Study finds plant life affected by declining bird numbers

Researchers at the University of Canterbury and the University of Auckland are calling for stronger action to protect and increase native bird populations after finding proof that low bird numbers are causing a decline in plant life.

A study carried out by four UC researchers, working with Sandra Anderson from the University of Auckland, has found that the New Zealand shrub Rhabdothamnus solandri, or New Zealand Gloxinia, was slowly declining on the mainland because the endemic birds that used to pollinate it have largely disappeared.

The New Zealand Gloxinia is a bird-pollinated shrub that grows in native forests throughout the upper half of the North Island. It has orange flowers that are visited by three endemic New Zealand birds – tui, bellbirds and stitchbirds – and, more recently, by native silvereyes.

Professor Dave Kelly, an ecologist in the University of Canterbury’s School of Biological Sciences, said bird species are declining worldwide, raising concerns that the ecological services they provide, such as pollination and seed dispersal, may fail.

“This could have a cascading impact on biodiversity,” he said.

“But the possible failure of these ecological services is of particular concern in Oceania where bird species have suffered extensive extinctions from human impact. New Zealand has lost 49 per cent of its land bird species, which raises concerns about whether bird pollination and dispersal are adequate.”

Co-researcher Ms Anderson said the replacement of lost endemic species by introduced species does not necessarily provide the same function, with the recently self-introduced silvereyes usually robbing nectar from the flowers without pollinating.

The researchers’ findings, published in Science Express this week, were based on comparative studies of pollination activity by the three endemic bird species carried out on test sites in Auckland and Whangarei, on three island nature reserves (Little Barrier and Tiritiri Matangi near Auckland and Lady Alice near Whangarei) and at five adjacent mainland sites (two near Auckland and three near Whangarei).

Professor Kelly said the studies found that mainland plants, where only silvereyes and some tui are present, were poorly pollinated compared with the plants on the island locations where all three endemic species of birds remained abundant. This poor pollination reduced seed production on the mainland by 84 per cent and there were 55 per cent fewer juvenile plants per adult plant on the mainland.

Professor Kelly said the researchers had uncovered a “gradual cascading effect” of the decline of birds on the plant community.

And Ms Anderson said this loss is largely “invisible” until it is irreversible.

“Such cascading effects have been of concern worldwide, but are rarely properly documented and often hard to prove,” said Professor Kelly.

“Our example 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.”

The researchers’ paper, titled “Cascading effects of bird functional extinction reduce pollination and plant density” has been published in the 3 February issue of Science Express, which can be found online at http://www.sciencemag.org/journals.