Pollen transfer from invasive Carpobrotus spp. to natives – A study of pollinator behaviour and reproduction success
Introduction
Biological invasions are recognized as one of the main drivers of biodiversity loss, and virtually all ecosystems are affected by accidentally or intentionally introduced species (Heywood, 1989, Wilcove et al., 1998, Levine and D’Antonio, 2003). Invasive plants are well known to change community composition and ecosystem processes (e.g. Parker et al., 1999, Brooks et al., 2004) but the mechanisms behind these changes are relatively poorly understood (Levine et al., 2003). Moreover, when mechanisms have been studied, the main focus has been on direct competition for abiotic resources such as space, water and nutrients (Levine et al., 2003). More studies of indirect effects and the altering of ecological relationships such as breaking of native mutualisms are needed for further understanding of the processes operating in invaded systems (Traveset and Richardson, 2006, White et al., 2006). Invasive plants are often visited by native pollinators (e.g., Memmot and Waser, 2002), suggesting that they may interfere with native plant species by using their pollinators. Negative effects of invasive plants on pollination success (reviewed in Bjerknes et al., 2007) can be mediated by a reduction in pollination quantity if the invasive plant is more attractive to pollinators (Chittka and Schürkens, 2001, Brown et al., 2002, Ghazoul, 2004, Moragues and Traveset, 2005, Larson et al., 2006, Totland et al., 2006), or by a reduction in pollination quality if interspecific pollen transfer (IPT) occur. Studies have shown IPT between natives to cause a reduced female reproductive success (e.g., Galen and Gregory, 1989, Caruso and Alfaro, 2000; but see Waites and Ågren, 2004), but only a few studies have investigated IPT between invasive and native species (Grabas and Laverty, 1999, Moragues and Traveset, 2005, Larson et al., 2006). IPT can affect female reproductive success by three main mechanisms: (i) chemical interference (allelopathy) when heterospecific pollen exude chemicals inhibiting pollen germination, pollen tube growth, stigma receptivity, or ovule development (Sukhada and Jayachandra, 1980, Thomson and Andrews, 1982, Murphy, 2000), (ii) mechanical interference when conspecific pollen is prevented from reaching the stigma surface (Rathcke, 1983, Waser and Fugate, 1986, Galen and Gregory, 1989) and (iii) hybridization when the number of ovules available for legitimate pollination is reduced through fertilization with other species (Ellstrand et al., 1999).
Pollinator behaviour is influenced by absolute and relative abundance of flowering species (e.g., Stephens and Krebs, 1986, Rust, 1990, Rasheed and Harder, 1997), by the abundance of different pollinator species (Inouye, 1978, Bowers, 1986), and by weather conditions. Pollinator populations also exhibit strong dynamics at a local scale (Roubik, 2001). Temporal variability in number of invasive pollen grains deposited on native stigmas, as found by Larson et al. (2006), is therefore highly expected. The amounts of invasive pollen transferred to native stigmas depend on the frequency of pollinator switching from the invasive to the native, the amount of invasive pollen adhering to switching pollinators, and the number of pollen grains actually transferred to native stigmas during each visit. In the present work we monitor pollen transfer from the invasive Carpobrotus spp. to three native species, and we examine the effects of invasive pollen on seed production and seed quality in a field experiment.
Section snippets
Study species and sites
Carpobrotus edulis and C. affine acinaciformis and hybrids between them (hereafter referred to as Carpobrotus spp.) are South African and belong to the Aizoaceae. The taxa are widespread around the Mediterranean basin and considered a threat to several plant species on Mediterranean islands (e.g., Suehs et al., 2001, Suehs et al., 2005, Hulme, 2004, Vilà et al., 2006). The showy flowers contain hundreds of stamens and produce large amounts of pollen (Blake, 1969). Three native species sharing
Pollinator movements
A. aestivus was visited by six, D. hirsutum by 15, and H. stoechas by 19 pollinator species, and total number of visits was 59 for A. aestivus, 538 for D. hirsutum and 478 for H. stoechas (Appendix A). Only a few of the pollinator species were observed to move directly from Carpobrotus spp. to the natives (Table 1). For all three plant species, any switching pollinator species was always arriving more often from a conspecific individual than from a Carpobrotus spp. flower (Wilcoxon matched
Discussion
There is a low risk that transfer of Carpobrotus spp. pollen will reduce seed production in the native species, since very low amounts of invasive pollen were found on native stigmas. However, our study shows that such impact could potentially become a problem because invasive pollen adhered on pollinators, pollinators moved from the invasive to natives, pollen was transferred to native stigmas, and invasive pollen affected seed production in one of three species. Pollinator behaviour change
Acknowledgements
Thanks to C. Ornosa and D. Gibbs for support with identifying insect species, to E. Descals and E. Rodríguez for introduction to microscope photographing, to A.-M. Karjalainen for assistance in the lab, and to P. Börjesson and two anonymous reviewers for valuable comments which improved the manuscript. This research was supported by a post-doc grant to A. Jakobsson from the Swedish Council for Forestry and Agricultural Research.
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