And Diversity of Its Bee Community in a Fragmented Landscape

Oecologia (2009) 161:813–823 DOI 10.1007/s00442-009-1429-3 COMMUNITY ECOLOGY - ORIGINAL PAPER Reproduction of Amorpha canescens (Fabaceae) and diversity of its bee community in a fragmented landscape Malinda W. Slagle · Stephen D. Hendrix Received: 5 August 2008 / Accepted: 27 July 2009 / Published online: 26 August 2009 © Springer-Verlag 2009 Abstract Loss of insect pollinators due to habitat frag- eVect on the diversity and abundance of bees visiting A. mentation often results in negative eVects on plant repro- canescens, indicating community-level characteristics can duction, but few studies have simultaneously examined inXuence the bee community visiting any one species. Site variation in the bee community, site characteristics and size, a common predictor of plant reproduction in frag- plant community characteristics to evaluate their relative mented habitats did not contribute to any models of fruit set eVects on plant reproduction in a fragmented habitat. We and was only marginally related to bee diversity one year. examined the reproduction of a common tallgrass prairie Andrena quintilis, one of the three oligolectic bee species forb, Amorpha canescens (Fabaceae), in large (>40 ha) and associated with A. canescens, was abundant at all sites, small (<2 ha) prairie remnants in Iowa and Minnesota in suggesting it has not been signiWcantly aVected by frag- relation to the diversity and abundance of its bee visitors, mentation. Our results show that the diversity of bees plant population size, and species density of the forb Xow- visiting A. canescens is important for maintaining fruit set ering community. We found signiWcant positive eVects of and that bee visitation is still suYcient for at least some the diversity of bees visiting A. canescens on percent fruit fruit set in all populations, suggesting these small remnants set at a site in both years of the study and in 2002 an addi- act as Xoral resource oases for bees in landscapes often tional signiWcant positive eVect of plant species density. dominated by agriculture. Abundance of bees visiting A. canescens had a signiWcant positive eVect on percent fruit set in 2002, but was only Keywords Fruit set · Bee visitors · marginally signiWcant in 2003. In 2003 but not 2002, the Habitat fragmentation · Prairie · Oligolectic bees plant species density at the sites had a signiWcant negative Introduction Communicated by JeV Conner. The majority of the world’s 220,000 angiosperm species in Electronic supplementary material The online version of this habitats as diverse as Neotropical lowland forests and tall- article (doi:10.1007/s00442-009-1429-3) contains supplementary material, which is available to authorized users. grass prairies depend on insect pollinators for sexual reproduction (Buchmann and Nabhan 1996; Hendrix and Kyhl M. W. Slagle · S. D. Hendrix (&) 2000). However, wild pollinator diversity, richness, and Department of Biology, University of Iowa, abundance are thought to be threatened by habitat fragmen- Iowa City, IA 52242, USA W e-mail: [email protected] tation and destruction resulting from agricultural intensi - cation and other land use changes (Banaszak 1992; Kremen Present Address: et al. 2007). Loss of wild pollinators may have implications M. W. Slagle for agriculture because they are potential pollinators of crop Litzsinger Road Ecology Center, Missouri Botanical Garden, 9711 Litzsinger Road, plants typically pollinated by honey bees that have been Saint Louis, MO 63124, USA negatively aVected by colony collapse disorder, Varroa e-mail: [email protected] mites, and other diseases (Hayes 2007; Norton 2007). 123 814 Oecologia (2009) 161:813–823 Despite the general consensus that fragmentation is populations (Molano-Flores and Hendrix 1999) and may be harmful to many native plant and pollinator species, the less able to sustain pollinators, leading to reduced repro- responses of bee communities to fragmentation and the duction for some prairie forbs compared to populations in eVects on the plants they visit and pollinate remain under large remnants (>40 ha) (Lewis 1999; Molano-Flores and active discussion and investigation (Cane 2001; Cane et al. Hendrix 1999; Hendrix and Kyhl 2000). However, the rela- 2006). Of particular interest are the relationships between tive eVects of reduced size and other changes in plant and plant reproduction and reduced pollinator diversity (Klein pollinator communities potentially resulting from fragmen- et al. 2003, 2008), decreased site size (Jennersten 1988; tation of Iowa prairies are unknown. Aizen and Feinsinger 1994; Ågren 1996; Morgan 1999; In this study we measured fruit set of a common prairie Brys et al. 2004), and altered plant population (Richards forb, Amorpha canescens Pursh. (lead plant, Fabaceae), in et al. 1999; Matsumura and Washitani 2000) or habitat scattered remnants in northwest Iowa and southwest Min- patch characteristics (Tscharntke et al. 1998; SteVan- nesota while simultaneously sampling its bee community Dewenter and Tscharntke 1999). EVects of fragmentation and measuring Xoral resource availability of A. canescens on pollinator diversity and abundance should in turn be and other plants at the remnants. We tested and compared examined by studying relationships to site size and Xoral as possible correlates of fruit set critical factors associated diversity (Heithaus 1974; Potts et al. 2003). However, with the fragmentation process including remnant size unraveling the complexity of plant-pollinator interactions (large or small), population size of A. canescens, species in fragmented landscapes is diYcult because few studies diversity and abundance of bees visiting A. canescens, and simultaneously quantify plant reproduction in fragmented plant species density, a measure of species richness at a landscapes and the multiple factors that can potentially remnant. We also examined for eVects of remnant size and aVect it. Such studies are needed to allow direct comparison measures of Xoral resources on diversity and abundance of of predictors of plant reproduction such as population size bees visiting A. canescens. In addition, we examined year- or remnant size with other potential predictors such as to-year variation in the inXuence of these factors on repro- pollinator diversity or abundance. duction in A. canescens because temporal variation in bee The reproduction of bee-visited plants in fragmented communities (Roubik 2001) and plant reproduction landscapes is also complicated by the degree of specializa- (Molano-Flores and Hendrix 1999) can be great, particu- tion of the bees. Plants dependent on oligolectic bees that larly in small habitat fragments. forage on a narrow taxonomic classiWcation such as a genus or subfamily of plants (Cane and Sipes 2006) may be par- ticularly vulnerable to the eVects of habitat destruction and Materials and methods fragmentation (Buchmann and Nabhan 1996), which may reduce the abundance (Goulson and Darvill 2004) and Study plant genetic diversity (Packer et al. 2005) of oligolectic bees because they are restricted to foraging sites containing their Amorpha canescens is a perennial semi-shrub commonly host plants (Buchmann and Nabhan 1996). In contrast, the found in remnant prairies and savannas throughout the tall- reproduction of plants dependent on polylectic bees, which grass prairie region from Indiana west to Texas and from visit many plant species in diVerent families, may be rela- Texas north to Manitoba (McGregor et al. 1986). Ramets of tively buVered against some of the negative eVects of habi- A. canescens that are presumably from the same rootstock tat fragmentation on pollinator richness (Waser et al. 1996) often occur as closely associated groups or clumps scat- because functional redundancy should allow for high levels tered up to 10 m apart in Iowa’s prairies (McGregor et al. of pollination service with relatively low levels of species 1986; S. Hendrix personal observation). The root system richness (Gamfeldt et al. 2008). Thus, understanding the can penetrate the soil up to a depth of 5 m with lateral eVects of fragmentation on a plant species’ reproduction spread beginning 1 m underground and extending outward is likely to require detailed knowledge of its pollinator radially for 1 m or more (Weaver and Fitzpatrick 1934). community. The inXorescences of A. canescens are compound racemes In Iowa the destruction of 99.9% (11.9 million ha) of the that range from one to 30 branches and contain up to 3,000 pre-settlement tallgrass prairie by about 1900 (Steinauer Xowers. Flowering begins in late June in northwest Iowa and Collins 1996) has created a system of prairie fragments and southwest Minnesota and continues through early which can be used to study the relationships between native August (Lewis 1999). Each Xower can give rise to a single- plant reproduction, pollinator communities, and plant popu- seeded indehiscent pod; fruits mature by early September in lation characteristics. These small remnants (<2 ha) typi- northwest Iowa and southwest Minnesota. cally contain relatively low species richness of Xoral The primary pollinators of A. canescens are solitary bees resources (Leach and Givnish 1996) and/or small plant that make up the majority of its pollen- and nectar-feeding 123 Oecologia (2009) 161:813–823 815 visitors (Robertson 1928; Parrish and Bazzaz 1979; M. Slagle, Table 1 Location, area, and size of 14 sites in northwestern Iowa and personal observation). Adult syrphid Xies are also frequent southwestern Minnesota, USA

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