HENK SIEPEL
STRUCTURE AND FUNCTION OF SOIL MICROARTHROPOD COMMUNITIES
CENTRALE LANDBOUWCATALOGUS
0000 0572 1341
Promotor: Dr. L. Brussaard Hoogleraar inde Bodembiologie
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AlfJOï'të"J
HENK SIEPEL
STRUCTURE AND FUNCTION OF SOIL MICROARTHROPOD COMMUNITIES
Proefschrift terverkrijging van de graadvan doctor inde landbouw- en milieuwetenschappen op gezagvan de rector magnificus, dr. C.M. Karssen, inhet openbaar te verdedigen op dinsdag 20september 1994 desnamiddags tehalf twee indeAula van de Landbouwuniversiteit teWageningen.
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CIP-DATAKONINKLIJKE BIBLIOTHEEK, DENHAAG
Siepel, Henk
Structure and function of soilmicroarthropod communities / Henk Siepel. -[S.I.:s.n.] ThesisWageningen. -With réf.- With summary inDutch. ISBN 90-9007450-3 NUGI 825 Subjectheadings: soilbiology /ecotoxicology /nature management
The investigations described in this thesis were carried out at the Department of Animal Ecology of the DLO- Institute for Forestry and Nature Research (IBN-DLO), The Netherlands
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STELLINGEN behorendebijhetproefschriftvanHenkSiepel: Structure and function of soil microarthropod communities
1.
Foresie isgeenr-geselecteerdkenmerk;het iseenmaniervanvervoer tussengeschiktebiotopendie inoverigensongeschikt terreinliggen. (Dit proefschrift, Binns E.S. 1982, Biol Rev: 57, 571-620)
2.
Hetgebruikvanzichasexueelreproducerende dieren inoecotoxicologischetoetsengeeftgeenrepresentatieve resultatenvoordesoort, laatstaanvoorgroepenvansoorten. (Dit proefschrift)
3.
Hetactuelegebruikvanvoedingsmogelijkhedengeeftalleeninzicht in deautoecologievaneensoort invergelijkingmethetpotentieelaan voedingsmogelijkheden. (Dit proefschrift)
4.
Dematevanchitinaseactiviteit indepopulatiesvanmicroarthropodesoorten indiceertdematevanongestoordheidvanhet afbraakproces vanorganischmateriaalopzwakzurebodems. (Dit proefschrift)
5.
Bijlagemobiliteitkunnenmeersoortensamenvoorkomendandater bronnenvanbestaanzijn. (Dit proefschrift, n.a.v. Arthur W. 1987, The niche in competition and evolution, Wiley & Sons, Chichester, UK)
6.
Uitsluiting doorconcurrentie iszeldzaam. (Dit proefschrift, Hardin G. 1960, Science
131:
1292-1297)
7.
Begrazingvannatuurterreinen dientvoornamelijk omtelatenziendat hetterreinbeheerdwordt;devermeendgunstige effectenopdenatuur zijnnooitbewezen.
8.
Onderzoeknaardosis-effect relaties indeoecotoxicologieheeftgeen betekenisalshetnietgekoppeldwordtmetautoecologische en systeemoecologischekennis.
9.
Oecologenonderschattendenatuurwaardenvanhet stedelijken landelijkgebied.
10.
Indicatorenvoorbiodiversiteit zijneencontradictio interminis.
11.
Vergrotingvandeonvoorspelbaarheid vanhetmilieubevordertbehalve demobiliteitvandeénegroepvooralookhetuitstervenvanandere groepen.
12.
Onderzoeknaarnatuurontwikkelingwordtzeldenvanafdebodem opgebouwd.
Wageningen,20september 1994
Voor mijn
ouders
Contents
Contents
Abstract
7
Voorwoord
9
1. General introduction
11
2. Life-history tacticsofsoilmicroarthropods (Biology
and Fertility
of Soils,
in
15
press)
3. Applications ofmicroarthropod life-history tactics innature managementandecotoxicology (Biology
and Fertility
of soils,
in
51 press)
4. Feeding guildsoforibatid mitesbasedontheir carbohydrase activities (with ElzedeRuiter-Dijkman) (Soil
Biology
and Biochemistry
25 (1993),
69
1491-1497)
5. Mitesofdifferent feeding guilds affect decompositionoforganic matter (withFransMaaskamp) (Soil
Biology
and Biochemistry,
in
81
press)
6. Coexistence insoilmicroarthropods
95
(subm.) 7. Competitive exclusionisnotageneral ecological principle
117
(subm.) 8. General discussion Summary Samenvatting Curriculum vitae
121 131 133 136
Abstract
Abstract Microarthropod species were classified according to life-history tactics and feeding guilds. Twelve life-history tactics were distinguished, based on well-defined life-history traits like the type of reproduction (thelytoky, arrhenotoky, sexual reproduction), oviposition (semelparity, iteroparity), development, synchronization (of the life cycle with environmental conditions), and dispersal (phoresy, anemochory). Examples are given of the distribution of these tactics among microarthropod species occurring in several biotopes, during decomposition of organic matter, and under several types of disturbance and pollution. Thelytokously reproducing species appeared tohavehigher numbers at sites with apersistent pollution. Feeding guilds were defined on the basis of gut carbohydrase (cellulase, chitinase, and trehalase) activities. Five species-rich guilds were recognized. In the presence of species able to digest the fungal cell-wall next to cell-contents (called fungivorous and herbo-fungivorous grazers), a higher C0 2 evolution during decomposition of pulverized litter was found than in their absence. In the presence of species able to digest cell-contents only (called fungivorous browsers and opportunistic herbofungivores), in such experiments a lower C0 2 evolutionwas found than intheirabsence. In a simulation model it was tested whether relatively inefficient use of food may be compensated by life-history traits or abilities by which short-term environmental extremes canbe overcome.Having the possibility to bridge a relatively long period of food shortage, or withstanding extremes in drought, frost or heat, or having a higher mobility, was indeed shown to result in a better survival of species that use their food relatively inefficiently. This results in the effect that species with different life-history tactics cancoexist on the same food sources. Contradictory to the principle of competitive exclusion, species with identical niches may coexist when the mobility of these species isnot unlimited.
Key-words: Acari, Collembola, Life-history, Feeding guild, Coexistence, Decomposition,Naturemanagement, Ecotoxicology.
Voorwoord
Voorwoord Hoewel ik al heel jong belangstelling had voor biologie en van alles liep te bekijken als mijn vader en opa aan het vissen waren, had ik nooit gedacht dat ik me met mijten en springstaarten bezig zou houden. Voor ik in oktober 1984 op het toenmalige Rijksinstituut voor Natuurbeheer kwam, wist ik net dat mijtenacht potenhebben (met uitzondering van de larfjes) en springstaartjes zes. In het onderzoek op graslanden naar indicatorsoorten en evaluatiemethoden, waar ik me op moest gaan richten, waren dit ook al geen voor de hand liggende groepen. Bij mijn aanstelling werd ik echter geconfronteerd met enige nog uit te werken datasets van deze microarthropoden. Deze datasets waren toch verzameld inhet kader van het monitoringonderzoek op graslanden omdat ze volgens Chris van de Bund heel precies de toestand van het grasland aangaven. Chris bracht dit zo overtuigend en stimulerend dat ik niet alleen de datasets uitwerkte eneenbemonstering vanmicroarthropoden inhet graslandenonderzoek opnam, maar uiteindelijk zelf ook naar de mijten en springstaarten ging kijken. In de loop van het onderzoek bleken inderdaad heel goede indicatoren te vinden onder de microarthropoden, maar dat gold ookvoor de grotere insekten en spinnen, dievoor eenpraktijktoepassing devoorkeur kregen. Maar de teerling was geworpen en ik was geïntrigeerd geraakt door de wereld onder het bodemoppervlak. De talrijkheid van de beestjes, hun soortenrijkdom, hun veelheid aan vormen, ook in ecologisch opzicht, maakten dat ikme nog gauw door Chris liet inwijden indeherkenning van microarthropoden voor Chris in 1987 de dienst verliet en ik formeel op zijn plaats kwam. Overigens bleef Chris ook daarna bereid de determinatievan soorten die ikvoorhet eerst zag teverifiëren en geen idee was te wild om er eens serieus over door te bomen. Ook de anderen die als klankbord hebben gediend binnen en buiten het maandelijks entomologenoverleg op het instituut wil ikbedanken voor hun opbouwend commentaar, hun suggesties en het critisch doorlezen van de concepten, met name Frits Bink, Walter van Wingerden, Mari Verstegen, Rob Hengeveld, en bij het modelwerk Hilko van der Voet en Rienk-Jan Bijlsma. Enkele belangrijke hoofdstukkenvan ditproefschrift waren ermisschien nooit gekomen zonder de hulp en inzet van Elze de Ruiter en Frans Maaskamp. Ruut Wegman wil ik graag bedanken voor het tekenen van de figuren. Prof. dr. Lijbert Brussaard wil ikbedanken dat hij als promotor op wil treden en voor zijn inspirerend enthousiasme waarmee hij deconcepten te lijf ging en van critisch commentaar voorzag, zodat ze ook voor niet direct betrokkenen leesbaar werden. Furthermore I like to thank Prof. dr. M.B. Usher, who strengthened me in my view that identifications in soil microarthropods should go to the species level as he stated in the
Voorwoord
meeting of soil biologists of the Nordic Ecological Society in Denmark. I am glad that he instantly affirmatory responded to my invitation for the graduation committee. Rest mij nog een woord van dank aan het thuisfront, Annet, Anika en Hanneke, geen oase van rust, maar wel een heel andere wereld om in te vertoeven na dagenvanmijten, analyses en computers.Het overwerk inde avonduren zal wel niet helemaal afgelopen zijn, maar wordt hoop ik wel minder nu het proefschrift afgerond is. Pa en ma bedankt voor de niet aflatende steun endevele oppasdagen.
Arnhem, 11juli 1994
w
10
General introduction
The soil fauna isvery rich inspecies;protozoans,nematodes,arthropods and annelids are themajor groups of animals,both innumber ofspecies, andinbiomass.Thisthesiswillfocusonthesoilarthropods,particularly the microarthropods, the group most rich in species. All soil mites (Acari), andprimarywingless insects,predominantly springtails (Collembola)areincludedinthesoilmicroarthropods.Mitesareasubclassofthe spiders, which in The Netherlands contains six orders: Metastigmata (ticks),Oribatida(mossmites),Acaridida,Mesostigmata(predatorymites), Actinedida (spidermites,harvestmites,watermites,etc.)andTarsonemida (with the Actinedida also called Prostigmata). Next to these commonly acceptednamesmanysynonymsareused (VanderHammen1972).Oribatidmites are the most abundant in forest soils, followed by Collembola and Mesostigmata. Actinedida and Tarsonemida are sometimes very abundant in grassland and arable soils.Acaridida formmost ofthe time aminority in numbersinthesoilfauna.Withrespecttospeciesrichness inthesoilthe Actinedida are the first inrank,butmany species live inthevegetation andarefoundinsoilsamplesonlyaccidentally.Oribatidaarenextinrank inspeciesrichness,followedbyMesostigmata,Collembola,Tarsonemida,and Acaridida. Speciesrichness isthoughttoberelated tosurface:thelargerasite the more species can be present (MacArthur & Wilson 1967). The highest species richness among oribatidsvaries from on average 33oribatid mite speciesper500cm2 (range23-45)inCentralEuropeanforests (Moritz1965) to45oribatid speciesper250cm2 (range37-52)inunmanagedDutchgrasslands (Siepel&VandeBund 1988).Karppinen (1958)foundanaverageof19 oribatid mite species (range 8-26) per 5cm2 inFinnish forests. Species richness for all microarthropods together isevenmuchhigher,up to 108 species per 500 cm2 (Siepel & Van de Bund 1988).Assuming that every specieshasanuniqueniche,accordingtothehypervolumemodel (Hutchinson 11
Microarthropod communities
1957),and that competitive exclusion leads to minimizing niche overlap (Hardin 1960), one must conclude that in the soil a tremendous variation in resources has to exist (Anderson 1975). Theoretically this may be possible,but are theanimal species able todistinguish and select those resources? The alternative solution isthatdifferent speciesmayuse the same resources (food, shelter, etc.) without mutual exclusion. A less efficientuse ofresourcesmightbecompensatedby certainadaptations in the life-history. Theobjective ofthisthesisistoinvestigatewhetheralimitednumber of resources allows the coexistence of ahigher number of species. This question requires operational knowledge about the structure and function of soilmicroarthropod communities. The structure is explained from differences in life-history tactics resulting in different abilities to overcome suddenharshenvironmental conditions.The function is explained fromdifferentfeedingguildsofmicroarthropods.Theseguildsaredefined onthebasis ofactivities ofcarbohydrase gutenzymes.The differenteffectsrepresentativesofthesefeedingguildshaveontherateofdecomposition of organic matter may be explained from differences in digestion capacities. Inchapter 2the life-history traits ofsoilmicroarthropods are dealt with topresent a solid base for the definition of life-history tactics. Severallife-history traitspass inreviewandtheecological significance ofthelytoky, eitherautomictic orapomictic,arrhenotoky, amphitoky,and sexualreproductionareexplained.Aclassificationoftwelve life-history tactics,basedonwell-defined,underivedtraitsispresentedandcompared with theclassificationsofMacArthur andWilson (1967),Grime (1977)and Southwood (1977)andwith the multidimensional classification of Stearns (1976). For every tactic the functional relationwith the most important biotopes isdescribed. Examples ofspecies from each systematic order are givenwhen available. In chapter 3 a key is presented as a tool for the identification of life-history tactics. Examples of the distributions of microarthropod species among life-history tactics arepresented for the soil fauna ofa forest, a grassland, and a saltmarsh. The practical applicability of the defined life-history tactics is subsequently illustrated in nature management and ecotoxicology. The common use of asexually reproducing species inecotoxicology is discussed. Inchapter4forty-nineoribatidmitesandoneacarididmiteareclassified in feeding guildsbased on their gut carbohydrase activities. Three carbohydrases were selected to differentiate between food sources. Cellulaseactivity indicatesdigestionofplantmaterial,includingalgae, trehalase activity indicates digestion of the cell-contents of fungi, 12
General introduction
lichens, andblue-green algae,andchitinase activity indicates thatalso the fungal cell-walls can be digested. Herbivorous grazers, herbivorous browsers, fungivorous grazers, fungivorous browsers, herbofungivorous grazers, opportunistic herbo-fungivores, and omnivores are proposed as feeding guilds, with grazers able to digest most of the ingested food components, andbrowsers able todigest the cell-contents only. Inchapter 5 the decomposition of litterby fungi in thepresence and theabsenceofrepresentativesofthefivemajorfeedingguildsisstudied. The implications of the new classification of feeding guilds for insight into the role of mites in the decomposition of organic matter are discussed. Differences inefficiency offood-processingmayresult inaselective advantage ofthemore efficientmites.Inchapter 6asimulationmodel is presentedtoevaluatepossiblewaysofcompensationforthelessefficient mites by differences in life-history tactics,mobility, or tolerance for short-term extreme conditions (drought, frost,heat, food shortage). Chapter 7dealswithaspecialapplicationofthesimulationmodel:the test of the 'principle of competitive exclusion' (Hardin 1960). In the simulationmodel two species are defined as identical. Ina first set of runsbothspecieshavenorestrictions inmobility (asinaLotka-Volterra model) and in a second set of runs both species are restricted in their mobility (asusually innature). Thepossible co-occurrence of 'identical species' isdiscussed and related to speciationrates. In the general discussion (chapter 8) the approach followed in this thesisandtheresultsobtainedareevaluated.Thetraitsaspeciescanuse tosurvive,eitherlife-historytraits,digestioncapacities,orpossibilities to overcome short-term extreme conditions, determine the potential range ofbiotopes that species can inhabit.Thismaybe abetter starting pointforthecoexistenceofspeciesunderstanding insoilthancomparisons of species communities andmicrohabitat characteristics.
REFERENCES
Anderson, J.M., 1975. The enigma of soil animal species diversity. In: Vanek,J. (ed.)Progress insoilzoology,JunkBV,DenHaag,pp.51-58. Grime,J.P.,1977.Evidence for theexistence ofthreeprimary strategies in plants and its relevance to ecological and evolutionary theory. American Naturalist 111,1169-1194. Hardin,G., I960.The competitive exclusionprinciple. Science 131,12921297. Hutchinson, G.E., 1957.Concluding remarks. Cold Spring Harbour Symposia inquantitive biology 22,415-427. 13
Microarthropod communities
Karppinen,E.,1958.ÜberdieOribatiden (Acar.)derfinnischenWaldböden. Annales Zoologici Societas ZoologiesBotaniesFennicae 'Vanamo' 19(1), 1-43. MacArthur, R.H. & E.O. Wilson, 1967.The theory of island biogeography. PrincetonUniversity Press,Princeton,N.J. Moritz,M., 1965.UntersuchungenüberdenEinflußvon Kahlschlagmaßnahmen aufdieZusammensetzungvonHornmilbengemeinschaften(Acari:Oribatei) norddeutscher Laub- undKiefernmischwälder. Pedobiologia 5, 65-101. Siepel,H.&VandeBund,C F . , 1988.Theinfluenceofmanagementpractices onthemicroarthropodcommunityofgrassland.Pedobiologia31,339-354. Southwood, T.R.E., 1977.Habitat, the templet for ecological strategies? Journal ofAnimal Ecology 46,337-365. Stearns, S.C., 1976. Life-history tactics: a review of the ideas. Quarterly Review Biology 51,3-47. VanderHammen,L.,1972.Spinachtigen,Arachnoidea. IV.Mijten,Acarida. Algemene inleiding in de acarologie. Koninklijke Nederlandse NatuurhistorischeVereniging,WetenschappelijkeMededeling91.Hoogwoud,The Netherlands.
14
Life-history tactics of soil microarthropods
SUMMARY. Inorder toprovide asoundbasis fordefining life-historytactics,severallife-historytraitsarereviewed.Theecological implication isexplained of thelytoky (automictic or apomictic), arrhenotoky, amphitoky, sexual reproduction, and semelparity, iteroparity, seasonal iteroparity andtherelationship betweensemelparity andjuveniledevelopment. Several forms ofsynchronization oflife-cycleswith environmentalconditions are classified, ranging from the ability to overcome harsh periods duringtheseasontoobligatediapausedormancy.Ecologically,thisinvolvesadaptationstoenvironmentalconditionsrangingfromirregularlyoccurring andunpredictable to regularly occurring andpredictable. Dispersal traitsaregroupedindirectionalmigration(phoresy)andundirectionalmigration (anemochory).A distinction ismadebetween facultative andobligatephoresyandbetweencarrier-specificandcarrier-unspecificphoresy. Amulti-dimensional systemof tactics ispresented,based onwell-defined underivedtraits.Thissystemiscomparedtoone-andtwo-dimensionalschemes ofMacArthur &Wilson (1967), Grime (1977), and Southwood (1977)and tothemulti-dimensionalsystemofStearns (1976).Foreverytactic,relationshipswiththemainbiotopeswhere itoccursamongspecies,aregiven. Examples ofspecies showingcertain tactics aregivenper taxonomieorder of microarthropods. The generality of the traits for various groups of organisms and of the classification of life-history tactics developed is discussed.
INTRODUCTION
Many papers have been published on life-history strategies of species duringthepastfewdecades.Classificationsoflife-historystrategiesare useful in analysing the effect ofnature management and of the pollution ofbiotopes (Grimeet al. 1988),becausespeciesandspeciesgroupscanbe compared.However,therearestillveryfewpapersonthelife-historystrategieswithin a group of species sharing commontraits. 15
Microarthropod communities
Thetheoryofr-andK-selectionofMacArthurandWilson (1967)ismainlybasedondifferencesamongspeciesinreproductivetraits,inparticular thenumberofeggslaidandthejuvenilesurvival inrelationtothestabilityof theenvironment. Intheclassificationoflifehistories,species wereplacedalongagradientrunningfromr-selectedtoK-selectedspecies. However,many species may be found in the middle of the gradient. Grime (1977,1979)foundthatplantshavecertain'K-selected'traitsunderharsh environments,differentfromtheenvironments typicalofK-selection sensu MacArthurandWilson (1967).Consequently,Grimeaddedstressselection(or adversity selection as itwasnamed laterbyGreenslade (1983)), defining theplantswithK-selected traitsinaharshnon-KenvironmentasS-selected.ThisresultedinatriangleofR-,C-,andS-selection,whereR-selectioncorrespondswithMacArthur&Wilson'sr-selectionandC-selectionwith their K-selection. Southwood (1977) drew attention to the role of the habitat as the templet for ecological strategies. In Stearns' (1976) classification of life-history tactics the templetbecomes multidimensional. Inthispaperaclassificationoflife-history tactics isdevelopedfor mites,whileCollembolaareusedtoevaluatethesuitabilityoftheclassification for a taxonomically unrelated group. Themain components of the lifehistoryofmicroarthropodsare:1.reproduction (sexualreproduction, thelytokous parthenogenetic reproduction, arrhenotokous parthenogenetic reproduction, semelparityand iteroparity),2.development (developmental stagesandfactorsthatcontroldevelopment),3.synchronization(diapause, aestivation,andquiescence),andA.dispersal (mobility,phoresyandanemochory). Patterns inthesetraitsaredescribedasasetthatconstitutes a tactic sensu Stearns (1976) "a set of coadapted traits designed, by naturalselection,tosolveparticularecologicalproblems.Acomplexadaptation." Special attention ispaid totherelationships betweenlife-history traits and it isdiscussed whether these are also relevant to other organisms.Theclassificationoflife-history tacticsdefinedherewillbe compared to those of MacArthur & Wilson (1967), Grime (1977, 1979), Southwood (1977, 1988)and Stearns (1976).
REPRODUCTION
Waysofreproductionwillbeemphasizedaslife-historytraits,ratherthan the number of eggs produced or the initial rate of population increase whichplay acentral role inthe theoryof r- andK- selection (MacArthur &Wilson 1967).Although eggproduction isan important trait,thevariationamongmicroarthropods inotherreproductiontraitsisquitehigh,and, 16
Life-history tactics
thereby, suitable for classifying tactics. Sexual and asexual reproduction Reproduction inmicroarthropods iseithersexualorparthenogenetic.Parthenogenesiscanhaveseveralecologicallyandgeneticallydifferentforms: 1)arrhenotoky:unfertilizedeggshatchintomales (haploid)andfertilized eggsbecome females; 2)thelytoky: asexual reproduction offemales giving female offspring, resulting in clones; 3) amphitoky: a rather peculiar trait combining both thelytoky and arrhenotoky inone species. Theadvantageofsexualreproductionoverasexualreproductionisinthe exchangeofalleleswhichresultsincombinationsofgeneticmaterial that maybemorefavourableforsurvival.Inavariablebiotope,geneticvariationprovidesachancethatatleastsomeoftheoffspringarewelladapted totheenvironmentalcircumstances ataparticular time.Diploidyofsexuallyreproducinganimalscanalsomaskrecessivemutations (orchromosomal alterations), resulting inagreater geneticvariability,whichmaybeof possible futurebenefit. Inarrhenotoky suchamasking isimpossiblebecause inthehaploidmale anymutationunfavourable ataparticular timefindsexpressionandconsequentlydisappears (Helle 1965). So,arrhenotoky resultsinfastselection and fixation of favourable alleles.Another advantage of arrhenotoky may beincolonization:oneunfertilizedfemaleinanewplacecanproducemale offspring,which might, depending on the adult female's survival and the male development rate,mate andproduce female offspring. Arrhenotoky occursamong theMesostigmatain,forexample, Macrocheles muscadomesticae (Pereira&deCastro1947),amongtheTarsonemidainTarsocheylidae,Heterocheylidae,Dolichocybidae,Trochometridiidae,Pyemotidae, Acarophenacidae,somePygmephoridae,someMicrodispidae,fewScutacaridae, most Tarsonemidae and Podapolipidae (Lindquist 1986), in Actinedida in Tetranychinae,Eryophioidea, (Jeppsonet al. 1975), Cheyletus trouessarti (Hughes1976)andamongtheAcarididain Histiostoma feronarium (Scheucher 1959). InCollembola andOribatida arrhenotoky doesnot occur. Apomicticthelytoky,i.e.thelytokywithoutchromosomereduction,fusion ofnuclei,oranysimilarphenomenon,mayhaveanumericaladvantageabove sexual reproduction or arrhenotoky because a female gives rise to only females, instead of also producing unproductive males (Williams 1975, Greenwood&Adams1987). Inaconstantbiotopeithastheadditionaladvantageofminimalgeneticloss,withdaughters inheritingtheapparentlyfavourable genome of their mother, and deviations from it (mutations) are rare. However, in a temporally variable biotope, thismay be adisadvantage,likesexualreproductioninaconstantbiotope,becauseofthechance ofchangesoftheadaptedgenome.Intheothertype,automictic thelytoky, 17
Microarthropodcommunities reductionofchromosomenumberstakesplaceandthehaploidnucleimayfuse intodiploidnuclei (OliverJr et al. 1973), resulting insomegenetic variation among the progeny, though usually less than in sexual reproduction (Wrenschet al. 1994). Dataontheoccurrenceofeachofthe typesof thelytoky, are sparse.Taberly (1987b)gives evidence forthe occurrenceofthelytokyoftheautomictictypeinPlatynothrus pelcifer and Trhypochthonius tectorum andmaybethistypeofthelytokyiscommonamong these primitive oribatid mites. Recently, Palmer and Norton (1990) publishedexperimentalproofofthelytokyinfifteenspeciesofprimitive Oribatida (Desmonomata),butdidnotmentionthetype.Therearenodata onthetypeofthelytokyintheMesostigmata(Walter&Oliver1989). Thelytoky occurs, for example, among the Oribatida in Platynothrus peltifer, Trhypochthonius tectorum (Taberly 1987a), Tectocepheus velatus (Grandjean 1941), Oppiella nova (Woodring & Cook 1962, sub Oppia neerlandica) and Microppia minus (Luxton 1981); among theCollembolain Isotoma notabilis, Isotomiella minor and some Tullbergiinae (Petersen 1980); among the Mesostigmata in Uropoda minima (Athias-Binche 1981), Rhodacarus denticulatus and Protogamasellus mica (Walter&Kaplan 1990); amongtheActinedida in Bryobia praetiosa, Brevipalpus obovatus (Jeppson et al. 1975), Tetranycopis horridus (Helle&Bolland1967)and Cheyletus eruditus (Hughes 1976); and among the Tarsonemida in the amphitokous specieslistedbelow,andin Tarsonemus virgineus (Lindquist1986). Amphitokyseemsasuitabletraitforthecolonizationofratherconstant biotopes:arrhenotokyforafastselectionofthegenomeadaptedtothenew environmentandthelytokytoquicklypopulatethisenvironment.Inacomparableway the sexual reproduction of the overwintering generationof speciesthatreproducebythelytoky inthesummercanbeunderstood.The best adapted genome of thenext summer season is thereby selected and throughout the favourable season the species exploits thebiotopemost efficientlybyreproducing thelytokously,as inholocyclicaphids (Heie 1980)and cladocerans (Ruvinsky et al. 1978). InRotifera, thelytokous generationsduringsummerarealternatedbyanarrhenotokousgenerationin autumn(Allan1976).Itisnotknownwhetheraseasonalpatternalsooccurs inmites.Dataonthisalternationofreproductiontraitsarelacking,but the phenomenon may be expected in some tarsonemid or eryophiidmites, becauseinthesetaxaboththelytokyandarrhenotokyoccur. Amphitokyhas,forexample,beenobservedin Tarsonemus (= Phytonemus) pallidus by Karl (1965a), in Tarsonemus (= Iponemus) confusus by Karl (1965b)andin Tarsonemus fusarii bySchaarschmidt(1959);thefirsteggs laidby the femalewere reported to give rise tomales, later onesto females. Thesuitabilityofthevariousreproductiontraitsinspaceandtimeis summarizedinFigure2.1. 18
Life-history tactics
Persistence ofpopulation maintenance
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