| Abstrakt: |
The flowers of two species of threatened New Zealand mistletoes (Peraxilla tetrapetalaandPeraxilla colensoi, Loranthaceae) have explosive buds that do not open unless force is applied by birds or two species of native short-tongued bees. Opened flowers are visited by a variety of birds and insects. Although both species ofPeraxillaconform to a pollination syndrome of ornithophily, bees may be effective alternative pollinators. We investigated the effectiveness of bees and birds as pollinators ofP. colensoiat one site andP. tetrapetalaat two sites in the South Island. Bees and other insects outnumbered birds as flower visitors at all three sites. By excluding birds with wire cages, we showed that two bee species regularly open flowers ofP. tetrapetala, but only rarely open flowers ofP. colensoi. Few pollen grains were deposited when either birds or bees opened buds, so opening buds was not by itself sufficient for adequate pollination. Instead, pollen continued to accumulate over the next 6 or 7 days, even inside cages that excluded birds. Both populations ofP. tetrapetalawere regularly pollen-limited, but in different ways. At Ohau, opened flowers gained enough pollen to produce seeds, but many buds were not opened and hence failed to set seed. In contrast, at Craigieburn, nearly all buds were opened, but many of these did not receive enough pollen. These results demonstrate that native bees can partially replace birds as pollinators of mistletoes, despite their apparent ornithophilous syndrome. Ongoing reductions in New Zealand forest bird numbers means that the service bees provide may be important for the long-term future of these plants. [ABSTRACT FROM AUTHOR] |
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