Carpenter bees are considered important pollinators on islands (e.g., McMullen 1989, 1993). Because of their resourceful foraging and nesting behavior (Jackson & Woodbury 1976, Roubik 1989, Scott et al. 1993), they may be extremely resilient to disturbances and unpredictable food sources. With a lack of a patterned flowering period and low availability of flowers per day, the cactus benefits from an asymmetrical relationship with Xylocopa whose populations would not be sensitive to reduced flower densities of a particular species.
Thus, the pollination biology of P. royenii contradicts the hy- pothesis that Neotropical chiropterophilous cacti are strictly polli- nated by bats. In addition, pollinating activity of carpenter bees may be affecting variation in floral characteristics. Our data and that of others suggest that the chiropterophilous pollination syndrome in columnar cacti appears to break down in extratropical regions and islands subjected to periodic, severe disturbances.