Surplus Nectar Available for Subalpine Bumble Bee Colony Growth
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PLANTÐINSECT INTERACTIONS Surplus Nectar Available for Subalpine Bumble Bee Colony Growth 1,2 SUSAN E. ELLIOTT Environ. Entomol. 38(6): 1680Ð1689 (2009) ABSTRACT Mutualisms may cause coupled population expansion or decline if both partners re- spond to variation in the otherÕs abundance. Many studies have shown how the abundance of animal mutualists affects plant reproduction, but less is known about how the abundance of plant mutualists affects animal reproduction. Over 2 yr, I compared reproduction of the bumble bee, Bombus appositus, across meadows that varied naturally in ßower density, and I compared reproduction between fed colonies and unfed control colonies. Colony reproduction (gyne, worker, and male production) was constant across meadows that varied naturally in ßower density. Forager densities per ßower did not vary among meadows, and daily nectar depletion was consistently low across meadows, suggesting that bees had ample nectar in all meadows. However, colonies directly fed with supplemental nectar and pollen generally produced over twice as many gynes as control colonies. Feeding did not affect male or worker production. Although colonies may beneÞt from food supplementation at the nest, it is possible that they may not beneÞt from additional ßowers because they have too few workers to collect extra resources. KEY WORDS Bombus appositus, bumble bee, Delphinium barbeyi, food limitation, ßower density Mutualisms are often considered from a unilateral and the costs of partner exploitation (Johnson and perspective, emphasizing the more sessile or larger Steiner 1997, Herrera 2000, Morales 2000, Egger and partner, whose Þtness is often easier to measure (e.g., Hibbett 2004, Ness et al. 2004, Rudgers and Strauss the plant in a plant-pollinator mutualism: Cushman 2004). In plantÐpollinator mutualisms, unless the pol- and Beattie 1991). Knowing how a visitor affects a host linator also acts as a seed predator, there is likely little mutualist can help explain the ecology and evolution to no cost of extra pollinator visits for seed production of the host, but if the visitor responds to changes in its (but see Young and Young 1992). Similarly, there is partner species (e.g., by an increase or decrease in probably no cost to bees for living in areas where there abundance), this could alter the nature (and our in- are surplus ßowers unless bee parasites are positively terpretation) of their relationship (Bronstein 1994b, correlated with ßower density (Carvell et al. 2008). Thompson 2005). For example, little is known about The beneÞts of pollinator abundance to female plant how the Þtness or population growth of mutualistic reproduction and changes in population growth mycorrhizal fungi, nitrogen-Þxing microbes, and pho- (through effects on seed production) may saturate tosynthetic zooxanthellae vary with the frequency of when abiotic resources become limiting for seed pro- their mutualistic partners (Bever 1999, Simms and duction (Burd 1994, Ashman et al. 2004). Plants may Taylor 2002, Hay et al. 2004). In plantÐpollinator mu- compensate for pollinator shortages if they can self- tualisms, ßowering plant reproduction often varies pollinate without inbreeding costs, reproduce asexu- with the number and diversity of pollinators (Pellmyr ally, or endure unfavorable periods via longevity or and Thompson 1996, Gomez et al. 2007, Sahli and dormancy by living longer or having seed banks (Pake Conner 2007). However, the sensitivity of pollinator and Venable 1996, Morgan et al. 2005). In contrast, reproduction to changes in ßoral resources or other most insect pollinators are short-lived organisms, so environmental variables is poorly understood, espe- they must reproduce even when resources are scarce. cially in North America (National Resource Council With very few exceptions, bee pollinators cannot re- 2006). produce without harvesting pollen and nectar to feed The outcomes of mutualisms are often highly con- themselves and their offspring (Michener 2007). text dependent (Bronstein 1994a). The beneÞts of However, is pollinator per-capita reproduction food- mutualistic associations can vary with partner quality, limited? resource availability, the presence or absence of pred- Positive correlations between bee and ßower den- ators, competition for access to a mutualistic partner, sities could indicate that bees have higher reproduc- tion, recruitment, or survival in areas with more ßow- ers (Steffan-Dewenter et al. 2002, Westphal et al. 1 Corresponding author: Pinyon Publishing, Montrose, CO 81403 (e-mail: [email protected]). 2006), but these relationships could also be inßuenced 2 Dartmouth College, Biology, Hanover, NH 03755. by other limiting factors such as parasitism and nest 0046-225X/09/1680Ð1689$04.00/0 ᭧ 2009 Entomological Society of America December 2009 ELLIOTT:BUMBLE BEE NECTAR SURPLUSES 1681 Fig. 1. Study meadows (a) in the East River Valley, Gunnison National Forest, CO, and total number of D. barbeyi inßorescences (b) in 200-m radii around each captive colony location (1Ð3; GPS coordinates ϭ 1: 2Њ56Ј8Љ E, 38Њ49Ј33Љ N; 2: 2Њ56Ј12Љ E, 38Њ49Ј21Љ N; 3: 2Њ56Ј17Љ E, 38Њ49Ј13Љ N). site availability. Recent studies from low-elevation ar- male offspring production) was limited by ßoral re- eas (Ͻ200 m) conÞrm that bee reproduction of soli- sources, by asking the following questions. tary and colonial bee species is greater in areas with 1. Does colony reproduction vary across meadows higher natural or experimental levels of ßoral re- that vary naturally in ßower density? If bee colony sources (Goulson et al. 2002, Pelletier and McNeil density is independent of ßower density and if 2003, Greenleaf 2005, Williams and Kremen 2007, colony reproduction is food-limited, colonies in Carvell et al. 2008). However, because bees at high meadows with more ßowers should produce more elevations have shorter growing seasons and conse- offspring, assuming that ßower number represents quently less time to establish nests, collect resources, pollen and nectar availability (Tepedino and Stan- and produce offspring, their reproduction may instead ton 1982). Alternatively, if meadows with more be time-limited (Pyke 1982). To determine whether ßowers have disproportionately more colonies, subalpine bumble bees have surplus ßowers for colony ßower availability per colony might be lower in reproduction, I measured bumble bee reproduction meadows with more total ßowers. If ßower avail- across meadows that varied naturally in ßower density ability per colony is greater in some meadows, and in response to supplemental feeding at the nest. ßowers in those meadows should have lower per- Time limitation of bumble bee reproduction was not ßower pollinator visitation rates, and nectar in addressed experimentally but is discussed within the those meadows should be depleted less quickly context of the experimental Þndings of this study. than in meadows with more ßowers per colony. Although bumble bees visit a variety of ßowers, they 2. Is bee colony reproduction food-limited? If bee may be functionally specialized if they derive most of reproduction is food-limited, colonies that are fed their resources from the most rewarding ßower spe- at the nest with supplemental nectar and pollen cies (Dramstad and Fry 1995, Stenstrom and Bergman should have higher reproduction than unfed con- 1998, Goulson and Darvill 2004). The focal bumble bee trol colonies. Also, fed colonies may reduce for- in this study, Bombus appositus L. (Apidae) may visit aging to avoid costs associated with foraging out- at least 20 ßower species throughout the season in the side the colony (Dukas and Morse 2003) or to study area (Pyke 1982), but it collects much of its beneÞt the colony by devoting more time to brood pollen and nectar from the abundant perennial wild- care or colony defense (Cartar 1992). ßower, Delphinium barbeyi Huth (Ranunculaceae) (Elliott 2009). Delphinium barbeyi, the second most Materials and Methods abundant ßower species in these meadows, is in bloom in the middle of season while B. appositus colonies are In 2006 and 2007, I studied reproduction in the provisioning and producing adult workers, gynes, and long-tongued bumble bee, B. appositus (Apidae), in males. During this time, D. barbeyi accounts for the subalpine meadows in the East River Valley near the majority of pollen collection and ßower visitation by Rocky Mountain Biological Laboratory in Gothic, CO B. appositus (described below). I tested the hypothesis (2,952- to 2,969-m elevation; Fig. 1). B. appositus is one that B. appositus reproduction (i.e., worker, gyne, and of the three most abundant bumble bee species in this 1682 ENVIRONMENTAL ENTOMOLOGY Vol. 38, no. 6 study area (Pyke 1982, Elliott 2009). Inseminated len collection by B. appositus is dominated by these bumble bee queens emerge from hibernation in early two species (S1). I used a one-way analysis of variance spring (late May to early June) and establish new (ANOVA) to compare the proportions of cream and nests, where they incubate their Þrst brood. About 6 orange pollen loads carried by foragers returning to wk later, the Þrst worker cohort emerges from their the colonies (described below) among the three study cocoons, and forager density increases sharply. During meadows, using colony as the unit of replication. this increase in bee abundance, the common perennial Bumble Bee Colonies. In 2006 and 2007, I moni- wildßower, Delphinium barbeyi (Ranunculaceae), ac- tored captive B. appositus colony reproduction in the