Pollinator Community Structure and Sources of Spatial Variation in Plant–Pollinator Interactions in Clarkia Xantiana Ssp

Oecologia (2005) 142: 28–37 DOI 10.1007/s00442-004-1693-1 PLANT ANIMAL INTERACTIONS David A. Moeller Pollinator community structure and sources of spatial variation in plant–pollinator interactions in Clarkia xantiana ssp. xantiana Received: 17 February 2004 / Accepted: 14 July 2004 / Published online: 26 August 2004 # Springer-Verlag 2004 Abstract The structure of diverse floral visitor assem- overall abundance of specialist pollinators in plant blages and the nature of spatial variation in plant– populations was not influenced by the broad-scale abiotic pollinator interactions have important consequences for gradient but strongly affected by local plant community floral evolution and reproductive interactions among associations. C. x. xantiana populations sympatric with pollinator-sharing plant species. In this study, I use pollinator-sharing congeners were visited twice as often by surveys of floral visitor communities across the geo- specialists compared to populations occurring alone. graphic range of Clarkia xantiana ssp. xantiana (hereafter These positive indirect interactions among plant species C. x. xantiana) (Onagraceae) to examine the structure of may promote population persistence and species coex- visitor communities, the specificity of the pollination istence by enhancing individual reproductive success. system, and the role of variation in the abiotic vs. biotic environment in contributing to spatial variation in polli- Keywords Pollination . Facilitation . Positive nator abundance and community composition. Although interactions . Geographic variation . Specialization the assemblage of bee visitors to C. x. xantiana is very diverse (49 species), few were regular visitors and likely to act as pollinators. Seventy-four percent of visitor species Introduction accounted for only 11% of total visitor abundance and 69% were collected in three or fewer plant populations (of The diversity and structure of pollinator communities can ten). Of the few reliable visitors, Clarkia pollen specialist have important influences on floral evolution and repro- bees were the most frequent visitors, carried more Clarkia ductive interactions among plant species. The great pollen compared to generalist foragers, and were less diversity in floral morphology and its utility in plant likely to harbor foreign pollen. Overall, the core group of systematics have supported the idea that specialized pollinators was obscured by high numbers of incidental pollinator relationships were important in the radiation of visitors that are unlikely to contribute to pollination. In a angiosperms by allowing for rapid reproductive isolation geographic context, the composition of specialist pollina- (Grant 1949; Stebbins 1970). Historical studies have tor assemblages varied considerably along the abiotic similarly indicated that flowering plant diversification gradient spanning the subspecies’ range. However, the coincided with the appearance of faithful animal pollinators (Crepet 1984; Eriksson and Bremer 1992). However, Electronic Supplementary Material Supplementary material is contemporary investigations of pollination systems sug- available for this article at http://dx.doi.org/10.1007/s00442-004- gest that plant–pollinator associations are diffuse because 1693-1 plants are visited by a broad spectrum of animals and D. A. Moeller likewise, pollinators often use a wide array of plant taxa Department of Ecology and Evolutionary Biology, Cornell (Herrera 1996; Waser et al. 1996). Following these ideas, University, an increasing number of community-level studies has Corson Hall, concluded that generalization is the rule and specialization Ithaca, NY, 14853, USA rare (e.g., Olesen and Jordano 2002) while new dis- Present address: coveries of highly specialized pollination systems have D. A. Moeller (*) been reported from poorly studied regions (e.g., Steiner Department of Plant Biology, University of Minnesota, and Whitehead 1996; Manning and Goldblatt 1997; 1445 Gortner Avenue, St. Paul, MN, 55108, USA Momose et al. 1998; Moog et al. 2002; Kato et al. e-mail: [email protected] 2003). The variety of findings suggests that pollination Fax: +1-612-6251738 systems range across a continuum of generalization 29 (specialization). However, for plants with diverse visitor tal gradients over which plant populations occur, habitat assemblages, it is often unclear whether most visitor suitability for pollinators, or local variation in plant species play an important role in pollination and therefore community interactions? The first two reasons may influence selection on floral traits or community processes. suggest that the composition of the pollinator community Pollination systems are often considered generalized is determined, in part, by edaphic or climatic factors while when many pollinators effect reproduction in individual the third reason may indicate that landscape features or plant populations or when plant populations vary exten- indirect interactions in plant communities have important sively in their associations with pollinators, without related influences on patterns of pollinator visitation. Distinguish- floral evolution. The challenge in systems where visitor ing sources of variation in plant–pollinator interactions assemblages are diverse is to distinguish important and the spatial scale over which important factors vary can pollinators from rare or ineffective visitors. There is provide insight into the evolutionary consequences of increasing interest in measuring levels of generalization in variable interactions between plants and pollinators. plant–pollinator networks using either data on the species This study focuses on Clarkia xantiana ssp. xantiana A. richness of visitor communities or calculations of the Gray (hereafter C. x. xantiana), an annual plant endemic to fraction of possible interactions between plant and poten- southern California. The subspecies is abundant across a tial pollinator species within a network that are established narrow geographic range, spans a strong gradient in (i.e., connectance). Most surveys have revealed extensive abiotic factors, and exhibits local variation in associations generalization or connectance (Waser et al. 1996; Olesen with pollinator-sharing congeners across its range, making and Jordano 2002) but have been unable to distinguish it an ideal system for disentangling sources of variation in pollinators from visitors. However, substantial variation in plant–pollinator interactions. In this study, I characterized pollination effectiveness among animal visitors has been the bee visitor communities of ten populations of C. x. shown in many systems (Primack and Silander 1975; xantiana spanning most of its geographic range (only bees Herrera 1987; Fishbein and Venable 1996; Olsen 1997; were observed visiting C. x. xantiana in this study). Gómez and Zamora 1999; Tepedino et al. 1999; Mayfield Information on the distribution of abundance of all visitors et al. 2001) suggesting that the total number of visitors within communities, the spatial predictability of visitors may not always be a good indicator of important among communities, and the composition of pollen loads mutualistic interactions. were used to assess the structure of pollinator communities Pollination systems can be examined in the same and the specificity of the pollination system. Second, I general framework used to understand community struc- examined spatial variation in plant–pollinator interactions ture and assembly. For example, the distribution of and the ecological correlates of variation in pollinator abundance of flower visitors within plant populations communities. Analyses of spatial variation were used to and their spatial predictability among populations can distinguish the relative contribution of broad-scale envir- reveal which animals regularly use floral resources onmental variation vs. local interactions between C. x. (component community, Root 1973) versus those that xantiana and pollinator-sharing congeners in influencing are incidental and less likely to make consecutive visits the abundance and diversity of pollinators. among conspecific plants and effect pollination. Studies of species diversity in natural communities have suggested that uncommon, immigrant taxa that are unlikely to Materials and methods maintain population growth often account for a large percentage of species in communities (Shmida and Ellner Study species 1984; Bell 2000). Likewise, in pollination systems, incidental visitors may obscure identification of the core C. x. xantiana (Onagraceae) is self-compatible but highly pollinator community when they account for a large outcrossing (Moore and Lewis 1965; Eckhart and Geber portion of visitor taxa. At present, data on the structure of 1999). Flowers consist of a four-petaled rotate corolla whole visitor assemblages and the role of individual visitor (wheel-shaped), two whorls of four stamens which species in pollination remain limited for diverse systems, produce large pollen grains connected by viscin threads, but should reveal which visitors make important contribu- and a pistil with the stigma typically exserted well beyond tions to plant reproductive output and most strongly the anthers. The subspecies occurs primarily on steep, influence selection on floral traits. sandy slopes in the southern-most portion of the Sierra In a geographic context, the spatial structure of variation Nevada (Lewis and Lewis 1955; Eckhart and Geber 1999). in pollinator abundance and community composition

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