The Impacts of Introduced Species for Birding

A significant contributing factor in the reduction in New Zealand bird diversity has been competition and predation by exotic introduced species. In the absence of natural checks and balances, introduced species can create massive problems as some of these species have demonstrated.

My “Big Five” of the predators and competitors are:

Stoats, Ferrets and Weasels

These are related within the genus Mustela

Rabbits which had been released in New Zealand in 1777 for meat and fur had ballooned into a major pest threatening pastures for stock. The following carnivores were released in a effort to control the rabbits.

Ferrets: introduced around 1870
Stoat and weasels: released about 1885

As often is the case when Man tries to “fiddle” with nature’s balance, the “hoped for” solution became a new and larger problem. The mustelids grew to a major treat to native wildlife and bounties were introduced to limit them. This problem continues today with New Zealand having the largest wild ferret population in the world. (See note 1 below)

The populations of stoats and weasels while not as dramatic re still significant. Ferrets and weasels tend to prefer the more open country, while stoats are more wide-spread. Only the stoat is found in Fiord land and Westland. One stoat tagged in the Eglington Valley was recaptured in Burwood Bush near Te Anau 2 days later having travelled nearly 60 km and across 2 major rivers.(See note 5 below) Stoats have had a significant impact on forest species such as young Kiwi, Kaka and Mohuoa.

Fortunately for birding, all three mustelids are absent from Stewart Island and other offshore areas.

Rats

Rats have always followed man in the voyages of travel and exploration. The Kiore, Rattus exulans was introduced by Maöri at the time of settlement in New Zealand. The other two species; Norway rat Rattus norvegicus and Ship Rat Rattus rattus came with the Europeans in the late 1770’s.

The biggest impacts of these species are in competition for food supplies due to their high reproductive rate and impacts on eggs and nestlings.

“In 1872 Von Fischer calculated a single pair of rats could in ten years produce 48,319,698,843,030,334,720 progeny” (See note 2 below)(Not bad for the long hand multiplication he had to rely on in that day!)

Ironically the mustelids appear to keep the rat populations in check.

The fluctuation of rodent populations (mice included) in the South Island are linked to beech forest mast (seed ) productivity. When the rodent population drops due to these variations in food supply the predators seek alternative prey: birds and reptiles. Hence their impact on birding and birding tours

Possoms

Bushtail possums are not a true possum, but a phalanger. They were imported from Australia to stimulate a fur industry in 1858. Estimates put their peak numbers at about 70 million, while in parts of their native environment they are considered endangered.

The damage they do is primarily to foliage and can devastate vast areas. (See note 3 below) It is estimated some 21,000 tonnes of vegetation per day is consumed. They consume about 15% of the annual foliage production of New Zealand's 15 most common plants.

Initially considered to be a plant-eating competitor, research has shown they can be direct predators of birds, such as Kõkako, eating both eggs and killing chicks. Perhaps up to 33% of nest predation on this rare species is attributed to possums. They have been documented attacking other native bird species such as New Zealand Wood Pigeon.

Possums, along with ferrets, threaten export markets due to their role as a vector in the Bovine TB cycle. Control efforts using indiscriminate 1080 poison threatens other species.

Research into reproductive control looks promising. (It has been suggested that visiting Australians should each take home a couple of possums in their luggage on their return and help the environments of both countries!) You have an excellent chance of seeing possums on a birding tour.

Wasps

Small in size, wasps are big in impact.

Four species have been introduced and established in New Zealand since the 1880’s and as recent as 1979. The most significant species is the German Wasp, Vespula germanica, accidentally introduced at the end of WW2. New Zealand has had some of the highest densities of social wasps in the world. The environmental impacts can be regionally significant without effective controls.

Wasps consume large quantities of native insects and honeydew. Honeydew is a product produced by a scale insect on native beech trees and a critically important food source for Kakas, Tuis, Bellbirds, and geckos. During the peak of the seasonal cycle wasps can virtually ‘wipe out” the honeydew supplies robbing the native species of an important food source. This in turn can have effects on breeding success. Wasps have been observed attacking and killing newly-hatched birds on nests. (See note 3 below)

Feral animals

Feral animals are also both competitors and predators of native birds.

Cats are obvious, with many leaving their “kills on the sill’ much to their owners' disgust. Cats have been significant predators of some nesting waders and shorebirds, as well as the forest species near homes and parks.

Pigs were introduced early to supply seafarers with a food supply. Captain Cook is documented to have let pigs loose in the New Zealand bush. Being omnivorous and destructive they can threaten both ground dwelling species and vegetation.

Goats don’t eat birds (well, not yet anyway) but can denude and trample areas of vegetation. Some on the biggest impacts have been on off-shore islands in the past. Efforts to control these pests have been successful in many localities.

Dogs, although they are Man’s best friend, can be very hard on flightless birds, and in particular the Kiwi. Kiwis are large enough to fend off cats and ferrets with their strong kicking legs but they stand little chance against an aggressive dog attack.

Dogs can sniff out the kiwi burrow easily and the little brush often placed over a burrow entrance is of little deterrent to a determined canine. Dogs are often abandoned in rural areas, and lost hunting dogs can be out several days before being recovered or finding home. They are a danger to kiwis.

The damage they can do is highlighted in Waitangi State Forest Massacre (See note 4 below)

“One tragic incident in 1987 involved a female German shepherd living wild in Waitangi State Forest. This dog killed an estimated 800 to 1,000 kiwi in just six weeks. Researchers first noticed the dog’s deadly activities only by chance. Thirteen of their radio-tagged kiwi were killed, all violently, and 10 were buried under leaf litter and soil. As the scientists regularly checked on the kiwi, the numbers of dead birds began to outweigh the numbers of live ones. With every week that went by, the horror mounted.

At first it was assumed that a pack of wild dogs must be loose in the forest. Finally, a single female was shot, and the massacre stopped. In the six weeks she lived in the forest, this one dog killed more than half of its kiwi. Most of them were not eaten; it appears she killed them for sport”.

Animals from the primary sector

Other Ruminants such as deer, sheep, and cattle form the back bone of the primary sector of New Zealand’s economy. Vast tracts of native bush have been cleared to provide for pastoral agriculture. However the effects of uncontrolled stock have been serious in fragile environments, in particular wetland habitats. Browsing, trampling and crushing of vegetation, nesting sites, and competition for critical food plants can all contribute to the negative effects on native species. Many of these factors often work together and the cumulative results are the adverse habituate loss and the diversity of New Zealand native bird species.

Note 1
Field Guide to New Zealand Wildlife, T Lindsey and R Morris

Note 2
Exotic Intruders, Joan Druett

Note 3
LandcareResearch Information sheets

Note 4
kiwirecovery.org.nz

Note 5
Personal communication with Peter Dilkes, DOC