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- Materials and methods
To help expand knowledge of the relative importance of tropical rainforest in providing pollination services for crops, our research aimed to assess the identity, source and contribution to pollination of the pollen vectors of two subtropical tree crops growing in a diverse agricultural landscape, with scattered patches of rainforest and savanna remnants, in north Queensland, Australia. These crops were macadamia Macadamia integrifolia Maiden and Betche, a native of the subtropical rainforests of south-eastern Australia, and longan Dimocarpus longan Lour., a native of the mountain chain from Myanmar (Burma) to southern China and possibly also south-western India and Sri Lanka (Wong 2000).
Macadamia flowers in Hawaii are reported to be mainly pollinated by honeybees Apis mellifera L. (Urata 1954; Shigeura, Lee & Silva 1970) but in south-eastern Australia stingless bees, native Trigona spp., are also thought to be major pollinators (Heard 1994). Small bees, like Trigona spp., are potentially more effective pollinators of macadamia than honey bees (Heard 1994) and macadamia flowers are not thought to be wind pollinated (Heard 1993). In other areas of Australia, outside the natural range of Macadamia integrifolia, the lycid beetle Metriorrhynchus rhipidius (MacLeay), the flower wasp Campsomeris tasmaniensis Saussure and the halictid bee Homalictus sp. are capable of contributing to macadamia pollination (Vithanage & Ironside 1986). Longan flowers are thought to be pollinated by Apis and Trigona bee spp. and possibly also by hoverflies (Syrphidae) (Boonithee et al. 1991) but the relationship between bee visitation and longan pollination has not been verified.
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- Materials and methods
Our results demonstrate that pollination rates in both macadamia and longan, as represented by initial fruit set, are enhanced substantially by proximity to tropical rainforest. Our study is the first to document that autogamous self-pollination, wind and insects can all contribute to longan pollination but that insects are the main pollen vectors in longan orchards. The positive relationship between the number of stingless bee visits to longan flowers and initial fruit set suggests that stingless bees are the major pollinating insects of longan flowers. The higher visitation of stingless bees to longan flowers near rainforest suggests that rainforest is a source of these bees.
Our findings are less clear-cut for macadamia. Even though we confirmed that macadamia flowers benefit from pollen transfer by vectors, and demonstrated that macadamia pollination by vectors was higher near rainforest, our design did not distinguish between insect and wind pollination. There were more honeybee visits to macadamia flowers in orchards near rainforest, which supports a honey bee-mediated advantage for orchards near rainforest, but there was no relationship between honeybee visitation and fruit set on a per raceme basis. This lack of agreement between the two analyses could be the result of the effect of pollen vectors that we did not observe (and the honeybee pattern is therefore weak) or (if honeybees are important) to a poor congruence between honeybee visits estimated at the site level and fruit set measured at the raceme level. The short period of the total flowering time that the racemes were observed could be responsible for either possibility.
The absence of stingless bees in macadamia orchards during our observations was at first surprising given their apparent importance in macadamia orchards in south-east Queensland (Heard 1994) and their occurrence during pre-study surveys. However, we later discovered that the orchards were sprayed with chemicals to control pests in the period between the pilot surveys and our observational study. Honeybees may have been more resistant than the stinglessbees to the pesticides used. This may explain why honeybees were the only pollinating insects remaining when we carried out our observations. We do not know whether stingless bee numbers recovered in time to pollinate the flowers in our study.
The amount of wind-blown pollen reaching receptive macadamia flower stigmas in our study may have been greater than anticipated. Wind may be a particularly important agent of cross-pollination on the Atherton Tableland, where strong south-easterly winds prevail from July to September, when macadamias are flowering. To our knowledge no-one has tried to separate the contribution of wind-blown macadamia pollen from pollen transfer by other agents, although this has been attempted for other crop species such as oilseed rape or canola Brassica napus L. (Hayter & Cresswell 2006). Urata (1954) suggested that the role of wind-blown pollen in macadamia pollination should not be discounted but most experiments carried out since then have not taken wind into account. Bags with mesh smaller than the diameter of macadamia pollen, used to exclude insects, also exclude wind-blown pollen, but the combined effects of excluding both have been attributed solely to the exclusion of insects (Vithanage & Ironside 1986).
Our observations did not allow for the possibility that nocturnal insects or vertebrates play a part in the pollination of these crops, but it has already been established that night-flying insects (and by default bats) have a negligible effect on the pollination of macadamia flowers (Vithanage & Ironside 1986; Heard 1993). The role of these pollinators has not been investigated for longan but as pollination of longan flowers is reported to be most effective between 08:00 and 14:00 (Wong 2000) it is unlikely that nocturnal pollinators contribute greatly to longan pollination.
Birds may make a substantial contribution to macadamia pollination (Heard & Exley 1994) but their role has yet to be quantified. In the macadamia orchards we surveyed, birds were only observed visiting flowers during the first hour of observations. The few birds observed targeted flowers at the top of the canopy several metres above the flowers used for our study. The hole size of the mesh used in the coarse mesh treatment applied to longan flowers was too small for vertebrate pollinators like birds to pass through. The fact that their exclusion by the coarse mesh treatment did not significantly reduce pollination rates, compared with the frame-only treatment, suggests that vertebrate pollinators did not contribute to longan pollination.
It seems clear that rainforest acts as a reservoir for stingless bees that enhance longan pollination, and it is likely that honeybees are also harboured by rainforest and that these can be important pollinators of macadamia. More detailed studies that separate the contributions of potential pollen vectors are required before the vectors responsible for the enhancement of pollination in macadamia orchards near rainforest can be determined. The importance of these pollinators might vary between years such that they are of even greater significance when other modes of pollen transfer are not adequate (Waser et al. 1996; Kremen, Williams & Thorpe 2002).
Our findings have important ramifications for crop management and rainforest conservation. Rainforest is commonly perceived as a source of pest insects that damage crops but the role of rainforest as a source of beneficial insects is rarely recognized (Blanche et al. 2002). Our study and others (Klein, Steffan-Dewenter & Tscharntke 2003a,b,c; Ricketts 2004; Blanche & Cunningham 2005) underscore the benefits of rainforests. This new perspective draws attention to the advantages to be gained from policy and management practices that allow rainforest vegetation to remain near crops, or be planted within crops, to improve visitation by beneficial insects. Such strategies have the potential to improve crop yields while conserving tropical rainforest.