,and mirevegetationonsub-AntarcticPrinceEdwardIsland.(E)Yeticrabs(Kiwasp.)ataScotiaarchydrothermalventsite.2500min depth. (F) The impact of ice scour on the rich benthic diversity near Rothera Research Station, Adelaide Island, Antarctic Peninsula.Photocredits:A–D,S.L.Chown;E,CourtesyofNERCChEsSoConsortium;F,K.Brown,BritishAntarcticSurvey. preference(Kennedy1999)influencedistributionsofthe al. 1998, Bargagli et al. 1999, Porazinska et al. 2002b, fauna (see also Hogg et 2006, Caruso et al. 2012b). Sinclair 2002, Poage et al. 2008) (see Table 2). Similar Notwithstanding these factors, Antarctic biodiversity patterns are manifest in the sub-Antarctic, though here variation and ecosystem functioning are predominantly interspecific (i.e., biotic) interactions may become more driven by abiotic factors (Convey 1996), including soil significant(e.g.,Vogel1985,Frenot1987,Chown1994, structure (Bo¨lter et al. 1997, Wall and Virginia 1998), BergstromandSelkirk1997,Smithetal.2001,Davieset chemistry (Porazinska et al. 2002a), and, in particular, al. 2011, Lebouvier et al. 2011, Terauds et al. 2011). wateravailability(Kennedy1993,Block1996,Powerset Thus, any discussion of spatial variation in diversity 210 PETERCONVEYETAL. EcologicalMonographs Vol.84,No.2 TABLE 1. Dependence matrix of terrestrial and marine mustcommencewithatreatmentofthedirecteffectsof physical factors vs. latitude, altitude/depth, and distance abiotic environmental variation. We do so here, then from the coast in Antarctica (from Peterson and Howard- Williams2001). discuss direct gradients, such as resource gradients (including soils and productivity), followed by indirect Terrestrialand Altitude/ Distance gradients, most commonly investigated with latitude or marinefactors Latitude depth fromcoast altitude as the independent factor. Although this Terrestrialphysicalfactor approach follows, in broad outline, Austin’s (1980) Totalsolarradiation 3 1 0 classification, the strong influence of solar radiation on Temperature 3 2 1 water availability means that we accord it initial Humidity 2 1 2 Wind 1 1 1 attention. Usually,or atleast in the caseof plants, it is Precipitation(amount) 2 1 1 consideredaresourcegradient.Throughoutweconsider Precipitation(type) 2 1 1 both strictly terrestrial and limnetic environments, Ablation(amount) 2 2 2 Ablation(type) 2 2 2 which may include a variety of streams, lakes, and Terrestrialgeochemistry 1 1 1 smaller water bodies such as cryoconite holes (see Seasonality 3 1 0 Bergstromet al.2006). Marinephysicalfactor Polynya 2 0 1 Solarradiation Current 2 2 2 Salinity 1 2 2 The two physical attributes that have the greatest Seaicecover 2 0 1 influence on Antarctic ecosystems are the presence of Iceshelfcover 3 0 0 liquid-phase water on land and of ice cover in marine Notes: Key: 0, no dependence; 1, weak dependence; 2, and freshwater systems. These are both influenced by medium dependence; 3, strong dependence. The original solar radiation and hence show a latitudinal response. measurements for latitude were in degrees, for altitude/depth were per 100 m, and for distance from the coast were in Other important factors include extreme seasonality kilometers. combined with steep latitudinal gradients of light–dark periodicity (Fig. 3) and exposure to high incident radiationandtodamagingwavelengthsoftheradiation S spectrum. W E VI E TABLE2. EnvironmentalstressorsforandresponsepatternsbyAntarcticterrestrialbiota. R Factor Protectionprovidedby Negativeeffectsofprotection Radiation(PARandUV) screeningpigments loweredlightresponseofphotosynthesis; costofproduction shadedhabitat,refraction/reflectancedueto loweredPARandtemperature plantformand/oroverlyingsnow Temperature(includingfreeze– insulationbysnow loweredPAR;increasedrespirationcost; thawconsequences) potentiallyshortenedgrowthseason northerngrowthaspect increaseddesiccation wetground(extracellularfreezingofwater surfaceicelayercanleadtoincreased protectsactivemossesfromsubzero soilinvertebratemortality temperatures(continent)) buttonform,lowstature potentialtoreducePARandthus photosynthesis darkpigmentation(enhancessolarheating) accelerateddesiccation Wind groundbetweenrocks,soilcrusts,endo-and reducedlightavailability chasmo-lithichabitats(stronglinkwith wateravailability),leelocation Desiccation(notestronglinkage south-facingaspect(continent) lowertemperatures withwind,radiationand endo-andchasmo-lithichabitat,soilcrusts loweredPAR;physicallimittoorganism temperature) size sublithichabitat loweredPAR permanentsnowbank/drift loweredPARandtemperature;increased respirationcost;atcontinentalsites insulationpreventswarminginearly summer flowingmeltwater substrateinstability;abrasion;burial Notes:Protectionagainstthemainenvironmentalstressors(showninthemiddlecolumn)forAntarcticterrestrialbiotaalso resultsinnegativeconsequencesforgrowthanddevelopmentasshownintherightcolumn. May2014 ANTARCTICBIODIVERSITYSPATIALSTRUCTURE 211 FIG.3. Solarradiationatthetopoftheatmosphereasafunctionoflatitudeandtimeofyearforthezone608–908S(adapted fromVincent1988).Valuesfollowingthelatitudevaluesaretotalannualradiation(MJ/yr). The existence of gradients in exposure to solar nominal full sunlight, 2000 lmol photons(cid:2)m(cid:3)2(cid:2)s(cid:3)1 radiation is implicit in many studies of terrestrial (Pannewitz etal. 2003a,Schroeter et al.2011). vegetation distribution, ecophysiology, and photosyn- Littleevidenceexiststhatnaturallyoccurringlevelsof thetic biochemistry. These include photosynthetically PAR or UV radiation are major limiting factors for active radiation (PAR), ultraviolet radiation (UV-A) autotrophs in Antarctica (Kappen et al. 1998a, Clarke and seasonally anthropogenically enhanced levels of andRobinson2008,Schroeteretal.2012).UVradiation R biologically harmful UV-B radiation. On an annual generally is lower in Antarctica than in tropical and E V basis there is a latitudinal decline in total radiation temperate areas; the mean UV index for January is ;3 IE incident on the Earth’s atmosphere, from a monthly atRossIslandand;4atAnversIsland,comparedto7– W S average of about 22 MJ/m2 in the northern Antarctic 12inNewZealand.IncreasedUV-Bradiationasaresult Peninsula to around 9 MJ/m2 at the South Pole. PAR of the ozone hole occurs between September and early comprisesonlypartoftheradiationspectrum(400–700 December,sohaslittleeffectwithinthemainAntarctic nm,typically43%ofthetotalradiationenergy),andthe continent, as vegetation activity occurs mainly within fraction that reaches ecosystems at Earth’s surface is theperiodDecembertoearlyFebruary.Enhancedlevels influencedbylarge-scalepatternsincloudinessaswellas do occur on the Antarctic Peninsula, but there is reflectance through albedo, with the result that the evidence that the bryophytes at least can rapidly radiation gradient experienced at ground level can be acclimate to changes in UV radiation (Newsham 2003, reversed. Green et al. 2005, Newsham and Robinson 2009, Insummertheintegrateddailyradiationisashigh,or Turnbull et al. 2009). Exposure does result in damage even higher, in continental Antarctica than in many to biochemical constituents of cells (e.g., DNA and temperateareas.Geography,however,canalsostrongly photosystems[Greenetal.2000,Georgeetal.2002,Lud influence local weather conditions, consequently affect- etal.2002,TurnbullandRobinson2009,Robinsonand ingtheproportionofradiationreachingecosystems.For Waterman 2013]), but these effects are transient. The example, as a result of different cloud cover and path dynamicbehavioroftheprotectivepigments,whichare length,islandsofftheAntarcticPeninsulareceiveabout typicallylostiftheplantsareshaded,suggeststhatthere 10% of radiation incident on the Earth’s atmosphere, is a metabolic cost to their presence. However, their while on the polar plateau this can reach 90%. The productioncostsaresmall,estimatedat2%ofdailynet almost continuous cloud in the northwestern Antarctic photosynthesis (Snell et al. 2009). While protective Peninsulameansthatradiationlevelsatgroundlevelare ‘‘sunscreen’’ pigments are found across many mosses, low and even, and temperatures relatively stable lichens, and microbes, and there is clearly greater compared to elsewhere in Antarctica. As a result the production in more exposed locations (Post 1990, Post active times of lichens are mainly confined to the two and Vesk 1992, Lovelock and Robinson 2002, Clarke summermonthsatBotanyBay(778S)comparedtothe andRobinson2008),studiesquantifyingtheenergeticor wholeyearatLivingstonIsland(628S)(Schroeteretal. resource investment in these strategies across exposure 2010). However, photosynthesizing organisms in the gradients are still required. Dynamic avoidance strate- former region can experience incident PAR exceeding gies such as vertical migrations to avoid high UV have 212 PETERCONVEYETAL. EcologicalMonographs Vol.84,No.2 also been demonstrated in Antarctic microbial mats hours(Pecketal.2006).Similarly,freshwaterpoolscan (Nadeau et al. 1999). Such investment is arguably vary by .508C annually and by 208C daily. Small, sufficient to generate a significant selection pressure, chemically stratified ponds in ice-free areas can have suggesting that spatial variation in exposure will be summer temperature differences between surface and mirrored in biological responses that affect organism bottom waters (;40–70 cm depth) of .108C (Healy et energybudgetsandlifehistorytrade-offs(Convey2011). al. 2006). At broader scales, pronounced variation is The interaction between insolation and desiccation is clear, as might be expected. For example, Howard- important. The mosses and lichens that dominate the Williamsetal.(2010)foundasignificantlineardeclinein limited continental Antarctic flora and survive in summer temperatures across Victoria Land. As an extremely xeric environments, such as rock surfaces, integrating measure of ‘‘probability of melt,’’ they also can avoid light stress by drying out rapidly, becoming calculated degree-days above freezing, finding a wide inactiveandincreasinglytolerantofradiationincluding variation at latitudes above 808 S, with values rapidly UV-B (Schlensog and Schroeter 2000, Kappen and droppingtowardzero athigherlatitudes. Valladares 2007, Turnbull et al. 2009). Interspecific Variabilityinabioticconditionsisrecognizedasakey differencesintheresponseofmossestodesiccationand feature affecting organismal responses (e.g., Tufto UVradiation (Smith 1999, Robinsonet al.2003, Dunn 2000), and possibly also biodiversity variation (e.g., and Robinson 2006, Turnbull and Robinson 2009, Archibald et al. 2010). However, the effects of Wasley et al. 2012), determine both their spatial temperature variation and its predictability per se distribution on small scales, and their likely different (ratherthanmeanannualtemperatures),havenotbeen future response to changes in the pattern and/or as widely studied in Antarctic organisms as elsewhere. magnitude of stresses experienced. On exposed surfaces Most of the work in this area is concerned with the that dry quickly, lichen development is limited to physiological implications of thermal variation at crevices,interstitialspaces,orsouth-facingsurfacesthat several spatial scales both on the continent and on the offer shade but also where a reliable water supply is sub-Antarcticislands(e.g.,Daveyetal.1992,Deereand present. One result of this is the development of Chown 2006, Rinehart et al. 2006, Teets et al. 2011). endolithic and chasmolithic communities, which are Nonetheless, adaptation to large diurnal temperature completelyconfinedtothenorthfacesofsuitablerocks variation means that the terrestrial biota may not S W (Friedmann 1982). Mosses and lichens that grow in respond clearly to small local temperature gradients, E mesicareaswheremeltwateristhemainsupplymayface especiallyifconfoundedbyotherenvironmentalfactors VI veryhighlightlevelswhenactive.However,thespecific (Smith 2003). High natural variability in temperature E R conditionswhenbiotaareactiveatagivenlocationare may also underlie the comparatively small effects of important. Temperatures, for example, are practically artificial warming often reported in climate manipula- identicalforactivelichensinthemaritimeAntarcticand tion experiments (Sinclair 2002, Bokhorst et al. 2007b, continentaldryvalleys,buttheirperiodsofactivityvary 2008) compared with changes in response to moisture drastically (Schroeter et al. 2010). Interstitial, sublithic, availability and freeze–thaw cycles (Wasley et al. and soil crust communities become relatively more 2006a,b, 2012, Yergeau and Kowalchuk 2008, Lenne´ important in drier areas, with translucent rock and soil et al. 2010). particles allowing light transmission while protecting Wateravailability from desiccation (Hughes and Lawley 2003, Cowan et al.2010). At a large geographic scale, a strong desiccation As a result of these activity patterns, there is no (water availability) gradient exists from the sub- and relationship at continental scale between light levels or maritime Antarctic into the continent (Walton 1984), interceptedquantityoflightandgrowthrate.Lichensin thelatterincludingsomeofthedriestsitesonEarth.The themaritimeAntarctichavesomeofthehighestgrowth availability of liquid water to organisms broadly rates in the world for this group, while those in the depends on the balance between annual precipitation continent can be one (Cape Hallett) or two orders of andlossesbyevaporation,sublimation,andfreezing.In magnitude (McMurdo Dry Valleys) slower (Sancho et locations such as the McMurdo Dry Valleys, most or al. 2007, Green et al. 2012). The duration of annual even all precipitation falling as snow may sublime, and metabolic activity, controlled by water availability, thusnotbecomeavailabletobiota.Precipitationoccurs appears to be correlated with richness (Green et al. mainly as rain in the warmer sub-Antarctic zones (and 2011a, Clarkeet al.2012). increasingly in the maritime Antarctic during summer), andsnowonthecontinent.Additional,thoughnotwell Temperature quantified,sourcesaredew(Bu¨deletal.2008)andcloud Antarctica contains among the most and the least ormist(especiallyforridgelinebiotasinmontaneareas). variable thermal environments, with the former being Thisbalancecanbemodifiedbysnowremovalbywind mostly terrestrial and the latter marine. Temperatures scour or addition by redistribution, or by release of on open rock surfaces can vary by .758C through the concentrated precipitation during local melt of ice or year and by at least 108C and even up to 508C within snow. The concentration of precipitation is particularly

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circulation or ''ocean conveyor belt. 205. ANTARCTIC BIODIVERSITY SPATIAL STRUCTURE. R. EVIEWS . Page 7 .. scour or addition by redistribution, or by release of illustrated by the substantial decline in cryptogamic.
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