Breeding system and pollen limitation in two supergeneralist alien plants invading Mediterranean shrublands

Many widely known invasive plants are well integrated into native plant–pollinator networks. Typically, these invaders have entomophilous flowerswhich are visited by a diverse array of pollinators. The type of breeding system and the role that pollination services play in the reproductive success of invasive plants have, however, received little attention. We studied the breeding system and pollen limitation of two entomophilous invasive plants, Carpobrotus affine acinaciformis and Opuntia stricta, in different Mediterranean coastal localities in north-eastern Spain. Both species are, to some degree self-compatible; however, because of frequent visitation, open pollination increased the seed set in both species byat least 50%.WhereasO. stricta showednopollen limitation, somepopulationsofC. aff.acinaciformishada lower seed set in open-pollinated flowers than in flowers where supplementary hand-pollination ensured out-crossing. This local pollen limitation inC. aff. acinaciformis could be due to the low efficiency of its visitors (mainly beetles) or its hybrid status. On the basis of previous studies onCarpobrotus sp. hybrid complexes, we suggest that the variability among sites in the seed set of open-pollinated flowers is caused by different degrees of hybrid introgression. Not withstanding, we found the C. aff. acinaciformis seed sets studied were higher than those reported in other regions. Further research is needed to assess the invasion potential of these hybrids in Mediterranean shrublands.