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Protecting our precious places

Flowering plants: Reduced level of pollination

An update from the Landcare Research Sustaining and restoring biodiversity programme funded by the Foundation for Research, Science and Technology.

Bright flowering trees and shrubs that form an intrinsic part of our landscape such as flax (harakeke), kowhai, mistletoes, rata and tree fuchsia (kotukutuku) are bird pollinated.

Birds are attracted to the flowers and pollination occurs when they carry pollen from flower to flower as they seek nectar.

Below: A tui pollinating scarlet mistletoe Peraxilla colensoi. Two native Peraxilla species need tui and bellbirds to open their specialised ‘explosive’ flowers. The large red flowers spring open only when tweaked by these birds and cannot be pollinated if they don’t open. Photo: Jenny Ladley

Tui pollinating mistletoe Photo: Jenny Ladley
A research team from Canterbury University has been studying the consequences of the reduced number of tui, kereru, bellbirds (korimako) and silvereyes (tauhou) on flower pollination.

Dave Kelly from Canterbury University says the aim is to determine the level of reduced pollination in plants such as scarlet mistletoe Peraxilla colensoi, yellow mistletoe Alepis flavida, tree fuchsia Fuchsia excorticata, and kowhai Sophora microphylla.

'Our research used three pollination treatments at flowering sites,’ says Dave. ‘We bagged some flowers to prevent them from being visited by any pollinators. We tagged other flowers and followed them to see if they were naturally pollinated. The third treatment was pollinating by hand which represents perfect attention from pollinators.

‘Some of the bagged flowers still set seed even though they couldn’t be visited. This self-pollination means birds are not always needed in some species. By hand, we got good pollination.

‘But when we looked at the fruit set for naturally visited flowers on the mainland, the levels of fruit set (i.e. seed production) were well down on the levels seen with hand pollination. In other words, not enough birds were visiting flowers to let the plants make all the seeds they were capable of, and therefore lack of birds was reducing seed production.’

Dave says bird pollination is not working well on the mainland with reduced seed production for a lot of bird pollinated plants. ‘For Peraxilla, the amount of seed production is reduced by about 50%.

‘In Northland, work by Sandra Anderson at the University of Auckland shows that the seed production for New Zealand gloxinia Rhabdothamnus solandri (taurepo) is down by about 90%. However, very close by on Little Barrier Island which still has high bird numbers, the seed production is fine in Rhabdothamnus.’

Increasing the number of bird pollinators

There is a conservation focus on kereru throughout the country as they are a critical species for dispersing seeds of trees with large fruit such as miro, puriri, tawa and tairare.

‘Our work shows that we also need to focus on birds that are widespread and not endangered like tui and bellbirds. They really matter,’ Dave explains.

‘Tui and bellbird densities are now low in some areas such as Canterbury and Northland respectively. Nobody knows why bellbirds are absent on the mainland north of Hamilton. In the 1950s and ‘60s, tui were common on Banks Peninsula but there’s practically none there now. Is it because of a shortage of food?

‘A benefit of QEII covenants is that they keep the function of forests going,’ says Dave. ‘They provide an environment where both flowering plants and bird pollinators thrive.

'To help provide food sources for bird pollinators, plant flowering species such as fuchsias, kowhai and flaxes, plant natives that flower through the winter, for example, five-finger and puriri, and establish mistletoes.'

Dave adds that predator control also increases bird densities. ‘An experiment we did at Craigieburn, a site with reduced mistletoe pollination, measured the effects of stoat control during the nesting seasons of 2000/01 and 2001/02.

‘Trapping stoats significantly reduced stoat numbers in the area compared to a nearby non-treatment area. Bellbird fledging increased by 10-fold and bellbird density increased by 80%. The good news is that it proved surprisingly easy to boost bellbird densities.

‘Unfortunately, mistletoe pollination and fruit set did not increase. The reasons for this are not clear, but we are now exploring plant-bird interactions in places with very high bird densities to try to understand how bird densities interact with mistletoe flowering density to determine pollination rate.’

For more information on the research ...

Find out about covenants protecting our bright flowering species ....

Open SpaceTM Magazine No. 72, March 2008 © QEII National Trust

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