Our already silent forests are dying.
Scientists have proved for the first time the alarming rates of decline in regeneration of native tree species that rely on kereru, or native pigeons, to disperse seeds.
In two forests, they have found regeneration has fallen by up to 84% over two years. However, they fear the problem could be far worse in other areas in which bird populations are much lower.
Canterbury university plant ecology professor Dave Kelly said researchers were taken aback by their findings. "It was a surprise for us how big the effect was and how long it was lasting for."
At one extreme, the researchers said regeneration of trees could fail completely, leaving forests full of dying adult trees and eventually lead to the collapse of mature forests.
Kelly, with Landcare Research ecologist Debra Wotton, studied native taraire and karaka trees in two forests less than 100 hectares in size.
Taraire rely exclusively, and karaka almost exclusively, on kereru to disperse their fruit, which are too big for other smaller birds to eat.
Although it was already believed that falling populations of kereru were having an impact on seed dispersal, it was the first time the link has been proved and assessed.
Kelly said both forests - one was Wenderholm Regional Park near Auckland and the other private land near Whangerei - still had kereru living amongst them.
"Kereru haven't disappeared. They are still there, we still see them, but they're less common.
"The decrease in kereru numbers is big enough to make a difference." He feared the situation could be worse in other forests in which predator control work was not undertaken.
"This is a better than average piece of New Zealand where people have tried to keep things working better and it's still not working particularly well."
The lifespan of karaka and taraire trees was thought to be hundreds of years.
"It's not a short-term crisis. It's one that we have got more notice on," Kelly said.
The research highlighted the importance of protecting native bird species. "It's not that we think these plants are going to disappear overnight, but it shows the impact on protected birds."
Conservation Department scientist Hugh Robertson said the research added weight to DOC's "integrated pest management approach" of targeting pests on conservation land.
"You just can't protect a forest by putting a fence around them and stopping the cattle and goats coming in."
However, DOC could manage only a "relatively small fraction" of the total conservation area in New Zealand, and gave priority to areas that were ecologically representative.
Private landowners, regional councils and the Animal Health Board also carried out valuable pest control work each year, he said.
Monica Awasthy, the co-ordinator of Wellington's Kereru Discovery Project, which focuses on conservation of the native wood pigeon, said the research was particularly timely as the United Nations had marked 2011 as The Year of the Forest.
"It's really driving home the message of how important kereru conservation is. We finally have the science to back it up."