Monte L. Bean Life Science Museum, Brigham Young University

POLLINATORS OF ASTRAGALUS MONOENSIS BARNEBY (FABACEAE): NEW HOST RECORDS; POTENTIAL IMPACT OF SHEEP GRAZING Author(s): Evan A. Sugden Source: The Great Basin Naturalist, Vol. 45, No. 2 (30 April 1985), pp. 299-312 Published by: Monte L. Bean Life Science Museum, Brigham Young University Stable URL: http://www.jstor.org/stable/41712134 Accessed: 19-08-2014 00:30 UTC

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This content downloaded from 128.120.194.194 on Tue, 19 Aug 2014 00:30:22 UTC All use subject to JSTOR Terms and Conditions POLLINATORS OF ASTRAGALUS MONOENSIS BARNEBY (FABACEAE): NEW HOST RECORDS; POTENTIAL IMPACT OF SHEEP GRAZING

EvanA. Sugden1

Abstract.-Important bee speciesinhabiting the study area are listed, including those observed and collected foragingonAstragalus monoensis , a rare . The significance ofeach species as a potentialpollinator is assessed,based on frequencyof occurrencein collecting,observed and publishedhost plant records, and morphology.Three pollinator categories are proposed: observed and/or collected on the plant, probable visitors, and possiblevisitors. New host plant records for these species are listed. Currentsheep grazing practices inthe A. monoensishabitat endanger pollinators infour ways: (1) destruction of potentialnest sites, (2) destruction ofexisting nests and contents, (3)direct trampling ofadult bees, and (4) removal offood resources. Exposure ofthe major bee species to each of these factors is assessed utilizing experimental data andpublished information.

Astragalus monoensis Barneby, "Mono baceae). Also prominentare Huhea vestita Rattleweed"or "Mono Milkvetch,"is a pe- Gray and Chrysothamnusparryi vulcanicus rennial legume endemic to the California (Greene) Hall & Clem. (),the pri- portionof the Great Basin (Barneby 1964, marylocal nectar sourcesfor early and late Munz and Keck 1959). It is listedas endan- parts of the summer,respectively (unpubl. gered by the Federal Government(Ayensu data). Otherassociates are Eriophyllumlana- and DeFilipps 1978) and rare and endan- tumvar. monoense(Rydb.) Jeps. (Asteraceae), geredby the CaliforniaNative Plant Society Phacelia frígida Greene (Hydrophyllaceae), (Smithet al. 1980). It occurs in the region Calyptridiumumbellatum var. caudiciferum east of the Mono Cratersbetween 7500 and (Gray) Jeps. (Portulacaceae), and Mimulus 8000 feet(2286^438 m) elevation(Smith et coccineusCongd. (Scrophulariaceae).Eriogo- al. 1980, K. Teare, pers. comm., unpubl. num umbellatum Torr. (Polygonaceae), a data). The numberof individualplants in widelyinsect-utilized nectar source, occurs at each populationvaries from several thousand severalsites. to lessthan 100. A majority of the known A. monoensis A detaileddescription of A. monoensisap- populationsoccur withthe above plant asso- pears in Munz and Keck (1959). It is a pa- ciates in isolatedsand flats,the soil of which pilionaceousbee flower,as characterizedby is composedof coarse graveland sand of vol- Percival (1965), and produces considerable canic pumice origin.The flatsare typically nectar,as evidencedby the regularityof vis- surrounded by a second-growthforest of itsto itsflowers by bumblebees. Males of the Pinus contortaDoug, ex Loud. (Lodgepole anthidiine("carder" or "mason") bees An- Pine) and Pinus jeffreyiGrev. & Balf. in A. thidiumclypeodentatum and the relatively Murr.(Jeffrey Pine). The forestfloor is essen- large Callanthidium formosum (Mega- tiallysterile with respectto pollen and nec- chilidae) also frequentthe flower. Pollen tar. Hence, the wildflowerpatches of the fromA. monoensisis also collectedby these sand flatsexist as resource"islands" for pol- and otherspecies. It is an obligateoutcrosser, linators.Beetle burrowsin many trees and requiringinsect transferal of pollen between logs provide abundantnest cavities that are flowersof different to set seed (Sugden utilized by solitary bees. Thomomystal- 1984,R. Barneby,pers. comm.). Plant associ- poides, the NorthernPocket Gopher,inhabits ates are relativelyfew, dominatedby the the sand flats,and its many abandonedbur- nectarless Lupinus duranii Eastw. (Fa- rowsare probablyutilized by bumblebees as 'DepartmentofEntomology, University ofCalifornia, Davis,California 95616.Present address: TheAustralian Museum, 6-8College Street, P.O.Box A285,Sydney South2000 N.S.W., Australia.

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nest cavities. Bombus huntii queens have legs) in the region (pers. inquiryof sheep- been observedentering such burrows, appar- herder).Solitary ground-nesting bee species entlynest-searching (pers. obs.). requirespecific substrate conditions to estab- The largest populations of Astragalus lish nests (Linsley 1958). Disturbanceof the monoensis,which occur on federalland, are soil by sheep hooves probablyalters the ac- subjectedannually to the influenceof sheep ceptabilityof manypotential nest sites. Pithy grazing.This usually occurs in midseason, stemsmay be brokenand trampledby sheep when the plants are in flowerand before if near the ground,removing potential cav- most seed is set. In the sand flat region, itiesfor species that utilize stick nests. 12,000sheep in herdsof up to 2,000 individ- (2) Destructionof existingnests and con- uals maybe stationedat springsor watering tents.Ground nesting bees constructdelicate burrowsand chambersin the soil. tanksnear the knownA. monoensispopu- Depending on the the brood chambers be lations (C. Chamberlain, pers. inquiry of species, may eitherclose to the sometimeswithin sheepherders).In the sand flats per se, the surface, threeto fourinches or several groundis oftencompletely denuded of vege- (Thorp 1969), feetbelow Soil disturbance tation where the sheep forage. Over the (Malyshev1936). courseof several the area may destroyall or partof such nests,damage days grazed may brood and and/or dis- extendin a continuousswath, 100 m or more existing provisions, nest-entrance whichfemale in width,to cover an entireflat place landmarks, (pers. obs.). bees utilize in orientationto the exact loca- The loose soil of the habitatis subjectto ex- tion of theirburrows. Many bees construct tremedisturbance by the spadelikeaction of nests in preexisting cavities above the sheep hooves,which may uproot or expose ground,often in hollow sticks. Such nests the crowns of mature plants and destroy maybe shatteredor disorientedby trampling obs.). effectsof seedlings(pers. Typical graz- and exposed to rainfall.Many of the bees ingon thehabitat are shownin Figure1. concerned here are the ofA. monoensisis solitary,nonaggressive Although toxicology species and have no means of nest defense unknown,foliage and flowersof the plantare often K. againstsheep. grazed(C. Chamberlain, Teare, pers. (3) Removalof food resources.Sheep graz- overa i.e. comm.).Sheep merelypassing site, ing greatlyreduces the densityof manyher- not do less than those al- feeding, damage baceous plants.Among these are the nectar lowed to forage (C. Chamberlain, pers. and pollen sourceson whichlocal bee popu- comm.). lations subsist.For many species, these re- This examinesthe in paper hypothesisthat, sourcesmay be importantto the completion additionto direct the disturbance, plant may of an entirelife cycle. (Due to itslimited dis- suffer setbackdue to the reproductive nega- tributionand population size, it is unlikely tive of intensive on its impact sheep grazing thatA. monoensisprovides all the annualnu- pollinators.These are various bee species, tritionalrequirements for individualsof any severalof whichnest in or near the ground single bee species. Its nectar and pollen- wheresheep are herdedand all of whichare yieldingplant associates are importantin this part of the sand flatcommunity. Postulated respect and are heavily impacted by sheep beloware fourtypes of impact: grazingas reportedhere.) (1) Destruction of potential nest sites. (4) Direct trampling.Male bees of several Bumblebeesoften constructtheir nests in species are knownto "sleep" singlyor in ag- abandonedrodent burrows, including those gregationson vegetationclose to the nest of Thomomysspp. (Thorp et al. 1983). Such sites of females (Linsley 1962, Rust et al. burrowsare abundantin and near the A. 1974) or in shallow, temporaryburrows or monoensishabitat. Sheep collapse the en- emergenceholes (Linsley1958). Early in the trancesof theseburrows and hence remove season, before nests are constructed,female manypotential nest sites fromdiscovery by bees may also restin similarpositions. In this bumblebeequeens in the spring.The fre- situation,particularly during the cool hours quencyof burrowencounter by sheep is in- of night or morning,when bees are torpid dicatedby the factthat rodent holes are the and unable to escape, theymay be in danger majorcause of livestockloss (throughbroken ofbeing trampledby sheep.

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Fig.1. (a) Typicalaspect of Astragalus monoensis habitat prior to grazing. Big Sand Flat, June 1980. Plants in foregroundareprimarily Lupinas duranii. (b) Same view following grazing by domestic sheep.

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Methods divided into three groupsbased on inferred probabilityof importanceas pollinatorsof Bee listed are those which species ap- Astragalus monoensis.Group I consistsof two or moretimes in occasional col- peared those species collected or observed 1979-1982. All foraging lecting during specimens on the plant. For these data and numberof werecollected with a standardinsect net and individualscollected and/or observedare in- are in the R. M. Bohart currentlydeposited dicated,with sex and caste, where appropri- Museum,Department of Entomology,Uni- ate, for those collected. Relative abundance of Davis. versity California, is given as the numberof discretecollection For this insect "abundance" is de- study, events(date X locality)resulting in an extant finedas the relativenumber of individuals voucherspecimen. Group II consistsof four in occasionalcollections from the appearing species which,although neither collected nor monoensishabitat. Astragalus observedforaging on A. monoensis,are pol- In host data fromthe liter- recording plant linatorcandidates by virtueof theirlocally a conservative was main- ature, approach great abundance, widely polylectic habits, tained. A host record to designated only and publishedor observedhost records,in- was countedas a record if genus species only cluding other Astragalusspp., Lupinus du- no other records fromthat species genus rami, the dominantflowering plant in the were in thesources consulted. For con- given community,and/ or otherpapilionaceous le- of more than one generic listings species, gumes. These bees are robust,moderate to recordswithout genus species designations large in size, and morphologicallycapable of werenot counted. pollinatingthe A. monoensisflower. Group Floras and referencesconsulted for plant III species have a low or intermediateabun- namesand identificationsare Munz and Keck dance. Some have been reportedto forageon and Willis (1959),Taylor (1981), (1973). otherAstragalus spp., and all of them have Plantdistributions were obtained by map- published papilionaceous legume host pingthe studysites into a gridof one square records. hectare units and near samplingrandomly Species fromTable 1 are listedin Table 2 thecenter of each unitwith a one me- square with host plant information.To these data ter quadratsampler. new informationhas been added fromper- Insectdensities were obtainedfrom direct sonal observation(Appendix I). For each bee observationof insectson flowers.Two identi- species the total numberof publishedhost cal transects 2.2 kmand be- walking covering plant families,genera, species, and species of tween 36 and 52 minutesin durationwere Astragalusare given. Papilionaceouslegume made at 11:00 and 14:00 daily approximately hosts are given as proportionsof the total hours.Densities are as counts kilo- given per numberof host plant generaand species. All meter hourto make with a per comparisons bee species appearingin Table 2 whose host second site utilizedin study anotherproject, specificityis knownhave been categorizedas where transectswere of a differentlength, "polylectic"(many flower hosts), with the ex- and as a partialcorrection factor for variable ception of Andrenacleodora ssp. cleodora.It durationof transects. is listed as an "oligolege of ceanothus" (re Average ambient temperature at 1 m strictedto a singlespecies or groupof closely above the groundis expressedas an average related species of host plants) (Krombeinet of threereadings per transect.Previous mini- al. 1979). mumtemperature is the previousnight's low From Table 1, it can be seen that a wide ambienttemperature at one meter.All read- diversityof bees is associatedwith A. mono- ingswere takenon a Taylormaximum-mini- ensis. Observed visitorsalone represent3 mum mercurythermometer calibrated in families,5 genera, and at least 7 species. saltedice waterto 0 C. Probable and possible visitorscomprise 5 families,4 genera,and 14 species. The pre- Results dominanceof megachilidsin thisassemblage, particularlyin the generaOsmia and Anthid- Table 1 listsbee species collected in the ium, is a reflectionof the general composi- sandflat region during 1979-1982. These are tionof the bee faunain the dry,high altitude

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Table 1. Beeassociates of Astragalus monoensis Barneby. Relative abundance refers to numberof collection eventsin which species appear, w = worker,q = queen,m = male.S = extantspecimens, O = fieldobservation, R.A.= relativeabundance. Number,sex, Species Date caste S orO R.A. GroupI- Observedorcollected on Astragalus monoensis Apidae Bombus(Pyrobombtis) huntii Greene 8.VII. 79 7w,2q S 5 14.VII.79 3w S B. (Bombias)nevadensis nevadensis Cresson 8.VII.79 4w,lq S 1 B.( Pyrobombtts ) vosnesenskii Radoszkowski 7.VII. 79 lw S 5 Megachilidae Anthidium(.Anthidium ) clypeodentatum Swenk 14.VII.79 2m,2f S 1 Anthidiumsp. 17.VII.79 10 - Callanthidiumformosum (Cresson) 8.VII.79 If,2m S 6 14.VII.79 2f,lm S Osmia(Monilosmia) caraCockerell 14.VII. 79 If S 1 O. (Nothosmia ) grindeliae Cockerell 7.VII.79 If S 3 14.VII.79 If 0 O. spp. 14.VII.79 1 0 16.VII/79 4 0 17.VII/79 4 0 GroupII- Probablevisitors Anthophoridae Anthophora(Anthophora) urbana urbana Cresson S 10 Apidae Apismellifera L. S 10 Bombus(Separatobombus ) morrisoni Cresson S 9 Megachilidae Hoplitis(Monumetha) albifrons argentifrons (Cresson) S 10 GroupIII- Possiblevisitors Andrenidae Andrena(Trachandrena ) cleodora cleodora (Viereck) S 2 A.(Plastrandrena) prunorum prunorum Cockerell S 2 CoLLETIDAE Colletesconsors consors Cresson S 2 Megachilidae Anthidium(Anthidium) banningense Cockerell S 4 A.(A.) mormonům Cresson S 3 A.(A.) tenuiflorae Cockerell S 4 Hoplitis(Monumetha) fulgida platyura (Cockerell) S 6 Osmia(Chenosmia) calla Cockerell S 3 O. (Monilosmia)densa densa Cresson S 3 O. (Acanthosmia)integra Cresson S 2

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Table2. Unweightedhost plant data for bee associatesof Astragalus monoensis Barneby. Bee specieslisted alphabeticallybygroup (see Table 1). Data for Apis mellifera omitted. Sources: Krombein et al. (1979),Moldenke andNeff (1974), Thorp et al. (1983), personal observations reported inAppendix I of this paper. Papilionaceous No. legumehosts as No. No. No. Astragalus proportionoftotal: Beespecies families genera species species Genera Species GroupI Anthidiumclypeodentatum 4 6 11 2 .50 .67 Bombushuntii 18 39 39 1 .21 .21 B.nevadensis nevadensis 9 16 16 1 .38 .38 B. vosnesenskii 38 117 142 1 .09 .15 Callanthidiumformosum 6 9 9 1 .44 .44 Osmiacara 7 11 18 1 .27 .39 O.grindeliae 3 3 3 1 .33 .33 GroupII Anthophoraurbana 3 103 174 1 .10 .11 B.morrisoni 16 35 35 1 .20 .20 Hoplitisalbifrons argentifrons 15 29 34 1 .07 .06 GroupIII Andrenacleodora cleodora 10 12 15 0 .08 .07 A.prunorum prunorum 32 89 101 0 .03 .03 Anthidiumbanningense 6 11 16 0 .45 .33 A.mormonům 8 14 26 1 .29 .38 A.tenuiflorae 9 15 19 1 .27 .37 Colletesconsors consors 4 5 6 0 .20 .17 Hoplitisfulgida platyura 17 34 49 1 .12 .12 Osmiacalla 12 22 33 1 .18 .30 O. densadensa 21 29 43 0 .17 .26 O. integra 3 4 4 1 .50 .50

sand flatregion. Three species of Megachile monoensisand has been regularlyobserved to and Dianthidiumparvum ssp. parvumoccur forageon Lupinus duranii. Its overall abun- regularlyon the flowerheads of ves- dance is matchedby B. vosnesenskii. titabut are not observedto forageon other Close in size to early season bumblebee plants.The latteris consideredan oligolege workersare individualsof Callanthidiumfor- ofcomposites (Krombein et al. 1979). mosum.This species is relativelycommon in the studyarea. It apparentlyexhibits an af- I Group finityfor A. monoensisand otherlegume spe- It is not surprisingthat bumblebeesvisit cies, based on its minimalhost records.Two Astragalusmonoensis. Although populations individuals of Anthidium clypeodentatum fluctuatein theA. monoensishabitat, as has were collected on A. monoensisin 1979. Al- been foundelsewhere (Bohart and Knowlton though its abundance is low, its papilio- 1952), theirlong-lived colonies, large body naceous legume preferenceis apparentlyat size, and caste polyethismmake themvigor- least as high as that of C. formosum, and it ous foragersand importantpollinators of has been recordedon at least one otherAs- many plant species. Bombus huntii and B. tragalusspecies. It has not been collectedon vosnesenskiiwere presentevery year of the L. duranii. study.Bombus nevadensis has been collected Osmia is the most speciose bee genus of only once and only on A. monoensis. This the sand flats,represented by at least 13 sym- speciesis at mostan intermittantmember of patric species (unpubl. data). Osmia grinde- the sand flat fauna,perhaps due to popu- liae may have some affinityfor A. monoensis, lation fluctuation.All threebumblebee spe- althoughits host recordsare too sparseto be cies are widely polylecticand readily visit usefulas indicatorsof generalpapilionaceous papilionaceouslegumes. Bombus huntiiwas legume preference.Its abundance is moder- the most commonly collected bee on A. ate. Osmia cara is rarein the area. Neitherof

This content downloaded from 128.120.194.194 on Tue, 19 Aug 2014 00:30:22 UTC All use subject to JSTOR Terms and Conditions April1985 Sugden: Pollinators of Astragalus 305 thesetwo species have been collected on L. good pollinatorsof papilionaceouslegumes. duranii. Only O. integrahas not been recordedon L. duranii. Andrena prunorumssp. prunorum GroupII and A. cleodora ssp. cleodora are the least likely pollinatorcandidates, but they have Honeybees,Apis mellifera,are presentin both been recorded on papilionaceous le- the sand flatsat relativelylow density,de- gumes.It appears possiblethat these species spite their apparentlygreat abundance in contributeto pollinationof A. monoensis. comparisonto wild bees of the studyarea Table 3 listsGroup I, II, and III bee spe- (pers.obs.). It is likelythat this relatively re- cies with known or probable nest type and source-depauperateregion supportsonly a respectivesusceptibility to the threetypes of few small feral colonies. Honeybees avidly grazing-associateddamage describedabove. collect pollen fromL. duranii (pers. obs.). Ground-nestingbumblebees are particularly Pollen tentativelyidentified as that of A. vulnerablein thisregard. Bombus nevadensis monoensishas been observedin pollen trap may be less vulnerable,because it is known samplesfrom experimental honeybee hives at to nest in abovegroundcavities on occasion. LittleSand Flat (unpubl.data). Bombusmor- Anthophoraurbana , Golletesconsors consors, risoniis a commonbumblebee in the study and Andrena spp. representspecies whose area. It readilyvisits L. duraniiand has been nestsare in jeopardy.The anthidiines,includ- recorded on other Astragalus species. Al- ing Anthidiumspp. and Callanthidiumfor- thoughits general preferencefor papilio- mosum,are apparentlyflexible in theirnest naceouslegumes seems somewhat lower than location.They maybe exposedto nestor nest thatof otherbumblebees, it remainsa good site damage, dependingon theirlocal nest- candidatefor pollination of A. monoensis. buildinghabits. Many cavitynesters, princi- Anthophoraurbana is a widely occurring pally Osmia spp., are presumablyexempt speciesthat forages on virtuallyevery major fromthis hazard because theirnests are most- pollen or nectar source plant in the sand ly in beetle holes in logš or standingtrees flats.It is capable of visitingA. monoensis (pers.obs.). Possibleexceptions are species in and probablydoes so, althoughits papilio- the Acanthosmioides subgenus, many of naceous legumehost recordappears moder- which nest in open, sandy sites in ground ate becauseof the large diversity of records. burrows.Burrows of Acanthosmioidesspecies Hoplitisalbifrons ssp. argentifronsis the were not foundat the studysites but likely mostcommon species associated with Hulsea occur there.An existingpopulation of O. (A.) vestita.Its foraginghabits are so flexiblethat integraat Panum Crater,several kilometers it frequentlyvisits plants fromtwo widely west of the nearestknown A. monoensispop- disparatetaxa on a singleforaging flight, e.g., ulation,nests at the base of Lupinus duranii L. duraniiand Mimuluscoccineus or Hulsea plants (pers. obs.). This species is also found vestitaand Chrysothamnusparry i var. vul- in groundnests associated with sparse vegeta- canicus(pers. obs.). An intensesurvey would tionon sand dunesin northwesternCalifornia probablyshow that H. albifronsvisits and (D. Gordon, pers. comm.). Nests of O. (A.) pollinatesA. monoensis. nigrobarbataalso occur at the base of plants (Rozen and Favreau 1967). Grazing hazards are for these bees GroupIII significant ground-nesting and othersthat may nest in hollow or pithy Bees in thisgroup are variablein theirpol- stems of herbaceous plants, which includes lination potential for A. monoensis. The manyof the speciesfrom Table 3. smallerspecies, Anthidium mormonům , A. te- Small solitarybees are limited in their nuiflorae,Osmia calla, and Hoplitisfulgida flightrange (Kapyla 1978) and are therefore ssp.platyura have each been recordedon As- more dependent on local floral resources. tragalus;A. tenuifloraeand O. calla have Since a large proportionof nectarand pollen been collectedon L. duranii. The three re- plantsmay be decimatedby sheep grazingin mainingmegachilids are relativelylarge, ro- the A. monoensishabitat, solitary bees de- bust species that would be expected to be pendenton such resourcesmay be subjectto

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Table3. Nestinghabits and type of exposure tointensive sheep grazing for bee associates ofAstragalus monoensis Bameby.Nest type of a congenormay be listedwhere information onan A. monoensisassociate is unavailableor supplementary.N = potential nest substrate disturbance, C = damageto nestsand contents, R = depletionof trophicresources, T = directtrampling, ( ) = questionable. Species Exposure Nest GroupI Bambushuntii N C (R)(T) Abandonedrat nest in building(Medler 1959); underground, surface,and aboveground domiciles (Hobbs 1967, Richards 1978);abandoned rodent burrows (pers. obs.). B. nevadensis N C (R)(T) Abovegrounddomicile (Richards 1978); surfaceand abovegroundhives (Hobbs 1965, Hobbs et al. 1962);rafters of building(pers. obs.). B. vosnesenskii N C (R)(T) Abandonedrodent burrows (Thomomys spp.) (Thorp et al. 1983). Anthiditimclypeodentatum (N)(C) R (T) A. maculosum: wooden trap nests (Horning 1971); probably beetleburrows intrees or other insect ground burrows instudy area. Callanthidiumformosum (N)(C)R (T) Woodentrap nests (Parker and Bohart 1966); C. illustre: floral scapeof Yucca sp. and oak stumps (Krombein etal. 1979). Osmiacara R (T) Driftwood(Graenicher 1935); probably beetle burrows instudy area. O.grindeliae R (T) O. pumila: wooden trap nests (Krombein 1967, Medler 1967). GroupII Apismellifera R Abovegroundcavities, hollow trees or logs(Seely and Morse 1976). B.morrisoni N C (R)(T) Abandonedrodent burrows? Anthophoraurbana N C R (T) Horizontaland vertical soil banks (Mayer and Johansen 1976). Hoplitisalbifrons R (T) Hoplitisspp.: elderberry stems (Clement and Rust1976); probablybeetle burrows instudy area. GroupIII Andrenacleodora N C R (T) Groundburrows (Krombein etal. 1979). A.prunorum N C R (T) Groundburrows (Krombein etal. 1979). Colletesconsors N C R (T) C.stepheni : ground burrows (Hurd and Powell 1958). Anthidiumbanningense (N)(C)R (T) Paperand wooden trap nests (Jaycox 1967); ground burrows (Krombeinetal. 1979). A.mormonům (N)(C)R (T) Beetleburrows, oak stumps, ground burrows (Hicks 1929). A.tenuiflorae (N)(C)R (T) Rockcrevices (Krombein etal. 1979). Hoplitisfulgida R (T) Elderberrystems (Clement and Rust1976); probably beetle burrowsinstudy area. O. calla R (T) O. atriventrisandO. clarescens: wooden trap nests (Fye 1965; Krombeinetal. 1979). O. densa (N)(C)(R)(T) unknown. O. integra N C R (T) Groundburrows (pers. obs.); O. longula: under stones (Parker 1975);O. nigrifrons: trap nests(Rust et al. 1974);O. nigrobarbata: ground burrows (Rozen and Favreau 1967). artificialresource depletion, which could source. Partial pollen depletionmight nega- constrainor curtailtheir brood-rearing activ- tivelyinfluence brood development.In sup- ities. Catastrophiclowering of nectar avail- port of these assertions,data have been col- abilitymay stressindividual adult bees and lected on temporarychanges in foragingbee induce or increasecompetition for this re- density on Hulsea vestita and Lupinus

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Fig.2. Distributionsof(a) Hülsea vestitaand (b) Lupinus duranii as mappedin June and July 1980. Both species areperennial. Figure 2a showsthe location of the two, one square hectare study plots utilized for insect density transectsand the approximate area that was grazed 2 August 1982 (hatched). durantiin responseto grazing of adjacent a severe freeze on the eveningof 1 August. areas. Extrapolationfrom this trendwould predict In 1982 the densityof flower-foragingin- a densityclose to zero 3 August.Densities for sects was recorded on two study plots at honeybeeson Lupinus duraniidiffered signif- LittleSand Flat beforeand aftersheep were icantlybetween pre- and postgrazingtran- introducedto the area. The distributionsof sects (p = 0.0413), displayingan increaseof the dominantplant species at the studysite several fold 3 August,despite relativelylow appear in Figure 2. On the afternoonof 2 ambienttemperature. Note that the relative August,approximately 75% of the flat was proportionof L. duraniiremoved by grazing grazedby a herdof 2000 sheep (Fig. 2a). Es- was greaterthan that of H. vestita(Fig. 2a,b). sentiallyall the abovegroundvegetation was Species occurrences on L. duranii also removedin thegrazed area (cfFig. 1). changedmarkedly (Table 4). Priorto grazing, Results for the most often observed no morethan 3 bee species per day were ob- bee/plantcombinations are shownin Figure served(average 1.4), comparedto 6 species3 3. Note that densitiesof bees increase dra- August.In addition,5 of the 6 species ob- maticallyafter sheep grazing.Transects from served3 Augusthad not been observeddur- beforeand aftergrazing were compareduti- ing the previous5 days,and, of the 3 species lizing the Mann-Whitney two sample rank observedprior to grazing,only Apis mellifera test.Differences were not significantfor na- was recorded3 August.Species occurrences tivebees on Hulsea vestita(p = 0.1071), al- on Hulsea vestitaremained unchanged from thoughthis analysisdoes not take into ac- pre- to postgrazingtransects, with the excep- countthe progressivedecline in bee densities tion of the appearance of a previouslyunre- priorto grazing.This was probablydue to a corded Osmia sp. and the conspicuousab- drop in ambienttemperature (Fig. 3) and a »sence of Apis mellifera3 August.Pre- and declinein nectaravailability, accentuated by postgrazingdensities of less frequentlyoccur-

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Fig.3. Insectdensities onflowers before and after nearby sheep grazing. Open bars: native bees on Hulsea vestita. Shadedbars: Honeybees onLupinus duranii. Each pair of bars represents one of two daily transects taken at 1030 and1300 hrs. Appearing above are average ambient temperature at one meter for each transect and the previous night'slow temperature. Native bees in decreasingorder of abundance: Hoplitis albifrons, Osmia montana , Megachilespp., Anthidium spp., Anthophora urbana , Xeromelecta californica , Hoplitis fulgida, Osmia spp., Bombus morrisoniyMisc. ringinsect/plant combinations were record- three hazard types. Sleeping behavior is ed and testedas above forthose cases where known for many bee and wasp species in data permittedanalysis. None were signifi- which adults gather,often in large aggrega- cantlydifferent between treatments. tions, on vegetation or in shallow ground The remainderof the flat was grazed on shelters (Linsley 1958, Evans and Linsley the eveningof 3 August,and insectdensities 1960). Bumblebeequeens are active and nest- droppedeffectively to zero on the following less in the early season,April throughJune, day. after emerging from winter hibernation. Apis melliferaand Bombus spp. may for- They oftenspend the eveningsin temporary age manykilometers from the colony (Gary sheltersnear the groundand thusexposed to 1979,Heinrich 1979, respectively) and there- physicalcrushing by passingsheep. Males are foremight be less affectedby the outcomeof produced in the late summer. Currently, local flowergrazing as mentionedabove. If, sheep are herded in the sand flat regionbe- however,grazing is extensive,bumblebees tweenthese times, i.e., late Junethrough Au- and honeybeesnesting in this area may be gust.Trampling hazard may be variablyim- forcedto flylonger distances to forage,in- portant for differentbumblebee species curringa greaterenergy cost throughlocal dependingon yearlyweather patterns, tim- foraging.If such costsare significant,colony ing of the colonycycle, and arrivalof sheep survivalmay be reduced.Potential effects of in the area. Most solitaryspecies have begun resourcedepletion may be intensifiedby nat- nesting by the time sheep grazing com- ural spatiotemporalvariation in floral re- mences. For these, tramplingexposure may sources,as demonstratedfor similar habitats interferewith nest orientation by femalesdue (Tepedinoand Stanton1980). to the disruptionof local landmarks.Sleeping The dangerof directtrampling is the least aggregations of males might also be knownand thereforemost speculative of the disrupted.

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Table4. Beespecies on Lupinus duranii at Little Sand Flat before and after nearby sheep grazing. Pregrazing Postgrazing July August Species 29 30 31 1 2 3 Anthophoraurbana X Apismellifera X X X X X X Bombushuntii X B.morrisoni X B.vosnesenskii X Callanthidiumformosum X Osmiamontana X unidentifiedspecies X

Conclusion insect-visited plant associates. For this rea- son, it could be undercompetition for insect In this field observationsand liter- study, pollination.Heavy grazingmight alter such a ature have contributedto an survey analysis relationship by increasing the pollina- of the most importantpollination relation- tor/flowerratio in favorof A. monoensisif of a bee- ships Astragalusmonoensis, rare, its flowerswere relativelyless grazed and if Pollinatorsof maximumim- pollinatedplant. bee populations did not proportionately portanceare bees thatforage from its flowers decline. and other thatwould be to species expected Furtherresearch into the floral ecology, do so based on local host abundance, plant autecology,and populationgenetics of Astra- and behavior. records,morphology, galus monoensisis necessaryfor the estab- The evidence herein presented supports lishmentof criteria necessaryfor its pro- the that includ- hypothesis pollinatingbees, tection,as suggestedfor other endangered those found in association with As- ing plant species (Järvinen1982). The impor- be affect- tragalusmonoensis, may adversely tance of such researchis emphasizedby a re- ed the movement of by regular large cent increase in grazing allotmentsfor the numbersof domestic the sand sheep through Mono Basin and adjacentareas (BLM 1982). flats.An investigationon the local nestingbi- ologyof thesebees would requirea special- ized and intensiveeffort. However, the exist- Acknowledgments ing literaturesuggests that the nestsof many of thesespecies are made withinthe rangeof An earlyversion of the manuscriptwas re- substratedisturbance by passingsheep. The viewed by JohnHarris, David Herbst,Lisa data presentedhere demonstrate that bees re- Palermo,Kathleen Teare, and Dean Taylor. spond to localized resource depletion by Later reviewsby my dissertationcommittee sheep grazing.The resultantlong-term ef- are gratefullyacknowledged: R. M. Bohart, fectson the populationsize and structureof R. W. Thorp,and G. Webster.Albert Grigar- A. monoensisremain to be investigated.If ick, Richard Rust, and Robbin Thorp (bees) pollinationbecomes limitingfor the plant, and Dean Taylor (plants)aided in species de- seed setmay decline. terminations.Kathleen Teare (U.S. Forest Other factorsare obviouslyimportant to Service,Inyo National Forest)and Charlotte the systemas it is perturbed.For example,a Chamberlain(Bureau of Land Management, certaindegree of soil disturbance,as would Bakersfield,California) kindly shared field be causedby movementof sheep throughthe notes. Bud Kaye provideduseful comments. habitat,may create favorableconditions for Fieldwork was supportedin part by a re- seed germination,such as increasedscarifica- searchgrant from the CaliforniaNative Plant tion,which has been shownto be important Society.This studyis an enlargementof a re- to manydesert plant species (B. Kay, pers. port submittedto the U.S. Forest Service as comm.).Astragalus monoensis usually occurs Contribution9 of the Mono Basin Research in low densitycompared to itsmost abundant Group.

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Literature Cited Krombein,K. V., P. D. Hurd,Jr., D. R. Smith,and B. D. Burks.1979. Catalog of Hymenopterain Ayensu,E. S.,and R. A. DeFilipps.1978. Endangered Americanorth of Mexico.Smithsonian Inst. and threatenedplants of the UnitedStates. Press,Washington, D.C. Vol. 2 Apocrita SmithsonianInstitution and WorldWildlife (Aculeata). Fund,Inc., Washington, D.C. 403 pp. Linsley,E. G. 1958.The ecology of solitary bees. Hil- Barneby,R.C. 1964.Atlas of North American Astraga- gardia27:543-599. lus.Part II. 57sec. Monoenses. Mem. New York 1962.Sleeping aggregations ofaculeate Hyme- Bot.Gard. 13:964-973. nopteraII. Ann. Entomol.Soc. Amer. Bohart,G. E.,and G. F. Knowlton.1952. Yearly pop- 55(2):148-164. ulationfluctuation ofBombus morrisoni atFredo- Malyshev,ò. i. îyjb.l nenesting habits ot solitary bees. nia,Arizona. T.Econ. Entomol. 45:890-891. Eos11:201-309. Bureauof Land Management,U.S. Dept. of the Mayer,D. F.,and C. A.Johansen. 1976. Biological ob- Interior.1982. 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E. 1979.Factors that affect the distribution of Reports74-1 to 74-6. Parts I, II, III,IV, and VI. foraginghoneybees. Proc. IVth Int. Symp. on Pol- Munz,P. A.,and D. D. Keck.1959. A Californiaflora. lination,Maryland Agrie. Expt. Sta. Spec. Misc. Univ.of California Press, Berkeley. Pubi.1:353-358. Parker,F. D. 1975.Nests of the mason bee Osmia tan- Graenicher,S. 1935. Bee-fauna and vegetation ofWis- neriSandhouse and O. longulaCresson with a de- consin.Ann. Entomol. Soc. Am. 28:285-310. scriptionof thefemale of O. tanneri.Pan-Pac. Heinrich,B. 1979.Bumblebee economics. Harvard Entomol.51:179-183. Univ.Press, Cambridge, Massachusetts. Parker,F. D., and R. M. Bohart.1966. Host-parasite Hicks,C. H. 1929.The nesting habits of Anthidium associationsin some twig-nesting Hymenoptera mormonůmfragariellum Ckll. (Hym.: Megachili- fromwestern North America. Pan-Pac. Entomol. dae).Entomol. News 40:105-110. 42:91-98. Hobbs,G. A. 1965.Ecology of species of Bombus Latr. Percival,M. 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Soc. Seeley,T., and R. A. Morse. 1976. The nest of the Hon- Washington73:43. ey Bee (Apismellifera L.). InsectesSociaux Hurd,r. D., andJ. A. rowELL.1958. Observations on 23:495-512. thenesting habits of Colletesstephani Tim- Smith,J. P. Jr.,J. Cole, J.O. Sawyer,and W. R. berlake.Pan-Pac Entomol. 34(3): 147-153. Powell,eds. 1980. Inventory ofrare and endan- Jarvinen,O. 1982.Conservation ofendangered plant geredvascular plants of California. 2d ed. Cali- populations:single large or severalsmall re- forniaNative Plant Society, Berkeley, California. serves?Oikos 38:301-307. Sugden,E. A. 1984.Grazing hazards for plants and pol- Jaycox,E. R. 1967.Territorial behavior among males of linators.Pages 251-255 in J. N. Tasei,ed., Proc. Anthidiumbanningense (Hymenoptera: Mega- VthIntl. Symp. on Pollination;1983 27-30 Sep- chilidae).J.Kansas Entomol. Soc. 40:565-570. tember;Versailles. INRA, Paris. Kapyla,M. 1978.Foraging distance of a smallsolitary Taylor, D. W. 1981.Checklist of plants of the Mono bee, Chelostomamaxillosum (Hymenoptera, Basin.Mono Basin Research Group Contrib. 3. Megachilidae).Ann. Ent. Fennica 44(2):63-64. ISBN0-939714-01-9. Krombein,K. V. 1967. 312-318in Tepedino,V. J.,and N. L. Stanton.1980. Spatiotem- Pages Trap-nesting of waspsand bees. Smithsonian Inst. Press. poralvariation in phenologyand abundance

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floralresources onshortgrass prairie. Great Basin Thorp,R. W.,D. S. Horning,Jr., and L. L. Dunning. Nat.40(3): 197-215. 1983.Bumblebees and cuckoobumblebees of Thorp,R. W. 1969.Ecology and behavior of Antho- California(Hymenoptera: Apidae). Bull. Califor- phoraedwardsii (Hymenoptera: Apidae). Amer. niaInsect Survey v.23. 79 pp. Midi.Nat. 82(2):321-337. Willis,J. C. 1973.A dictionaryofthe flowering plants andferns. Vili ed. Cambridge Univ. Press. AppendixI. New host records for bees associated with Astragalus monoensis Barneby. Location for all records: MonoCraters region, Mono County, California. Astragalus monoensis records included in text, Table 1. f = female, q = queen,m = male.Host plant abbreviations: C.p.v. = Chrysothamnusparryi vulcanicus (Greene) Hall & Clem. (Asteraceae),H.v. = Hulseavestita Gray (Asteraceae), L.d. - Lupinusduranii Eastw. (Fabaceae), P.f. = Phacelia frigidaGreene (Hydrophyllaceae), E.u. = Eriogonumumbellatum Torr. (Polygonaceae), C.u. = Calyptridium umbellatum(Torr.) Greene (Portulacaceae), M.c. = Mimuluscoccineus Congd. (Scrophulariaceae). Host Number,sex, Species plant Date orcaste Andrenidae Andrena(Trachandrena) cleodora cleodoraViereck E.u. 28.VI.'81 2f E.u. l.VII/81 4f A.(Plastrandrena) prunorum prunorum Cockerell E.u. 28.VI/81 2f Anthophoridae Anthophoraurbana urbana Cresson H.v. 26.VI. '79 If L.d. 7.VII/79 If C.p.v. 8.VIII.'79 If Apidae Bombtis(Pyrobombtis) huntii Greene H.v. 8.VII. '79 lw P.f. 24.VII.79 2w,lq C.p.v. 8.VIII. '79 lw L.d. 21.VIII.'81 lw B. (Separatobombus ) morrisoni Cresson P.f. 13.VII. '79 3w,lq P.f. 14.VII.'79 3w,lq P.f. 24.VII.79 lw P.f. 8.VIII.79 2w L.d. 24.VII.79 4w L.d. 8.VIII.79 lw L.d. 21.VIII.'81 4w C.p.v. 8.VIII.79 3w C.p.v. 21.VIII.'81 lw B. (Pyrobonibus)vosnesenskii Radoszkowski P.f. 14.VII. '79 lw P.f. 24.VII.79 lw L.d. 8.VIII.79 lw C.p.v. 21.VIII.'81 lw Colletidae Colletesconsors consors Cresson P.f. 13.VII. '79 3f P.f. 14.VII.79 3f Megachilidae Anthidium(Anthidium) banningense Cockerell P.f. 14.VII.79 If L.d. 4.VII. '80 Im A.(A.) mormonům Cresson P.f. 4.VII.'80 If A.(A.) tenuiflorae Cockerell H.v. 8.VII.79 If H.v. 14.VII.79 If L.d. 14.VII.79 If

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AppendixI continued. Host Number,sex, Species plant Date orcaste Callanthidiumformosum (Cresson) L.d. 24.VII. '79 If C.p.v. 21.VIII/81 lm Hoplitis(Monumetha ) albifrons argentifrons(Cresson) M.c. 15.VII. '80 If Osmia(Chenosmia) calla Cockerell L.d. 8.VI. '79 5f O. (Monilosmia ) densa densa Cresson P.f. 24.VII. '79 If L.d. 13.VII.79 If O. (Nothosmia)grindeliae Cockerell M.c. 9.VI.'80 If O. (Acanthosmia ) integra Cresson M.c. 9.VI.'80 If

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