Robertson, A. W.; Kelly, D.; Ladley, J. J.; Sparrow, A. D. 1999. Effects of pollinator loss on endemic New Zealand mistletoes (Loranthaceae). Conservation Biology 13: 499-508.
Abstract
The endemic mistletoes Peraxilla colensoi and P. tetrapetala (Loranthaceae) have declined considerably in New Zealand since 1840, reputedly because of introduced herbivores, but coincident with a major decline in native bird densities. We show that at three South Island sites (Wakefield, Craigieburn and Ohau) there are now too few bird pollinators visiting the flowers to allow full fruit set. We studied pollination rates in P. colensoi at Wakefield and P. tetrapetala at Craigieburn over four flowering seasons, and P. tetrapetala in one season at Ohau. Supplemental hand pollination increased fruit production 1.25 - 5.3 fold at Craigieburn and Ohau but not at Wakefield. Excluding birds with mesh bags always decreased fruit set significantly at Wakefield, but had little effect at Craigieburn and Ohau. Bellbirds (Anthornis melanura) and Tuis (Prosthemadera novaeseelandiae) visited flowers significantly more often at Wakefield than at Craigieburn. A lack of pollen tubes in the style, not self-incompatibility or resource shortage, caused the low fruit production in unmanipulated flowers at Craigieburn. Thus, at the two P. tetrapetala sites (Craigieburn and Ohau) fruit set was chronically pollen limited, whereas P. colensoi at Wakefield was not pollen limited. Data from other Peraxilla sites also suggest pollination failure. This study suggests that the conservation of Peraxilla species will require maintenance of native bird populations. Tuis and Bellbirds are important pollinators and dispersers of many other New Zealand plants and the breakdown of these mutualistic relationships may have widespread consequences.