Size constraints and flower abundance determine the number of interactions in a plant–flower visitor web

Abstract

The number of interactions with flower visitor species differs considerably among insect pollinated plants. Knowing the causes for this variation is central to the conservation of single species as well as whole plant Á/flower visitor communities. Species specific constraints on flower visitor numbers are seldom investigated at the community level. In this study we tested whether flower size parameters set constraints on the morphology of the potential nectar feeding visitors and thus determine the number of visitor species. We studied three possible constraints: the depth and width of tubular structures hiding the nectar (nectar holder depth and width) and the size of flower parts that visitors can land on (size of the alighting place). In addition we assess the role of flower abundance on this relationship. We hypothesized that the stronger size constraints and the smaller flower abundance, the smaller the number of visitor species will be. Our study of a Mediterranean plant Á/flower visitor community revealed that nectar holder depth, nectar holder width and number of flowers explained 71% of the variation in the number of visitor species. The size of the alighting place did not restrict the body length of the visitors and was not related to visitor species number. In a second step of the analyses we calculated for each plant species the potential number of visitors by determining for each insect species of the local visitor pool whether it passed the morphological limits set by the plant. These potential numbers were highly correlated with the observed numbers (r 2 0/0.5, p B/0.001).For each plant species we tested whether the observed visitors were a random selection out of these potential visitors by comparing the mean of the observed and expected proboscis length distributions. For most plant species the observed mean was not significantly different from the random means. Our findings shed light on the way plant Á/flower visitor networks are structured. Knowing the constraints on interaction patterns will be an important prerequisite to formulate realistic null models and understand patterns of resource partitioning as well as coevolutionary processes.M. Stang, P. G. L. Klinkhamer and E. van der Meijden, Institute of Biology Leiden, Leiden Univ., PO Box 9516, NL-2300 RA Leiden, the Netherlands (stang@ rulsfb.leidenuniv.nl).Plants pollinated by animals differ greatly in the number of interactions with visitor species, varying from one to more than hundred animal species (Moldenke 1975, Jordano 1987, Ellis and Ellis-Adam 1993, Waser et al. 1996. The mechanisms leading to this variation are still poorly understood (Johnson and Steiner 2000). Especially the importance of species specific constraints on this variation has seldom been investigated at the community level (Waser et al. 1996, Vazquez 2005. In order to illustrate the role of constraints, we will use traits that are thought to have an important impact on flower visitors even if they are rarely tested as a factor determining the number of visitor spe...

Biodiversity, Species Interactions and Ecological Networks in a Fragmented World

“…Given that network substructure is likely to be related to both body size and spatiotemporal context, future work needs to focus on the potential impact of habitat fragmentation on the robustness of the underlying structural mechanisms in food webs and mutualistic networks, although species traits (e.g. abundance) other than size might be more important in the latter (but see Stang et al, 2006Stang et al, , 2009.…”

“…Petchey et al, 2010;Vázquez et al, 2009). For instance, flower abundance can account for much of the variation in linkage level of plants in pollination networks (Stang et al, 2006; but see Olesen et al, 2008). The importance of abundance for the functional roles of species in antagonistic networks is well known, but remains largely unexplored in mutualistic networks.…”

“…The species traits that determine functional groups in ecological networks can differ within and between types of networks. In pollination networks, functional diversity defined by morphological traits might be vital for the persistence of diverse plant communities (Faegri and van der Pijl, 1979;Fontaine et al, 2006) and can constrain interaction patterns (Stang et al, 2006). In addition, functional groups can also be defined by behavioural traits (e.g.…”

Indirect interactions between crops and natural vegetation through flower visitors: the importance of temporal as well as spatial spillover

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Impact of Intensive Sylviculture System (Tptii/Silin) on Understory Bird Community at Triwira Asta Bharata Forest Concession, East Kalimantan

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