Blown in the wind or border slippage?
From Biosecurity 69, August 2006.
What natural dispersal of exotic species to New Zealand has to do with biosecurity
By Craig Phillips, Helen Townsend and Cor Vink AgResearch/Better Border Biosecurity
New Zealand’s geographical isolation is an enormous help in defending our national borders against unwanted organisms, but do we overrate the protection it affords us? Most new incursions are blamed on breaches of our border biosecurity systems – typically, a suspicious eye is cast at the half-million shipping containers we import each year, and MAF cops flak for not adequately managing the risks involved. But another entry pathway, natural wind-borne dispersal to New Zealand, might sometimes be involved. What do we really know about organisms that arrive in the wind, thus evading even the very best pre-border and border biosecurity systems?
The answer is: surprisingly little. The pioneering work of Fox (1978) provided compelling, circumstantial evidence that moths and butterflies frequently cross the 2,000 km of ocean between Australia and New Zealand, perhaps as often as 20 times per year (Tomlinson 1973). Many appeared to be in excellent physical condition, showing little evidence of having completed a two-to-three day international journey. Close et al (1978) indicated that other insects, fungal spores, seeds and pollen can also readily cross the Tasman. However, the light-trapping methods used by Fox would have sampled only a small subset of all potential trans-Tasman travellers. In windy conditions, light traps only work well for the strongest fliers amongst light-attracted organisms – such as larger moths and butterflies. They do not work at all for species that cannot or do not respond positively to light.
The prevailing impression that moths, butterflies and fungal spores are the only organisms likely to cross the Tasman in the wind is, therefore, based on a biased, though extremely valuable, set of observations (see sidebar on next page for examples of some other taxa that probably naturally dispersed to New Zealand). Moreover, the perception that wind-borne immigrants ‘blow over’ in the same way that smoke from Australian bush fires sometimes tinges our skies suggests we are badly underestimating the sophisticated adaptations possessed by many organisms for dispersing in the wind.
Evolved for air travel
Most aerial travellers do not become airborne by accident. Millions of years of evolution has equipped them with adaptations enabling them to detect weather conditions suitable for dispersal, become airborne, stay aloft, modify their altitude and direction relative to the wind, and survive long periods aloft.
Night-flying moths will continue flying during the day if they find themselves over water, while wingless mites can adjust their body posture to modify their rate of ascent and descent through the air, and thus their dispersal distance. The greasy cutworm has become magnificently adapted to migrate northwards for over 1,600 km each northern spring, then return against the prevailing winds the following autumn (Showers 1997).
The diversity of windborne organisms
Many insects are capable of flying long distances (e .g . Farrow (1984) recorded 24 species in insect orders such as Odonata, Hemiptera, Coleoptera, Diptera, Lepidoptera and Trichoptera that had flown at least 450 km from mainland Australia to a remote island in the Coral Sea).
The winged stage of aphids disperse large distances (e .g . corn-leaf aphid (Rhopalosiphum maidis) annually reinvades Canada and northern United States from the south (Irwin & Thresh 1988)).
First instar nymphs of scale insects disperse by wind (e .g . wind currents were primarily responsible for rapid establishment of the cottony cushion scale (Icerya purchasi) throughout the Seychelles Islands (Hill 1980)).
Spiders disperse by ballooning on silk threads (e .g . 28 spiders were collected 880 km from land in the Pacific (Bell et al 2005)).
Spider mites disperse by ballooning and other mites disperse without silk (Bell et al . 2005).
Fungal spores are wind dispersed (e .g . spores of Antirrhinum rust (Puccinia antirrhini) and poplar leaf rusts (Melampsora spp .) have been dispersed by wind across the Tasman (Close et al 1978)).
Pollen and seeds are readily wind dispersed (e .g . Casuarina pollen found in peat and surface samples from various parts of New Zealand has its source in eastern Australia (Close et al 1978)).
The extent of these migrations defied the imaginations of many early researchers, who fruitlessly dug up ground in the northern United States searching for overwintering cutworm caterpillars, while in reality the moths were enjoying warmer climes nearly 2,000 km to the south. Monarch butterflies make similar annual migrations between Canada and Mexico, a distance of over 3,600 km.
Diversity spread by air
An enormous diversity of organisms has been found to disperse aerially (see sidebar for some examples). Riley et al (1995) used aerial nets in China and found numerous insects, fungi, pollen, seeds and spiders were present in the air, and it’s notable that less than one percent of the arthropods they caught were moths or butterflies.
Another surprising feature of the published data is that a large proportion of airborne organisms are found at relatively low altitudes of around 100–300 m. The idea that trans-Tasman travellers are unlikely to survive the ravages of high altitude, cold and strong winds may not be generally true. Organisms exploit the wind for dispersing to new regions and finding new habitats, and the success of this strategy is evidenced by its prevalence in nature. Surely this is something we need to seriously consider as we refine New Zealand’s biosecurity systems?
Not only do diverse organisms disperse through the air, but they can do so in abundance. Using radar and aerial nets, Riley et al (1995) monitored airborne organisms and estimated that in each cubic hectare of air there were 500,000 aphids, 100,000 brown plant hoppers, 1,000 spruce budworms, 2,000–3,000 corn earworms and fall armyworms, and 700 rice leaf rollers. The point here is that New Zealand may not just be receiving one or two lucky individuals during trans-Tasman dispersal events, but a fairly serious sprinkling of new arrivals. Of course, such visitors may only have tiny chances of establishing self-sustaining populations here, but their establishment probabilities may well increase both with their frequency of arrival and with the number of individuals involved.
There have been a few studies of airborne dispersal to other remote locations.
A light trap monitored for a year on tiny Willis Island situated 450 km to the east, and upwind, of northern Queensland caught 115 taxa in 12 insect orders. Eighty-four percent of these were considered to be visitors unable to inhabit the island (Farrow 1984). Adults of three moth species and of painted lady butterflies have been recorded on Macquarie Island, a sub-Antarctic island 990 km southwest of New Zealand and 1,200 km southeast of Tasmania – a distance they probably travelled in less than 10 hours (Greenslade et al 1999). Greasy cutworm adults have been recorded at South Georgia, at least 1,750 km from the nearest possible source, and aphids are known to regularly cross the Baltic Sea into Sweden.
The meteorologist AI Tomlinson (1973) even suggested trans-Tasman travellers are more likely to be deposited in parts of New Zealand where westerly winds become weakened, including Tasman Bay, Marlborough Sounds, Taranaki and south Auckland.
Airborne for 30 million years
Airborne dispersal of new species to New Zealand could well involve a greater diversity and number of organisms than is currently recognised. This east-to-west tide of natural immigration to New Zealand probably began running at least 30 million years ago when our prevailing westerly winds started. Why should we pay attention to it now?
One of the reasons is because the New Zealand that airborne organisms have been visiting for the past 30 million years has changed, and recent human modifications have created a new New Zealand for overseas immigrants to visit. Some species that previously had little prospect of establishing here are being presented with their first-ever opportunities as their exotic hosts flourish in New Zealand. For example, Withers (2001) recorded 57 Australian insects which feed on eucalyptus trees here, and many probably naturally dispersed to this country. Australia has become similarly modified and, through the establishment of its own exotic flora and fauna, is becoming the departure point for new aerial travellers that have not previously had any chance of naturally dispersing to New Zealand (e.g., poplar rust). Also, some species that have previously found our country unsuitable for establishment might be able to colonise as the effects of climate change become more evident.
Of course we cannot expect MAF to stem this 30 million year tide of natural immigration. But better knowledge of the diversity and numbers of organisms that naturally disperse to New Zealand would help us to more effectively allocate resources for incursion responses.
Better understanding enables better targeting
Examples of exotic organisms that probably naturally dispersed to New Zealand
Migratory locust (Locusta migratoria)
Meteorus wasp (Meteorus pulchricornis)
Yellow flower wasp (Radumeris tasmaniensis)
Wolf spiders (Venatrix goygeri, Geolycosa tongatabuensis)
Lynx spider (Oxyopes gracilipes)
Garden orbweb spider (Eriophora pustulosa)
Wheat aphid (Macrosiphum miscanthi)
Australian crop mirid (Sidnia kinbergi)
Felted pine coccid (Eriococcus araucariae)
Species likely to have naturally arrived in New Zealand under wind power will probably continue doing so, and may not warrant major responses. The resources saved could then be used to help eradicate species that can only get to New Zealand by hitchhiking with people and imports, and that we have a chance of excluding in the future. Moreover, with better knowledge of natural dispersal to New Zealand, it might be possible to provide primary producers with warnings about the imminent arrival of new pests, and proactively provide them with management information and tools.
The Better Border Biosecurity research collaboration currently has a small research project on natural dispersal being led by Dr Suvi Viljanen of Crop and Food Research. Over the next few years, this project should help shed new light on the incursions by unwanted organisms that cannot be blamed on breaches of New Zealand’s biosecurity systems!
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