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Mistletoes and mutualism

In some of the beech forests of New Zealand, bright red or yellow mistletoe flowers stand out in the summer. The colour attracts native birds, which drink the nectar and pollinate the flowers at the same time. Yet Professor Dave Kelly and Jenny Ladley of the University of Canterbury are finding out that all is not well with the plants and birds.

About 100 species of New Zealand flowering plants are pollinated by birds. Some birds and plants benefit from each other in a relationship called mutualism. Birds get food (nectar and fruit) from the plants, while the plants get pollinated and have their seeds spread to new places. When things are going well, the bird and plant partners thrive, but if one of the two has problems, the other might suffer too.

Many New Zealand birds have declined over the last hundred years, mainly because their habitats have been damaged or the birds have been eaten by introduced predators. Dave and Jenny wanted to know if this was affecting bird-pollinated plants, so they looked at endemic flowering plants called mistletoes. These hemiparasites grow on beech tree hosts, taking water and nutrients without harming them.

Explosive flowers

Two species of New Zealand mistletoe (Peraxilla tetrapetala and Peraxilla colensoi) depend on tūī and bellbirds for pollination. Their flowers only pop open when one of these birds comes along and tweaks them with its beak. Another mistletoe, Alepsis flavida, does not have these explosive flowers but still relies on tūī and bellbirds for cross-pollination.

Dave and Jenny study mistletoes at Craigieburn, near Arthur’s Pass, and other sites in the South Island. They go back to the same places every year to count and measure plants. They have found that fewer seeds are being made, but plant numbers don’t seem to have gone down. Possums eat mistletoes at other sites, but not at Craigieburn, though there are plenty of possums.

Pollination experiments

Dave and Jenny recorded how many Peraxilla flowers had been opened by birds. They found the number of pollinated flowers was low, so they set up experiments to investigate. They chose some flowers and treated them in three different ways:

  • Left some for birds to pollinate.
  • Covered some with bags so there was no pollination.
  • Pollinated some by hand, then covered them to keep birds away.

They came back later in the year to find out which of the flowers had produced seeds.

As expected, the unpollinated flowers did not have seeds. The hand-pollinated flowers produced more seed than the bird-pollinated ones, showing that birds were not doing a good job. Dave and Jenny suspected there just weren’t enough birds. They predicted that, if they could increase bird numbers, pollination would increase, so they trapped the stoats that normally eat bellbird eggs and chicks. This resulted in many more bellbirds in the study area.

When Dave and Jenny repeated the pollination experiment, they found that more birds did not result in more pollination or more seed at Craigieburn. This surprised them, so they are doing more work to examine the problem at other locations. They have also gone on to study other bird-pollinated plants such as tree fuchsia, kōwhai and Rhabdothamnus, and their evidence suggests that all seem to be suffering from low pollination due to low bird numbers.

Research into the status of several bird-pollinated plants suggests that they need help if they are to survive. Mistletoes can be helped by not cutting down the trees they live on, by not cutting mistletoe for Christmas decoration and by getting rid of the possums that eat the plants. For some species at least, increasing bird numbers should help increase pollination and numbers of seeds produced and so increase new plants.

Nature of science

The experiments with mistletoes rely on manipulating variables and comparing results with what happens when there is no manipulation. This includes comparing hand pollination with natural bird pollination and comparing pollination in an area where stoats have been removed with pollination in an area where stoats are not controlled.

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