Tim Blackburn and Phill Cassey: Exotics pose threat to our local dawn chorus

COMMENT

Pamela Wade bemoaned the poor quality of New Zealand's native songbirds, comparing them unfavourably to the "heart-lifting glory" of the exotic species introduced by the pioneers in the 19th century.

When gardeners could augment their beds with the best that other countries had to offer, why, she asked, should we not be able similarly to enhance our parks and gardens with pretty and tuneful birds from around the world?

Perhaps Wade has taken to heart the conservation message that "biodiversity is good" and thinks that releasing yet more exotics would enhance New Zealand's backyards - but her views are, at best, silly and outdated and, at worst, irresponsible.

If the local dawn chorus is so lacking, has she asked herself why? One reason could be that so many of the original indigenous songbirds have become extinct.

What the locals woke up to 250 years ago, before the pioneers of the 19th century, we will never know. We can gain some clues, however, from the writings of the early explorers. It was said that as Captain Cook sailed up the coast, the noise of birdsong from the forests was deafening.

Quantity does not imply quality, of course, but clearly the chorus that greets the dawn today is greatly depleted of native singers.

Wade might also consider why so many native bird species have become extinct. In almost every case, the answer is the effect of exotic species - species introduced by the hand of humanity. The worst of these invaders is humanity itself, which has hunted its way through moa, adzebills, rails and ducks - species that were once found nowhere else in the world, and are now found nowhere.

Humans like to hunt large, meaty animals, but it is the small songbirds that are the main contributors to dawn choruses. These have suffered just as badly since humans reached New Zealand, thanks to the efforts of rats, cats, stoats, ferrets, weasels, possums and hedgehogs. No one will now walk their dogs to the sound of piopio or huia, or any of the eight species of songbird that we know fell extinct before Wade was born. Worldwide, exotic species have been responsible for thousands of bird extinctions.

Of course, Wade was not advocating the introduction of mammalian predators. But species are not just eaten to extinction. Natives can be out-competed by exotics.

In autumn, flocks of starlings that strip kahikatea of their fruit deprive native species of an important winter food source. Exotics may bring diseases to which the natives have no immunity.

Avian malaria carried to the Hawaiian Islands in the blood of introduced birds means that most native songbirds, which have no natural immunity, are now restricted to the high altitudes that the mosquito vectors of the disease cannot tolerate.

Perhaps the budgies that Wade is so keen to see brightening her garden will bring the disease that finally finishes off the kakapo, or will eat kakariki out of house and home? Maybe not, but what she advocates is effectively to play a game of Russian roulette with her national heritage. Indeed, with global heritage, for while exotic species may raise local diversity, their effects on unique and distinctive biotas, such as that of New Zealand, decrease the diversity of the world as a whole.

Wade's analogy with garden plants is also poorly chosen. As the Department of Conservation battles gorse, broom and pine, who can say which of the species in her garden will spread to become New Zealand's kudzu or prickly pear?

Controlling exotic pests costs the global economy billions of dollars a year. With good reason, Customs operates strict controls on what is imported - once the can is open, it is difficult to get the worms back in.

Why should birds get special treatment? Local farmers would not be too delighted when flocks of the exotics whose virtues she extols descend on their crops, as starlings, sparrows and finches already do.

Wade's arguments are, in the final analysis, based solely on her preference for some sounds over others. As scientists, we love the music that the tui produces, and know researchers who can tell individual males apart by their unique songs.

We could, likewise, contrast the beauty of the bellbird, fantail, and grey warbler with the asthmatic wheeze of the yellowhammer, or the monotonous chip of the house sparrow, but that would be as pointless as arguing for the primacy of Bach over Mozart.

The songs of all these species are beautiful, and a failure to find them so is more indicative of a deficiency in the ear of the beholder than in the voice of the singer.

Moreover, these singers and the chorus they produce are found in only one place in the world. People come here to experience the beautiful environment, and that includes special creatures like the tui. To find blackbirds and chaffinches in the forests is as appealing as finding a branch of McDonald's tacked on to the Taj Mahal.

Wade should move on from 19th-century attitudes to the introduction of exotics and, celebrate all that is distinctive about New Zealand.

* Drs Tim Blackburn and Phill Cassey are scientists studying biological invasions and extinctions, and the relationship between the two.