Effect of Carpobrotus spp. on the pollination success of native plant species of the Balearic Islands Eva Moragues, Anna Traveset * Spanish Research Council, Institut Mediterrani dÕEstudis Avanc ¸ats, IMEDEA (CSIC-UIB), Miquel Marque ´s 21, 07190 Esporles, Palma de Mallorca, Balearic Islands, Spain Received 31 March 2004 Abstract Invasive plant species are often considered as potential competitors of native species due to their usually greater capacity for col- onization and expansion, but we still have scarce information on whether invasives can also compete for pollination services with natives. In the present study, we hypothesized that the showy flowers of the highly invasive Carpobrotu spp. can compete with native species (Cistus monspeliensis, Cistus salviifolius, Anthyllis cytisoides and Lotus cytisoides) with which it shares habitat and flowering time, influencing pollinator visitation. To test this, we censused insects visiting the flowers of native species in the field and recorded the number of flowers visited in adjacent areas with and without the presence of Carpobrotus. We also assessed the presence of exotic pollen on stigmas of native species and evaluated its effect on reproduction. We detected potential competition for pollinators only in one native species (L. cytisoides), a facilitative effect in two other species (C. salviifolius and A. cytisoides), and a neutral effect in a fourth one (C. monspeliensis). Moreover, such effects appear not to be consistent in time. The presence of Carpobrotus pollen on native stigmas was almost negligible, and hand-pollination experiments showed that such exotic pollen does not interfere signifi- cantly with native pollen, not affecting seed set. Our results indicate that the role of the invasive Carpobrotus in promoting or con- straining the natural pollination dynamics is likely to have species specific effects on the native flora. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Invasive species; Competition for pollinators; Facilitation; Pollen interference; Balearic Islands 1. Introduction An increasing number of studies have demonstrated that invasive alien species are important competitors with native species for resources such as nutrients, water or light (e.g. Wardle et al., 1994; Weihe and Neely, 1997; Mack and DÕAntonio, 1998) or simply for space (New- som and Noble, 1986). Such competition often nega- tively affects the population growth of native species (e.g. Huenneke and Thomson, 1995; Randall, 1996; Williamson, 1996; Mack and DÕAntonio, 1998; Gordon, 1998; Zavaleta et al., 2001). In contrast, little informa- tion exists on the extent to which invasives compete for pollinator services with the native flora, with likely harmful consequences for the latter. The only three sys- tems studied so far consist of the invasive Lythrum sal- icaria and the native congener L. alatum (Grabas and Laverty, 1999; Brown and Mitchell, 2001; Brown et al., 2002), the invasive Impatiens glandulifera inducing pollinators from native species such as Stachys palustris in central Europe (Chittka and Schu ¨ rkens, 2001) and the invasives Cakile maritima and Carpobrotus spp. that coexist with the native Dithyrea maritima in sourthern California (Aigner, 2004). Many flowering plants de- pend upon pollinators for successful reproduction, and consequently the abundance and behaviour of pollen vectors may be an important ecological factor influenc- ing seed production (reviewed in Burd, 1994). 0006-3207/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocon.2004.09.015 * Corresponding author. Tel.: +34 971 611 718; fax: +34 971 611 761. E-mail address: atraveset@uib.es (A. Traveset). www.elsevier.com/locate/biocon Biological Conservation 122 (2005) 611–619 BIOLOGICAL CONSERVATION