Effects of mixed-species pollen load on fruits, seeds, and seedlings of two sympatric columnar cactus species
Two sympatric species of columnar cacti on Curaçao, Netherlands Antilles, share bat pollinators, overlap in flowering phenology, and are floral homologues. Leptonycteris curasoae curasoae and Glossophaga longirostris elongata (Chiroptera: Glossophaginae) visit flowers of Stenocereus (syn. Ritterocereus) griseus and Subpilocereus (syn. Cereus) repandus on the same nights; these visits may promote interference competition between cactus species. I studied the effect of heterospecific (mixed) pollen loads on fruit and seed set, fruit and seed size and mass, germinated seeds at 5 weeks, and seedling survival at 7 months in relation to hand pollination with intraspecific pollen and natural pollination. Hand pollination seemed to limit pollen loads available for pollination. Under these conditions, natural pollination tended to produce the most fruits and seeds, and the largest fruits (but lightest seeds); mixed pollination was the least effective treatment (fruit set was significantly greater for St. griseus in natural than mixed pollination; fruit volume was greater in natural than mixed pollination for S. repandus, and seed number and fruit mass were larger in intraspecific than in mixed pollination). Aspects of natural pollination, possibly repeated visits, compensated for the negative impact of interspecific pollen loads under pollen limitation, with positive impacts on the carrying capacity of cacti for frugivores. Seed mass from natural pollination was negatively correlated with seed number only for St. griseus. Germination success was not correlated with seed mass, but seedling survival at 7 months was for S. repandus. The two species do not seem to compete through pollen interference when pollinator visits are relatively frequent.