Sparrows and rats and mice all evolved with the spread of seed producing plants throughout the world, and they are adapted to eat seeds. Of course, so are humans – our principle foods are grass seeds, wheat, rice, corn and barley. Like rats, we are omnivorous, and enjoy a few other items too, even on the Barrier. Rats have long incisor teeth, which grow continuously throughout their life, so they’re not worn away by gnaw, gnaw, gnawing. Rat’s molars grow too, and the age of a rat can be determined by the amount of wear on them, but few live long enough to need a dentist.

Kiore (Rattus exulans) first came to New Zealand c.750 years ago or perhaps much earlier, on board the first Maori waka. The demise of many of our large ground dwelling invertebrates (eg. large wetas and ground snails), small ground dwelling birds (ground wrens), bats, lizards and frogs, can be attributed to the spread of these predators into our pristine forest ecosystems (1, 2). Hundreds of years later, when Norway rat (Rattus Norvegicus: 1790s) and ship rat (R. rattus: 1850’s) arrived here on European ships, a further wave of extinctions occurred. Indeed, the first rat – kiore – was one of the species which suffered in competition with its larger and more belligerent cousins, and kiore populations became more or less restricted to off-shore islands. Rats not only caused declines in pollinating and seed-dispersing birds, such as kereru, tui and bellbird, but they also ate seeds and seedlings of forest trees. Even dry kauri and beech seeds were eaten, sometimes in large amounts (2). Although all three rats are primarily herbivorous (vegetarian), only recently have their continuing and disastrous effects on our flora been demonstrated (3; Fig 1).

The effects of rats are selective, because the big-seeded trees, such as nikau palms, taraire and puriri are targeted. These trees are common, and very long-lived, so the effects of rats gnawing on their seeds is hardly obvious in our human time span – but it is happening never-the-less. Effects are often clearest on islands where tree populations are small and regeneration failure cannot be reversed by seed influx from elsewhere. Recently, by comparing islands with and without rats, and using rat-exclosure plots, Campbell and Atkinson (4, 5) have demonstrated that rats have changed the vegetation on some off-shore Islands. Kiore have substantially reduced recruitment of karo (Pittosporum crassifolium), milk tree (Streblus banksii) and maire (Nestegis apetala) on off-shore islands (Fig 2).

Reduced seed survival of nikau palm (Rhopalostylis sapida), puriri (Vitex lucens) and kohekohe (Dysoxylum spectabile) has also been demonstrated. In fact 24 native tree species are now known to be vulnerable to rats on islands, and a further 11 species are probably rat-affected.

Both nikau palms and puriri trees are rare on Tiritrii Matangi island, compared to Great Barrier. In fact only two puriri trees survived until the recent revegetation program started. Rat numbers fluctuate greatly from years to year, and it is known that kiore reached plague proportions on that island when farming stopped and the growth of rank grass provided an abundant food source. Numbers reached 200 rats per hectare in the grassland, and 120 per hectare in the bush patches – bark was gnawed off shrubs and seedlings decimated (6). It is quite possible that plagues such as this, combined with persistent high abundance, almost eliminated nikau and puriri on Tiri.

In a study on Great Barrier Island, Deline Samaka (7) demonstrated that there were significantly more nikau, milk tree and taraire seedlings in rat trapped areas at Windy Hill, than in non-trapped areas of the same vegetation type. The same was true for nikau and puriri at Glenfern sanctuary (Fig 3). Although Samaka’s work is based on small sample sizes, it is well supported by other data, and clearly implies that seed predation by rats is slowing the successional sequence from kanuka to broadleaf forest. Thus removing rats in these areas has a double whammy effect – it not only protects the seed pollinators and dispersers, but it also ensures that dispersed seeds stand a chance of becoming trees.

John Ogden