Birdlife has been declining in the New Zealand bush for many years, mainly due to introduced predators such as rats and stoats. Professor Dave Kelly and Jenny Ladley of the University of Canterbury have been looking at how this affects plants that rely on the birds for pollination and seed dispersal.
Until recently, it was thought that the most important job birds did for plants in New Zealand was spread their seeds. Scientists knew that some native plants were pollinated by birds, but they assumed that, if the birds weren’t around, insects would do the pollinating instead. But no-one had actually tried to find out if this was true, so in the mid-1990s, scientists started to study bird pollination and were surprised by what they found – many more plants than they had realised were bird pollinated.
Dave and Jenny and others found that, for many native plants, low native bird numbers means not enough pollination. This means that not enough seeds are produced, so populations are declining. They have found that, with a few exceptions, insects and introduced birds do not make up for this lack of pollination. This result is different to what most ecologists thought was happening.
One of the bird-pollinated plants that Dave and Jenny have studied has the scientific name Rhabdothamnus solandri. It doesn’t have a common name, so it just gets called Rhabdothamnus (say it rab-doh-THAM-nus).
Rhabdothamnus is a forest shrub that grows in the top half of the North Island. It attracts birds with its orange flowers and lots of nectar. The main bird pollinators are bellbirds and stitchbirds. Silvereyes don’t have long enough beaks to pollinate the flowers, but they sometimes rip into the sides of the flowers and steal the nectar.
Dave and Jenny started their research with some surveys. They knew that, by about 140 years ago, bellbirds and stitchbirds had disappeared from where Rhabdothamnus grows around Auckland. However, these birds can still be found on islands such as Little Barrier and Tiritiri Matangi near Auckland and Lady Alice Island near Whangārei. Scientists working with Dave and Jenny counted and measured Rhabdothamnus plants on the mainland and on the islands. They found half as many young plants on the mainland as on the islands. They also watched plants and found that flowers on the islands were visited by birds much more than on the mainland.
Dave and Jenny also did experiments in which they hand pollinated some flowers using paintbrushes and left other flowers to be pollinated by birds. On the islands, there was little difference between numbers of seeds produced by hand and bird pollination. On the mainland, they got many more seeds from hand-pollinated plants than from ones left for the birds. This showed that, on the mainland, there weren’t enough birds around to do the pollination, whereas there were on the islands. Other experiments showed that seedlings would grow as well on the mainland as on the islands – the problem was not with seed dispersal, but with pollination.
Dave and Jenny have also studied other native bird-pollinated plants, such as mistletoes, kōwhai and tree fuchsia. They keep finding the same story – flowers are not getting enough pollination from birds. This results in fewer seeds, so not enough new plants grow to replace those that die. One way to help these native plants is to protect the birds that they rely on. This helps show how a forest ecosystem works – the birds and animals have complex relationships, and changing the numbers of one species has a knock-on effect on other parts of the ecosystem.
Nature of science
What scientists used to think about bird pollination in New Zealand was based on knowledge gathered from other parts of the world. It is only recently that scientific studies have actually collected data on New Zealand bird pollination, and these have changed ideas that were previously only assumed.