Great Basin Naturalist

Volume 55 Number 1 Article 2

1-16-1995

Pollinator sharing by three sympatric milkvetches, including the endangered species montii

S. M. Geer Utah State University, Logan

V. J. Tepedino Utah State University, Logan

T. L. Griswold Utah State University, Logan

W. R. Bowlin Utah State University, Logan

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Recommended Citation Geer, S. M.; Tepedino, V. J.; Griswold, T. L.; and Bowlin, W. R. (1995) "Pollinator sharing by three sympatric milkvetches, including the endangered species Astragalus montii," Great Basin Naturalist: Vol. 55 : No. 1 , Article 2. Available at: https://scholarsarchive.byu.edu/gbn/vol55/iss1/2

This Article is brought to you for free and open access by the Western North American Naturalist Publications at BYU ScholarsArchive. It has been accepted for inclusion in Great Basin Naturalist by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. Greal Basin Naturalist 55(1), © 1995, pp. 19-28

POLLINATOR SHARING BY THREE SYMPATRlC MILKVETCHES, INCLUDING THE ENDANGERED SPECIES ASTRAGALUS MONTH

S. M. Geerl •3, V. J. Tepedin02-4, T. L. Griswold'. and W. R. Bowlin!

ABsTRAGr.-Insects visiting Howers ofthe endangered Heliotrope milkvetch, Astragalw montii, were compared with those visiting tv.'O common sympatric congeners, A kcntf'ophyta and A miser, on three sites on the Wasatch Plateau of central Utah for 2 yr. We recorded 27+ species ofbees, most of which were uncommon, visiting the three species. All three species were primarily visited by native of the genera O.rmia (15 species) and/or Bombu.s (4 species). Most O.rmia species visited the three species of Astragalus indiscrim.inantly; bumblebees preferred A. miser and avoided A montii. Our hrpothesis that A. montii flowers would receive fewer total visits and be visited by fewer bee species than their common congeners was rejected: A montii was intermediate to the N'O common species in its attractiveness to bees. Also rejected was our h}'pothesis that the greater similarity between A. montii and A. kentrophyta in flower size, flower morphology, and microhabitat would be associated with greater similarity offlower visitors than either had with A. miter. The data suggest that, rather than competing with each other for pollinators, the three species ofAstragalus fucilitate each olber's visitation rates.

Key words: Asb"agalus, milkvetch, endnngered pkmt, repnxwcoon, pollinatWn, [adUlation, bee diversity, conservation, Fahaceae, Osmia.

Many insects such as dipterans and lepidop­ sparse and likely to attract foraging bees only terans use flowers only as fuel stations (Elton inCidentally. We expect such species to be pol­ 1966); they collect nectar and bum it as they linator-vulnerable and, therefore, to be highly search for suitable spots to lay eggs. Such self-compatible and perhaps primarily self­ insects may merely pass through areas where pollinating (Karron 1987). It is less clear flowers are sparse. Bees, in contrast, are central­ whether in other categories of rarity, place foragers (Orians and Pearson 1979) that especially endemics (Rabinowitz 1981, Kruck­ must consistently reap profits in both nectar and berg and Rabinowitz 1985), are also pollinator­ pollen, for they forage not simply to under­ vulnerable. Endemics have narrow habitat write their own movements, but to provide food specificity but may be locally abundant. to rear their progeny as well (Stephen et al. One such endemic, the rare Heliotrope 1969). Because bees are under strong, selective milkvetch, AJtragalus montii Welsh, is limited pressure to be profitable foragers, they are to a few isolated populations in limestone attracted to dense patches offlowers (Heinrich gravel outcrops on the Wasatch Plateau of 1976, 1979, Thomson 1982). Bumblebees, for central Utah at about 3350 m. There it grows example, quickly recognize and ""1'loit partic­ with two common congeners, A. kelltrophyta ularly rewarding flower patches (Heinrich var. tegetanus (5. Wats.) Dom, hereafter A. 1976, 1979); other bees probably do so also. kelltrophyta, and A. miser var. oblongifolius Density-dependent foraging behavior by (Rydh.) Cron.. hereafter A. miser. In all three bees has important implications for certain rare species, seed production requires, or is in· plants. Rabinowitz (1981) distinguished seven creased by, pollinator visits to flowers (Geer and types of rarity in plants using the following Tepedino 1993). Information on the identity three criteria: (1) local abundance, (2) habitat and biology of these pollinators is important, specificity (narrow or 'vide), and (3) geographic for A. rna-ntH occurs on rangelands that are range (large or small). Those species with both grazed by domestic livestock and sprayed with narrow habitat specificity and small local pop­ insecticides to control grasshoppers. Successful ulations (regardless of geographic range) are management of this rare species requires

1000000roentof Biology, Utah State Univernty. LogaD. UT 84322-SJ05. 2USDA, AR$ Bee Biole>gy and Systematics Laboratory, Utah State Univenity, Logan. UT 84J2Z.5310. 3p~t ~ddreu: Wallowa Whitman National Forest. High\l'aY 82..Ba:< 88401, Eoterprise, OR 97$28. 4Author to whom cor~ shoald be a.;kIre$~.

19 20 CREAT BASIN NATURALIST [Volume 55 knowledge of how such spraying may affect its than do those of its congeners. The three pollinators. species overlap in bloom time for about 3 wk In this report we compared composition (Fig. 1). and abundance ofpollinator fauna ofA. l1wntii Heliotrope milkvetch is a subacaulescent with those of its two sympatric congeners. 1-5 em tall that arises from a branched Because there may be wide variation in a caudex. Flowers are deep purple with white species' pollinators between years and sites wingtips. There may be a dozen to a hundred (Tepedino and Stanton 1981, Herrera 1990, or more flowers (7.8 ± 1.5 mm long, N = 10; Eckhart 1992), we censused pollinators of A. Ceer unpublished) per plant, two to eight per montii and its congeners for 2 yr at three sites. raceme (Barneby 1989). It does not appe:il' to We hypothesized that A. montii would (1) reproduce vegetatively (personal observation). attract fewer individual pollinators, (2) have In 1989 and 1990 A. montii commenced flow­ lower pollinator species diversity than its two ering with fmal snowmelt beginning as early common congeners, and (3) share more species as June and continuing for about 4 wk until of /lower visitors with A. kentrophyta than mid-July (Fig. 1). with A. mise,. because similarity in plant and The mmmon species A. kentrophyta started flower size, flowering time, and microhabitat to flower approximately 1 wk before A. montH is greater with the former than with the latter. and continued to flower through early August. It is prostrate, with stems that fork repeatedly SPECIES AND STUDY AREAS and closely to form low convex cushions cov­ ered with small blue-while to purplish flowers All three species of Astragalus are small (6.6 + 1.2 mm long, N = 10; Geer unpublished), perennial herbaceous legumes. A montii is re­ only two per raceme (Barneby 1989). stricted to three mountaintops on the Wasatch The other common congener, A. mise1; com­ Plateau in central Utah. Although Ise!y (1983) menced flowering 1-2 wk after A. montii and proposed that A. montii be reduced in status continued flowering until September. It is taller to a variety of A. liml1lJcharis Barnehy, it was (2-20 em) than A. montii or A. kentrophyta. listed as endangered under the Endangered Flowers are larger (11.4 ± 1.4 mm long, N = Species Act in 1987 as A. montii and remains 11; Ceer unpublished) and vary in number per so (Anonymous 1991). Therefore, we refer to raceme (3-15; Barneby 1989) and in color; this taxon as A. montii. 1I0wers may be white, pink, or lavender. A. kentrophyto and A. miser are widespread All Astragalus species have papilionaceous species that occur with A. montH at three sites blossoms composed of a showy standard or on two of the mountains; the third mountain is banner petal, a keel that protects the joined less accessible and was not included in the stamens and pistil, and two wings that, along study. A. kentrophyta is widespread and abun­ with the keel, typically serve as a landing plat­ dant in the Rocky Mountains, mostly between form (Kalin Arroyo 1981). To trip A. miser 2280 and 3650 m. A. miser; one of the most 1I0wers, bees land on the keel and force their common species of Astragalus in the Rocky way under the banner (personal observation) as Mountains, is locally abundant from sagebrush they do for other species of Astragalus (Creen foothills to the spruce-fir belt (Barneby 1989). and Bohart 1975, Faegri and van del' Pijl 1979). The three species co-occur at 3250 to 3350 m Visitors to A. montii or A. kentrophyta spread in an Engelmann spruce (Picea engelmannii the wing petals with their midlegs and take Parry)/subalpine fir (Abies lasiocarpa [Hook.] nectar, or comb pollen from the anthers to Nutt.) community. A. mantii and A. kentrophyta their abdominal pollen haskets with their are intermingled in limestone gravel outcrop­ forelegs (personal observation). Stylar hairs pings where A. miser is found only occasional­ (termed a brush mechanism) aid in the collec­ ly. A. miser is most abundant nearby where tion of pollen by transporting it from the keel soil is deeper and less rocky. A. mOlltii and A. outward (Kalin Arroyo 1981). miser occur at similar local densities on Sexual reproduction by A. miser and A. Heliotrope Mountain (9.3 + 0.1/m2 and 12.6 kentrophyta requires insects to transfer pollen; ± 8.3/m2; Ceer unpublished data). There are A. montii is capable of unassisted self-pollina­ fewer A. kemrophyto (2.6 ± 0.8/m2; Ceer un­ tion (autogamy). However, fruits produced published), but individuals mver more ground autogamously by A. montii may be inferior in 1995] POLLINATORS OF SYMPATRIC MILKVETCHES 21

wind, and no precipitation). Initially, sight identification ofsome taxa was attempted so as A, miser to reduce impact on the pollinator community...... __ .. __ ... __ ..•...... It soon became obvious that it was impossible to identifY Osmia aud other individuals with­

A. montii out laboratory examination. Subsequently, all flower visitors were collected whenever possi­ ble. Few insects other than bees visited the

A. kentrophyla flowers. .. __ _ _ . -_ -_ Diversity of bee visitors to each Astragalus

1 10 20 30 10 20 30 10 20 species was calculated using Simpson's diver­ June July August S. sity index, D =1-k }=1 (p.)2l' where p.1 = the proportion of individuals that belong to each Fig. L Blooming dates for three co-occurring species of &tragalus at the SSH site. Solid line = 1989; dashed line bee species (Southwood 1978). Simpson's = 1990. index gives little weight to rare species and more weight to common ones. Similarity of the bee fauna visiting Astragalus species was quality to those produced by geitouogamous estimated using Czekanowski's similarity or xenogamous hand , or open-pol­ index: C = NJ/(a+b+ ...n), where N is the linated control treatments (there are fewer s number of plant species being compared, J is seeds per fruit and seeds are smaller; Geer the number of bee species shared by those and Tepedino 1993). Thus all three species plant species, and a, b, etc., are the total num­ probably benefit from insect visitation. ber of bee species visiting each plaut species (Southwood 1978). C is based on species METHODS s presence alone. We also calculated Cj , which Insect visitors were collected for about 3 wk adjusts for the number of individuals per in 1989 aud for 2 wk in 1990 at the following species (Southwood 1978). The indices range three sites, starting when A. rrwntii was in peak from 0 (no similarity) to 1.0 (complete similari­ bloom: the head of Mill Stream on Ferron ty). They were calculated between pairs of Mountain (HMS), south side of Heliotrope species and among all three species. Mountain (SSH), aud east end of Heliotrope Probable pollinators of the three Astragalus Mountain (EEH). In 1990 collections from all species were ascertained by examining flower three Astragalus species were made only at visitors and recording areas of their bodies on the SSH site because only two insect collec­ which pollen was found. Specimens were then tors were available instead of four, as in 1989. relaxed and pollen was removed using an We concentrated on the SSH site in 1990 to insect pin or by dabbing it with acid-fuchsin make the number of collector hours there gel (Beattie 1971). The pollen was placed on a equivalent to the 1989 effort. In 1990 visitors glass slide with acid-fuchsin gel, warmed until to A. kentrophyta were collected at the SSH liquid, aud a cover slip applied (modified from and HMS sites, and visitors to A. miser were Faegri aud Iverson 1964). One slide per leg or collected at the SSH aud EEH sites. Following two slides per abdomen were made for each are approximate direct distances between sites: insect. All slides were viewed at 100X magni­ HMS to SSH = 3.6 km, HMS to EEH = 2.4 fication aud the pollen compared to a pollen km, aud EEH to SSH = 1.2 km. reference collection of species in bloom at the Pollinators were collected with a standard study sites. butterfly net and killed in cyauide jars. Cold temperatures, strong winds, and frequent pre­ RESULTS cipitation (snow and rain) prohibited pollina­ tors from flying during all but brief windows Bees were scarce at the study sites in both ofcalm, sunny weather, so opportunistic collec­ years (Table 1, Appendices 1, II). Bee visitors tion was necessary to ensure an adequate sam­ per plant species rauged from about 0.5 to just ple size. Collections were made from all three over 3 per hour, a small number considering species contemporaneously, whenever weath­ that mauy flowers of each species were being er permitted (i.e., temperatures > 13 0 C, little monitored. Bee numbers were higher in 1990; 22 GREAT BASIN NATURALIST [Volume 55

TABLE 1. Number ofperson hours spent collecting and number ofbee individuals collected or obseIVed visiting flow~ ers ofAstragalus montU (Asmo), A. kentrophyta (Aske), and A. miser (Asmi) at three sites on the Wasatch Plateau in 1989 and 1990. SSH, EEH = south and east side Heliotrope Mountain, respectively; HMS = head of Mill Stream, Ferron Mountain. SSH EEH HMS Asmo Aske Asmi Asmo Aske Asmi Asmo Aske Asmi 1989 Hours 24 8 10 30 24 22 8 16 16 Individuals 28 9 10 30 19 10 5 II 18 Individualslhour 1.2 l.l 1.0 1.0 0.8 0.5 0.6 0.7 l.l Species 7 3 5 7 7 3 3 4 7

1990 Hours 30 15 15 12 12 12 12 Individuals 57 7 35 40 24 16 24 Individualslhour 1.9 0.5 2.3 3.3 2.0 1.3 2.0 Species 10 5 II 5 5 6 3 when categorized by site and Astragalw; species visiting A. montii than the other two species, visited, six of seven categories had more indi­ but this difference is probably because we col­ viduals per hour in 1990 than in 1989. lected at three sites for A. nwntii but at only The initial hypothesis, that A. nwntii would two for each ofthe other two species. have fewer individual flower visitors than Calculations using species diversity, D I, would its common congeners, received little also failed to yield expected trends (Table 2). support (Table 1, Appendices 1, II). In 1989 In 1989 diversity of visitors to flowers of A. there was little difference among species in montii was very similar to diversity recorded for visitors per person bour at SSH. At EEH A. A. kentrophyta and A. miser. Comparisons for montii flowers were visited more often than 1990 are more tenuous because of the differ­ the other species. Conversely, at HMS A. ences among species in number of sites sam­ montii flowers received the fewest visits. In pled. However, diversity offlower visitors was 1990 comparisons ofnumber ofvisitors among bighest for A. miser and similar for A. nwntii all tbree Astragalus species could be made and A. kentrophyta. Diversity in 1990 was only at tbe SSH site where A. nwntii had an generally lower than in 1989, although num­ intermediate number of visitors per hour. At ber ofindividuals captured was greater. EEH, A. nwntii again bad more visits per hour The most frequent visitors to these Astra­ than A. kentrophyta, and at HMS it had fewer galus species in botb 1989 and 1990 were visits per hour than A. miser. Osmia bees (Table 3). For the small-flowered The prediction that species richness and A. montii and A. kentrophyta, in both years species diversity of bees visiting the three > 70.0% of all visitors were Osmia bees. Only Astragalw; species would be lowest for A. montii for A. miser in 1990 did the percent Osmia was also provisionally rejected. The number of visitors drop below 50%. A. miser was more species captured on A. montii commonly ex­ frequently visited by bumblebees, especially ceeded those captured on the other species, at SSH. The abundance of bumblebees caused both when more bours were spent collecting SSH to have the lowest percentage of Osmia from A. nwntii than the other species (1989 individuals recorded at any site in both years. SSH) and when collecting hours were equal Even so, Osmia bees were always more than (1990 HMS; Table 1). Only once, when fewer 60% of tbe total flower visitor fauna recorded hours were spent collecting on A. montii than in any site-year. on the other Astragalus species (1989 HMS), Because ofgreater similarities in flower size, was A. montii visited by the fewest species of color, and microclimate, we expected A. montH bees. When all sites were considered, total and A. kentrophyta to have more visitors in number of species collected on A. montii in common than either did with A. miser. This 1989 exceeded those captured on A. kentro­ was not true in either year. The three pairings phyta and equaled those captured on A. miser ofAstragalus did not differ much in tbe num­ (Table 2). In 1990 more species were caught ber ofbee species they shared, though results 1995J POLLINATORS OF SYMPATRIC MILKVETCHES 23

TABLE 2. Number of individuals, number of species, and species diversity (D) ofbees found visiting three species of Astragahts at three sites on the Wasatch Plateau. In 1989 collections were made for each species at all three sites; in 1990 collections were made at all sites for A. montii, but at only two sites for the other two species. For comparative pur­ poses, collection data for the latter two species are shown in 1989 for all three sites and for only the two sites collected at in 1990. D = Simpson's diversity index.

Individuals Species Dl Astragalus speCIes 3 sites 2 sites 3 sites 2 sites 3 sites 2 sites 1989 nwntii 63 13 0.87 kentrophyta 39 28 9 8 0.79 0.81 miser 38 28 13 II 0.88 0.87

1990 montii 1I3 13 0.62 kentrophyta 31 7 0.60 miser 59 12 0.79 lIn 1989 only indiviuals that were collected were used in calculations, because uncaptured OsmiG individuals were not identiflllble to species.

TABLE 3. Percent visitors that were Osmia bees to the flowers ofthree Astragalus species (abbreviations as in Table 1). Data shown grouped by species across sites, and by site across species, for 2 yr. For comparative purposes, 1989 data are shown in entirety (3 sites or 3 species) or only for the 2 sites or 2 species sampled in 1990.

Asmo Aske Asmi SSH EEH HMS

~ ------~Across sites·· - ---- ~. - _. - --- -Across species ------

1989 3 (sites/species) 88.9 71.8 73.7 62.3 88.1 76.5 2 (sites/species) 78.6 64.3 85.7 87.0

1990 93.8 74.2 47.5 62.6 87.5 95.0 varied somewhat with year and with index used movements only two were interspecific. In (Table 4). In 1989 the three pairings ofAstraga­ 1990, 4 of 21 observed interplant movements lus species bad about the same number ofbee were between species. Interspecific visits species in common. In 1990 A. miser and A. occurred most commonly where species grew nwntii had about twice the number of species intermingled. in common as did the other pairings. Neither DISCUSSION coefficient of similarity, Cs or Ci, consistently supported the hypothesis; in 1989, but not Two hypotheses make predictions about the 1990, C, and C were highest for the A. man­ i abundance and diversity ofvisitors to the flow­ tii-A. kentrophyta comparison. ers of rare plants. For entomophilous plants, Many bees visiting Astragalus flowers car­ Levin and Anderson (1970), Straw (1972), and ried pollen on their bodies: 43% of the bees Karron (1987) proposed that pollinators should captured, primarily females ofthe genus Osmia, be more flower constant to abundant plant had been collecting pollen. Pollen loads com­ species than to rare ones, that this differential prised primarily Astragalus pollen (all means flower constancy would result in more suc­ >80%; Table 5). It is unknown whether loads cessful reproduction by «majority" species commonly contained more than one species of than by "minority" species, and that over time Astragalus because pollen grains could not be minority species would become extinct because distinguished to species with the light micro­ of dwindling recruitment or would evolve scope. some method of self-reproduction (Levin 1972). Our observations of foraging bees suggest A corollary of this hypothesis is that both the some interspecific movement. In 1989 few number and diversity ofvisitors to the flowers Osmia individuals flew between A. montii and of rare plants should be lower than they are to A. miser or A. kentrophyta; of 74 interplant abundant ones. 24 GREAT BASIN NATURAUST [Volume 55

TABLE 4. Number ofbee species (8) collected on each Astragalus species, and number of species shared (C) and simi- larity indices for each pairing for each year. Cs = Czekanowski's similarity inde.'( for bee species presence·absence: Cl = index: weighted. by individuals captured..

1989 1990 Astragalus species pair S C C, C, S C C, C, montii 13 6 0.50 0.34 13 7 0.56 0.37 miser 13 12 montii 13 6 0.55 0.43 13 4 0.40 0.35 kentrophyta 9 7 kentrophyta 13 5 0.45 0.43 12 3 0.32 0.53 miser 9 7

All three species 20 4 0.35 0.27 21 3 0.28 0.30

In contrast, the facilitation hypothesis (re­ trips. Gross (1992) also reported that bees for­ viewed by Rathcke 1983) predicts that rare aging on closely related legumes commonly species growing with attractive, more abun­ moved between species. Thus, there was no dant species may actualJy reproduce more detectable rare species disadvantage and no successfully because the latter draw many evidence that endemics, at least those growing more pollinating insects into the area than in close proximity to abundant congeners, are would otherwise be present. If so, rare and pollinator-vulnerable. abundant sympatric species should have simi­ The shared microhabitat and similarities in lar visitor diversity, and visitor abundances flower size and morphology of A. montii and should reflect respective frequencies of the A. kentrophyta led us to expect that facilitation plants. This study indirectly assessed the would be more likely between these two species importance of facilitation and competition. A and, therefore, that they would have more visi­ direct assessment is difficult because (1) the tors in common than either would with A. miser. experiments necessary to distinguish between For example, Thomson (1978, 1981, 1982) found alternatives cannot be conducted when the that, in two-species mixtures, the degree of "plant protagonist" is protected by the Endan­ intermingling and the similarity in structure gered Species Act; and (2) A. manti; did not and appearance of congeners' flowers deter­ occur in the absence of its congeners on our mined the importance of competition and study sites, so visitation rates of "facilitated" mutualism. The more similar the flowers, the and "unfacilitated" populations could not be more likely that visitation rates to rare species compared. would be bolstered by the presence of abun­ Our results supply consistent, though indi­ dant species and the more likely that visitors rect, support for the facilitation hypothesis. would be shared. Our data supported this Except for bumblebees, which foraged almost expectation for 1989 but not for 1990 (Table 4). exclusively from large-flowered A. miser, bees In 1990 Cs for the A. montH-A. kentrophyta did not discriminate against A. montH but comparison was intermediate to the other rather seemed to treat all three Astragalus comparisons; for Cj it was lower than the other species as one taxa. First, A. montH did not comparisons. Thus, results for the similarity consistently attract fewer visitors per hour analyses also tend to support the hypothesis than did the other species. Indeed, visitation that most bees do not distinguish among these rates to A. montH were higher than to the Astragalus species when foraging, and that the other species in three ofsix site-years (Table 1). Astragaltls species tend to facilitate each Second, neither species richness nor species other's visitation rates. diversity of pollinators was consistently lower Only bumblebees seem uninfluenced by for A. montii than for the other species (Table 2). Astragalus flowers in the aggregate. They In fact, an equal or greater number of species clearly preferred flowers of A. miser and visited A. montH than visited the others in avoided those of the other Astragalus species. both years. And finally, bees were observed Flowers of A. miser are large, probably more moving between species on individual foraging rewarding, and provide a landing platform from 1995] POLLINATORS OF SYMPATRIC MILKVETCHES 25

TABLE 5. Percent Astragalus pollen grains in pollen loads, and location of pollen IOHds l.'arried by bees collected on threeAstragalm species at three sites on the Wasatch Plateau in 1989 and 1990. Location of Mean % pollen Astragalus Number of Astragalus species pollen loads pollen (+SE) Abdomen Legs montii 45 82 + 4 42 3 miser 19 90 + 1 19 kentrophyta 5 95 + 1 5 which large, energy-demanding bumblebees plants. Land managers must eliminate losses can readily forage. Otber large-flowered Astm· of bees to insecticide applications made for galus species also attract numerous large bees nmgeland grassboppers and minimize physical such as bumblebees (Bombus spp.) and antho· damage to nest sites. The present insecticide­ phorids (Green and Bohart 1975, Sugden 1985, free buffer zone (currently 4.8 km) around rare Karron 1987). In comparison, bumblebees plant populations should continue to be main­ seemed unable to land on the small, weakly tained. Areas where bees nest in soil should supported A. mon.tii flowers whicb are borne also be protected from livestock trampling, above the foliage; they did occasionally exploit off-road vebicle use, and foot traffic (Sugden the tiny A. kentrophyta blossoms while perched 1985). Such diversity, comparable to or greater on the foliage ofthat cushion plant. tban that of other subalpine areas in North Factors other than flower abundance can America (Moldenke and Lincoln 1979), is to influence the flight path of foragiug bees. be marveled at and preserved. Because hees are central-place foragers (Orians and Pearson 1979), travel time and energy ACKNOWLEDGMENTS expended between flower patches and nest We are grateful to tbe many people who are also important. Thus, bees may patronize a assisted in this study. Etta Sechrest and Mike flower patch because of its proximity to their Cram were reliable field and laboratory assis­ nest, even though flowers are more abundant tants. John Healey, Don Riddle, and Bob elsewhere. For example, Osmia bees mated Thompson of the U.S. Forest Service and Larry and nested at the sheltered EEH site where England, U.S. Fish and Wildlife Service. helped relatively few A. kentrophyta or A. miser plants in a variety of ways, from locating plant popu­ grew; the population of A. montii was small lations to putting a roof over our heads. The but dense. Nevertheless. bees visited flowers manuscript was constructively reviewed by M. at least as frequently at EEH as at the other. Barkworth, K. Harper, and E. Sugden. Tbis more flower-rich, sites (fables 1, 2). Thus, suit· study was funded as part of the APHIS Grass­ ability of nesting babitat at EEH, rather than bopper 1PM Project. It is Journal Paper #4436 Astragalus flower abundance, may best account from the Utah Agricultural Experiment Station. for the abundance of bees there. The effect of wild bee nesting sites on seed production of LITERATURE CITED surrounding vegetation is poorly studied and ANONYMOUS. 1991. Endangered and threatoned wildlife warrants additional attention. and plants: 50 eFR 17.11 and 17.12, July 15, 199L Rigorous subalpine communities of the Publication unit, U.S. Fish and Wildlife Service, Wasatcb Plateau, with frequent higb winds. Washington. DC. thunderstorms. and below-freezing tempera­ BltRNE8Y, R. e. 1989. F'abales. Pages 12-167 in A. Cronquist, A.. H. Holmgren, N. H. Holmgren, J. L. tures during the blooming season, support a Reveal, and P. K. Holmgren, cds., IntennQuntain flora surprisingly rich bee fauna. In 2 yr we collected 3, Part B. New York Botanic Garden, Bronx, NY. 27+ bee species foraging on Astragalus flow­ BKATIlE, A. J. 197]. A technique for the study of insect­ ers during 2-3 wk (Appendices I, II). These borne pollen. Pan~Pacific Entomologist 47: 82. bees are invaluable pollinators of native plants EcKHART, V M. 1992. Spatio-temporal V'driation in abun­ dance and V"driation in foraging behavior of the polli­ both rare and common. Tbeir welfure must also nators of gynodioecious Phacelia /ineam (Hydro­ be considered in management plans for rare phyllaceae). Oikos 64; 573-586. 26 GREAT BASIN NATURALIST [Volume 55

ELTON, C. S. 1966. The pattern of animal communities. LEVIN, D. A., AND W. W. ANDERSON. 1970. Competition Methuen, London. 432 pp. for pollinators between simultaneously flowering FAEGRI, K., AND J. IVERSON. 1964. Textbook of pollen species. American Naturalist 104: 455-467. analysis. Hafner Co., NY. 237 pp. MOLDENKE, A. R., AND P. LINCOLN. 1979. ecol­ FAEGR!, K., AND L. VAN DER PUL. 1979. The principles of ogy in montane : a community analysis. pollination ecology. 3rd revised edition. Pergamon Phytologia 42: 349-379. Press, Oxford. 244 pp. ORIANS, G. H., AND N. E. PEARSON. 1979. On the theory of GEER, S. M" AND V. J. TEPEDINO. 1993. Breeding systems central place foraging. Pages 155-177 in D. J. Horn, of the rare Heliotrope milkvetch (Astragalws montii G. R. Stairs, and R. D. Mitchell, eds., Analysis of Welsh: ) and two common congeners. Pages ecological systems. Ohio State University Press, 334-344 in R. Sivinski and K. Lightfoot, eds., Proceed­ Columbus. ings ofthe Southwestern Rare and Endangered Plant RABINOWITZ, D. 1981. Seven forms ofrarity. Pages 205-218 Conference. New Mexico Forestry and Resources in H. Synge, ed., The biological aspects ofrare plant Conservation Division, Santa Fe. conservation. Wiley, New York, NY. GREEN, T. W, AND G. E. BOHART. 1975. The pollination RATHCKE, B. 1983. Competition and facilitation among ecology of Astragalus cibarius and Astragalus uta­ plants for pollination. Pages 305-329 in L. A. Real, hensis (Leguminosae). American Journal of Botany ed., Pollination biology. Academic Press, New York, 62,379--386. NY. GROSS, C. L. 1992. Floral traits and pollinator constancy: SOUTHWOOD, T. R. E. 1978. Ecological methods. 2nd edi­ foraging by native bees among three sympatric tion. Chapman and Hall, London. 524 pp. legumes. Australian Journal ofEcology 17: 67-74. STEPHEN, W P., G. E. BoHART, AND P E TORCHIO. 1969. The HEINRICH, B. 1976. Foraging specializations of individual biology and external morphology of bees. Agricultural bumblebees. Ecological Monographs 46: 105-128. Experiment Station, Oregon State University, _-,_.. 1979. "Majoring" and "minoring" by foraging Corvallis. 140 pp. bumblebees, Bombus vagans: an experimental analy­ STRAW, R. M. 1972. A Markov model for pollinator con­ sis. Ecology 60: 245-255. stancy and competition. American Naturalist 106: HERRERA, C. 1990. Daily patterns of pollinator activity, 597-<;20. differential pollinating effectiveness, and floral re­ SUGDEN, E. A. 1985. Pollinators of Astragalus motwensis source availability, in a summer-flowering Mediter­ Barneby (Fabaceae): new host records; potential ranean shrub. Oikos 58: 277-288. impact of sheep grazing. Great Basin Naturalist 45: ISELY, D. 1983. New combinations and two new varieties 299--312. in Astragalus, Orophaca, and Oxytropis (Legumino­ TEPEDINO, V. J., AND N. L. STANTON. 1981. Diversity and sae). Systematic Botany 8: 422. competition in bee-plant communities on short~grass KALIN ARROYO, M. 1: 1981. Breeding systems and pollina­ prairie. Gikas 36: 35-44. tion biology in Leguminosae. Pages 723-769 in R. M. THOMSON, J. D. 1978. Effect of stand composition ofinsect Polhill and P. H. Raven, eds., Advances in legume visitation on two-species mixtures of Hieracium. systematics. Part 2. Royal Botanic Gardens, Kew, American Midland Naturalist 100: 431-440. UK. _---". 1981. Field measures of constancy in bumble­ KARRoN, J. D. 1987. The pollination ecology of co-occur­ bees. American Midland Naturalist 105: 377-380. ring geographically restricted and widespread species _-,-,,' 1982. Patterns of visitation by animal pollinators. ofAstragalus (Fabaceae). Biological Conservation 39: Oikos 39: 241-250. 179-193. KRUCKEBERG, A. R., AND D. RABINOWITZ. 1985. Biological aspects of endemism in higher plants. Annual Review Received 29 April 1993 of Ecology and Systematics. 16: 447--479. Accepted 2 June 1994 LEVIN, D. A. 1972. Competition for pollinator service: a stimulus for the evolution of autogamy. Evolution 26,66iH;74.

ApPENDIX I. Species of bees collected and observed visiting flowers ofA. montii (Asmo), A. miser (Asmi), or A. kentro­ phyta (Aske) at three sites in 1989. Entries represent number of males/females collected. Observations are in parenthe­ ses. Site abbreviations as in Table L SSH EEH HMS 1-21 June 14-25 June 14-22 June Bee species Asmo Aske Asmi Asmo Aske Asmi Asmo Aske Asmi ANDRENIDAE And-rena transnigra Vier. 0/1 Andrena spp. (I) 1995] POLUNATOHS OF SYMPATRIC MILlCVETCHES 27

ApPENDIX 1. Continued.

SSH EEH HMS 1-21 JUlie 14--25 June 14-22 June

Bee species Asmo Aske Asmi A.'Hno /\'ke Asmi /\,mo A.. ke Asmi

APIDAE !J<>mI= bifarius e.-. 0/1 Bombus f/avifrotlS c,. (0/2) l30mbus huntii Greene (0/2) 0/2 80mbus nevadensis Cr. Oil

IIALICfIDAE Evylaeus niger (Viereck) Oil 0/1

M EGACnILIDAE A"tllidhu.. umuiflnme Ckll. (2)1.ftl (1) (1)1/2(1) 1/2(2) Meguchile '!''Pp. (1ft» (1ft» Osmia cyannpoda Ckll. 1ft> Osrnia Iwrdii White 0/1 Osmia loRb'Ula Cr. 0/1 Os-mia nigrifrons Cr. 0/1 0/4 0/2 0/.3 OS7uia ail nigrifrons 0/.3 0/1 0/1 Osrniaparadi.sica Sanh. 1ft) 212 Osmia pen.stemonis CklJ. 0/1 O""'io pike; Ckll. 0/1 Osmiu pusilla Cr. 0/1 1ft) O~mia sladeni Sanh. 2/0 I/O 4ft> 3/0 2ft> Osmia sladeni &/or alpestris 0/2 0/.3 0/2 0/1 0/2 0;5 Gsrnia tanneri Sanh. 1/.3 1/2 0/1 0/1 o.,mw, spp. (519) (1/4) (0/3) (819) (111) (1/3) (111) (0/1) (Oil)

ApPENDIX lI. Species ofbees collected and observed visiting Dowers ofA fnontii (Asma) at three sites and A. miser (Asmi) and A. kentrophyw (Aske) at two sites each in 1990. Entries represent number of males/females collected. Observations are in parentheses. Site abbreviations as in Table 1.

SSH EEH HMS 19 June-4 July 19-29 June 21-29 June Visitor Asmo Aske Asmi /\,mo Aske Asmo

ANDRENIDAE A,ulrena nigrihirta (Ashm) 0/1 Andrena tramnigra Vier. 0/1

APIDAE Apis melli/era L. 0/1(5) Bambus bifadus Cr. Oil Bam-bus flavtfrons Cr. 0/1 0/1 130mbus l1ttntu Greene 013(6) Bombus nevadensis Cr. 013(5)

MEGACHIUIJAE Anthulium tenuif/oroe Ckll. l/O(2) 0/1(3) I!ol,litis j"lgida e.-. 3/0 Megoci/e mekuwpMea Smith I/O 1ft> Megachile perihirta CklJ. 1ft> 28 GREAT BASIN NATURALIST [Volume 55

ApPENDIX II. Continued. SSH EEH HMS 19 Jun6-4 July 19-29 June 21-29 June Visitor Asmo Aske Asmi Asmo Aske Asmo Asmi

MECACHILlDAE (continued) Osmia longu1a Cr. 2/0 Osmin. Cr. IAJ Oamin aff. nigrifrons 0/1 0/1 Osmia paradi.sica 8anh. 1/0 0/2 3/0 1/2 I/O Osmw penstemonis Ckll. 0/1 O",.is pwrilla Cr. 0/1 Osmia sculleni Ckll. 2/0 lill 0""",slodem Sanh 19/13 4/0 lill 8/16 718 3/6 1/21 Os";"su/xwstrali> Ckll. 4/0 1/lI Osmia tanneri Sanh. 9/2 IAJ 1/lI 91l Oil