Pollination dynamics in a changing tropical forest landscape

Abstract

Little is known of pollination in the canopies of tropical trees or plants in which flowering is a rare event. I investigate reproductive success as a function of floral resources, pollinator guilds and the breeding habit in two tree species - Nothapodytes nimmoniana and Myristica dactyloides, and one monocarpic shrub Strobilanthes kunthianus, each representing a different forest type in the Western Ghats of India. Using three distinct approaches, I ask if pressures on the plant populations (from harvesting of plant resource or habitat degradation) have an impact on pollination outcomes. In the two canopies, the pollinator assemblage consisted of small diverse insects with some degree of specialisation (Coleopterans on Myristica and Dipterans on Nothapodytes). Bees were abundant in the montane grassland mass bloom of Strobilanthes, assuring 100% pollination efficiency. There was no pollen limitation in the three species, but in Nothapodytes, I provide evidence for a shift in insect diversity and a possible decrease in fruiting in populations where floral density is altered either naturally or from disturbance. An indirect measure of fruiting success in Myristica highlighted the influence of non-forest matrix and over-harvesting on mutualisms. In two species, I examine the evolutionary setting in relation to pollination dynamics: polygamous individuals seem to guard against decline in mates or pollinators in Nothapodytes and a set of adaptive floral traits render Strobilanthes protection from fruiting failure. I suggest that species with multiple pollinators may not be resilient to fluctuations, since generalists are just as sensitive to disturbances as specialists. Also, in forest canopies, there is a frequent overlap of functional guilds: a change in faunal assemblage could have a cascading effect on pollination processes. Habitat conservation is the key to maintaining connectivity between pollinator foraging resources that may be located in different forest types.