The Openness of a Flower and Its Number of Flower-Visitor Species
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TAXON 56 (3) • August 2007: 729–736 Olesen & al. • Flower openness and number of visiting species The openness of a flower and its number of flower-visitor species Jens M. Olesen1, Yoko L. Dupont1, Bodil K. Ehlers1 & Dennis M. Hansen2 1 Department of Biological Sciences, University of Aarhus, Ny Munkegade B1540, 8000 Århus C, Denmark. [email protected] (author for correspondence) 2 Institute of Environmental Sciences, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland Using a sample of 1,403 flowering plant species, we tested the hypothesis that flower openness and flower- visitor generalization level of a plant species correlate positively. The “flower-visitor generalization level” Ln of a flowering plant species n, here defined as number of flower-visiting animal species attracted to the flowers of n in a given study site, varied enormously among plant species. Its frequency distribution was extremely skewed. Within a study site, L also increased with number of flower-visitor species A. In order to correct for this, we expressed L relatively, as the proportion of the total flower-visitor fauna in a study site that visited a given plant species (relative generalization level, L/A). We listed the top-10 most generalized species (both according to L and L/A) in the “world”, i.e., out of our sample of 1,403 plant species. Flower openness is defined as accessibility to the interior of the flower. We placed the blossom classes of Fægri & van der Pijl along a gradient, albeit not very well defined, of decreasing flower openness (dish-bowl, bell-funnel, head-brush, tube, gullet, flag) and tested for any relationship to their generalization level. The classes differed slightly but significantly in their level of L/A. Tube, bell-funnel, and dish-bowl had the highest generalization level and flag, gullet, and head-brush the lowest. Thus, flower openness and generalization level were not correlated. We discuss other factors influencing generalization level such as accessibility to pollen and nectar, morphology and behavior of visitor, and species diversity of the different functional types of visitors. KEYWORDS: blossom class, ecomorphology, generalization level, pollination the ecosystem. It will be an enormous task! … for total INTRODUCTION investigation of flower-pollinator interactions in only one Floral ecomorphology is the study of relationships ecosystem, complete dedication by the investigator will between the morphology of flowers and aspects of their be needed” (Baker, 1979: 437). ecology related to floral morphology, i.e., flower visitors, Here we address one important aspect of floral eco- and vice versa for the ecomorphology of flower visitors. morphology, viz. the relationship between flower open- Thus, flower and visitor together constitute an ecomor- ness of a plant species and its number of flower-visitor phological entity (Turner, 2002). A very closed flower, species. By openness we mean accessibility to the interior for example, may only interact with a single long-tongued of the flower and its rewards. We expect this relationship or physically strong visitor species. However, an entity to be significantly positive. It is so obvious an ecomor- may also encompass several highly interacting species, phological hypothesis in pollination biology that it has together constituting an interaction network compartment reached the status of truism. Nobody has, however, tested (Dicks & al., 2002). For example, plant species with an it on a large species sample. open flower morphology may attract a richer coterie of Flower openness. — Flower openness is a contin- flower-visitor species and together all species involved uous character, but for which no appropriate parameter make up a more complex entity. Consequently, floral is available. For this reason, we have to make do with ecomorphology has to take place on the level of the existing typological classification schemes. (sub-)community, i.e., we need data about the complete Carl von Linné, typologist par excellence, classified interaction environment of a set of flowering plant species plants according to their reproductive organs. Flower biol- sharing a visitor fauna in order to make valid statements ogists more fond of forms and colors of flowers and their about their ecomorphology (Jordano, 1987; Jordano & al., pollen, nectar, scent, and flower visitors went along other 2006). However, this is certainly a time-consuming task. roads of classification. Some dived into the diversity of Baker pleaded for a move towards ecosystem pollination flowers and inflorescences ignoring their visiting fauna, biology, but also noted that this was very laborious: “The whereas others focused upon characters assumed to be same sort of tallying, on a much more exact and quantified important to the attraction of animals. This debate about basis, needs to be done with all of the other species in choice of focus was imbedded in a broader typological- 729 Olesen & al. • Flower openness and number of visiting species TAXON 56 (3) • August 2007: 729–736 evolutionary controversy and has been ongoing since the species); (4) gullet (pollen is placed dorsally on pollinators; days of the flower biologist Delpino (1868–1874) (Olesen, e.g., Salvia and Acanthus species); (5) flag (pollen is most 2000; Waser, 2006). Delpino and other, much later, writers, often placed ventrally on pollinators; e.g., Fabaceae and in particular Vogel (e.g., 1954), van der Pijl (e.g., 1961), Corydalis species); and (6) tube blossoms (e.g., Primula and Fægri & van der Pijl (1971), discussed pollinator types species) (Fig. 1). Combinations are also often found, e.g., or classes, and pollination syndromes, illustrating and an open dish-shaped capitulum of a composite, where each listing important character states. Pollination syndromes flower is tube-shaped. In this study, 1–3 were regarded as were defined as suites of states expected to be adaptive open types and 4–6 as closed. to various classes of pollinators. The wind blew from complete rejection of any notion about classification to heralding its importance (van der Pijl, 1961). Classes of natural entities are commonly taken too MATERIAL AND METHODS literally by scientific and public readerships, which most Thirty-seven pollination networks extracted from 22 often was not the intention of their inventors. These sci- published and 7 unpublished studies were collated and con- entists, all being skilled naturalists, of course knew the stituted our network database. A pollination network depicts exceptions to their classificatory schemes. For example, graphically all flowering plant and flower-visitor species van der Pijl (1961) stressed that a plant can be more or less within an area and all their interactions. We included all adapted to a certain kind of pollinator. He says: I will try to “total” networks to which we had access. “Total” does not, revive interest in the classes according to the pollinators, of course, indicate that networks are completely sampled classes with bad boundaries, but with a clear centre, foci with respect to species and links, which no networks are of differentiation in different substrata, but leading to and never will be. It only refers to sampling width, i.e., convergence, a style [p. 44], and he goes on: The [earlier] all species involved in flower visitation are included irre- interest in demarcation and classification shall here be spective of taxonomic and functional affinity (Olesen & replaced by more positive elements [p. 48]. Jordano, 2002). All networks are also “temporarily cumu- Blossom classes. — The blossom classes of Fægri lative” (sensu Schoenly & Cohen, 1991), i.e., data originate & van der Pijl (1971) are characterized by a set of floral from one site only, but are, in general, sampled over an states, some of which may be adaptations to pollinators extensive period, most often one season. All networks or other flower visitors. They may be pooled into broader are described in detail in their individual references and classes of flower openness (Fig. 1) (à la Ramírez, 2003). their most important descriptors are tabulated in Olesen The former authors define a blossom as the pollination & Jordano (2002), Bascompte & al. (2003, 2006), Jordano unit. In most species flower and blossom are the same, & al. (2003, 2006), and Olesen & al. (2006). The networks but, for example, in the Asteraceae the blossom is both the originate from highly disparate sites all over the world. capitulum and the individual flower, whereas in others it However, lowland rainforest sites are unfortunately missing. is only a part of the flower such as in Iris, where it is one Sampling effort of flower-visitor species varied a lot in the third of the flower. Fægri & van der Pijl (1971) base their different network studies. Sorting of species according to classification of blossom classes upon what is termed “the their blossom types was based on information from floras, functional structure with regard to the … processes of pol- monographs, the World Wide Web, etc. In some plant spe- lination” (p. 99). They operate with three main categories: cies, scoring of blossom type was ambiguous. Asteraceae open (most angiosperm flowers); closed blossoms, e.g., species got a score of ½ in the dish category and ½ in the the flowers of Trollius europaeus (Hagerup & Petersson, tube category, and Apiaceae species got a score of ½ in the 1956), Linaria vulgaris (Newman & Thompson, 2005), dish category and ½ in the head/brush category. Informa- some Annonaceae species (Silberbauer-Gottsberger & al., tion on flower-visitor generalization level L was retrieved 2003; Gottsberger & Silberbauer-Gottsberger, 2006); and from the network database. L is generalization level and traps, e.g., the inflorescence of Arum (Knoll, 1926) and is measured as total number of flower-visitor species to a the flower of Ceropegia (Vogel, 1961). Here, we exclude plant species during the entire study period. In the dataset traps because of a shortage of data. The open blossoms a few plant species have several entries because they appear consist of inconspicuous (abiotically pollinated blossoms in more than one network.