A decline in bird life is having a negative knock-on effect for some of our native plant species, researchers from the Schools of Biological Science at Auckland and Canterbury Universities have found.
It has long been suspected that declining bird numbers could have wider effects on the ecosystem, especially for such services as pollination and seed dispersal. However, scientific proof of this cascading effect has been difficult to gather.
But now, 4 University of Canterbury researchers (Professor Dave Kelly, Jenny Ladley, Sue Molloy and Jon Terry), working with Sandra Anderson from the University of Auckland, have studied plant life on 3 pest-free island sanctuaries where endemic pollinating birds (bellbirds, tūī and stitchbirds) are abundant. When compared with mainland sites where bellbirds and stitchbirds are now absent, they found the loss of native birds has led to dramatically fewer seeds and seedlings in the bird-dependent plant Rhabdothamnus solandri (New Zealand gloxinia), a forest understorey native shrub.
Gloxinia nectar taken without pollination occurring
Gloxinia grows in native forests throughout the upper half of the North Island. It has orange flowers, which are traditionally pollinated by tūī, bellbirds and stitchbirds and, more recently but less efficiently, by native silvereyes.
The team studied the pollination activity by the 3 endemic bird species carried out on test sites in Auckland and Whāngārei, on 3 island nature reserves (Little Barrier and Tiritiri Matangi near Auckland and Lady Alice near Whāngārei) and at 5 adjacent mainland sites (2 near Auckland and 3 near Whāngārei).
Professor Kelly said the studies found that, on the mainland, where only silvereyes and some tūī are present, plants were poorly pollinated compared with the plants on the island locations where all 3 endemic species of birds remained abundant. This poor pollination reduced seed production on the mainland by 84%, and there were 55% fewer juvenile plants per adult plant on the mainland. This showed that regeneration of the plant has decreased on the mainland in the 140 years since bellbirds and stitchbirds vanished off the mainland, following the introduction of ship rats and stoats.
The researchers also observed that silvereyes are too small to handle the gloxinia flowers properly and often take the nectar from the flowers without pollinating the plant. This means that, although silvereyes are still relatively abundant on the mainland, they do not properly replace the function of the missing endemic species.
Although species of bird are declining worldwide, Professor Kelly says bird declines are of particular concern in Oceania where bird species have suffered extensive extinctions from human impact. “New Zealand has lost 49% of its land bird species, which raises concerns about whether bird pollination and dispersal are adequate for a whole range of plant species.”
Gradual cascading effect of bird losses on plants
Professor Kelly said their studied example of a “gradual cascading effect” of the decline of birds on the plant community is a very clear case, which serves as a warning. “It may be that similar slow plant declines as a result of the decrease in birds have begun elsewhere, but the relevant studies have not been done to detect them. We think it would be important to do these studies because early conservation action is much more effective while species are still widespread. Also, proof that bird losses have a negative impact on ecological services like pollination will help support stronger action to protect and enhance bird densities.” He says that there are various actions that can increase bird densities, such as eradicating pests from additional islands to create new bird reserves, and intensively managed ‘mainland islands’ like Waitākere’s Ark in the Park, where pest control allows bird densities to increase again.
Dr John Craig, a Research Associate at the University of Auckland, said New Zealand has known about the losses of native birds and other animals but “few people seem to anticipate the associated and inevitable outcome – the forests and ecosystems that we see today will be different for future generations”.
“What the authors have demonstrated for one small shrub is likely just part of a trend of cascading effects that will become increasingly apparent.”
Dr Eckehard Brockerhoff, a Senior Scientist at Scion, commented on the research, adding that, although it is not specifically covered, there is “a possibility that host-specific insects that may be associated with [the shrub gloxinia] are also affected by this, in a further proliferation of this cascading effect”.
“These are very important findings that will certainly receive much attention in New Zealand and overseas.”
Read more about this research in the journal Science.