Proximity to rainforest enhances pollination and fruit set in orchards

Abstract

Summary Tropical rainforests are potential reservoirs of insects that could enhance crop pollination, but only a few instances of the provision of such services by tropical rainforest insects have been reported. Our field study aimed to determine the relative importance of such insects to the pollination of macadamia Macadamia integrifolia and longan Dimocarpus longan crops on the Atherton Tableland, north Queensland, Australia. We quantified initial fruit set, a measure of pollination success, in treatments designed to assess the relative importance of the possible modes of pollination. The treatments were applied in orchards that varied in distance from rainforest, in order to compare the effects of the contrasting pools of available pollen vectors. We also recorded the insect species present and estimated the number of visits each made to flowers in crops near and far from rainforest. For both crops there was an interaction between pollination treatment and distance from rainforest. Maximum fruit set was only achieved when pollen vectors had access to flowers and orchards were close to rainforest. Exclusion of pollinators near rainforest reduced initial fruit set to a greater extent than exclusion of pollinators far from rainforest. We confirmed that pollen transfer in macadamia is by autogamous self‐pollination and by pollen vectors, but our design did not distinguish among pollen vectors. The only abundant insects in macadamia orchards were honeybees Apis mellifera. There were more honeybee visits to macadamia flowers in orchards near rainforest than far from rainforest, but we detected no relationship between honeybee visits and initial macadamia fruit set in our sample of observations on a per raceme basis. More detailed studies are needed to identify the pollen vector responsible for enhanced pollination of macadamia near rainforest. We established for the first time that pollen transfer in longan is by a combination of autogamous self‐pollination, wind and bees. Longan flowers were visited by stingless bees and honeybees but only stingless bees had a positive relationship with initial longan fruit set and higher visitation rates near rainforest than far from rainforest. This suggests that enhanced pollination in longan near rainforest resulted primarily from a more abundant supply of stingless bees from the rainforest. Synthesis and applications. By demonstrating that tropical rainforest can act as a reservoir of pollen vectors that benefit crops, our study highlights the existence of a largely unrecognized resource available to agriculture. At the same time our results make a significant contribution to the growing database of studies that underscore the importance of tropical rainforest conservation. Policy and management aimed at sustainable use of this resource would satisfy the goals both of agriculturalists, to improve crop yields, and conservationists, to conserve tropical rainforest.