Effects of habitat fragmentation on pollination: Pollinators, pollinia viability and reproductive success

Abstract

1 For 3 years, I examined the effects of fragmentation on pollination of an endemic, euglossine bee-pollinated, epiphytic orchid, Catasetum viridiflavum, on 10 islands created during the construction of the Panamá Canal and at five sites in nearby large tracts of mainland forest. 2 I investigated the viability of pollinia over time, availability of pollinators and male and female reproductive success (measured as removal of pollinaria and fruit set, respectively) at different sites. 3 Viability of pollinia declined dramatically over time, although some remained at least partially viable for up to 60 days. Bees moved between trapping stations at different sites within a day, and viable pollen therefore could have been transferred between populations. 4 Although euglossine bees were equally frequent in fragments and continuous forest, those recorded on islands were visitors from mainland sites, and not island residents. 5 Neither male nor female reproductive success were significantly different between island and mainland forest types in 1997. The higher reproductive success in certain sites within both forest types throughout the flowering season could not be explained by variation in overall population floral display. Larger inflorescences had higher male and female reproductive success. 6 In 2 of 3 years, plants on islands had significantly lower fruit set than plants on mainland sites. Female reproductive success differed dramatically across years and sites. 7 The low fruit set in 1996 and 1998 on islands in comparison with mainlands, and the observation that bees are not island residents, conform with expectations of physical isolation of forest patches. In spite of the evidence of fragmentation in this system, patterns of pollinator behaviour and of reproductive success suggest significant connectedness among sites. This 3-year study shows that a single year study may not have highlighted the complexity of the system.