5.4 Long-term effects on ecosystems
In 1998, the New Zealand Ecological Society held a special scientific meeting to examine the ecological consequences of using poisons in general, for the control of mammalian pests (see the New Zealand Journal of Ecology Vol. 23, No.2, 1999). The major review paper at that meeting concluded that the ecological benefits of using toxins for ecosystem management outweigh the costs7, while noting that research on toxin use should continue.
Research, summarised in Chapter 4, also shows that 1080 degrades quickly in the environment, although degradation is significantly slower in cold dry weather. Accumulation in, or contamination of, soil or the environment does not occur because common bacteria and fungi degrade 1080 into harmless by-products. Likewise, research has shown that microbial degradation of 1080 in water is very fast. Animals that consume sub-lethal doses metabolise and excrete 1080 within one to three days, which means it does not persist or accumulate in the food chain. It can persist in carcasses long enough to cause secondary poisoning in predators such as rats, stoats, feral cats and dogs.
If 1080 does not persist in the environment, what are the long-term benefits of its use? There is evidence that significant reductions in possum numbers can benefit individual forest species as well as forest systems in general44. Possums reached the northern forests of Waipoua in the 1960s and were causing visible damage to the canopy by the late 1980s. In 1990, an aerial 1080 operation reduced possum numbers by about 87% and ground trapping has since kept them at low levels. Eight tree species were monitored between 1990 and 1994. All eight "indicator" species showed significantly more defoliation in a non-treatment area compared to the treatment area.
Several species of native mistletoe are favourite foods for possums, as well as providing nectar for birds such as tui and bellbirds. After being almost wiped out by possums in many forests, mistletoes do recover after 1080 operations, as long as possum densities are kept low. Preferred possum food species, such as northern rata, fuchsia, kohekohe and Hall's totara, have shown significant recovery after heavy reductions in possum numbers, while similar trees in non-controlled areas have not. A 30-year possum control programme using 1080 in the Otira Gorge of Westland has shown positive results. Giant southern rata, once threatened through defoliation, now flower regularly, and possum-vulnerable fuchsia trees, along with other native plants, are thriving.
Forest recovery can be complicated, however, by the impacts of other pest species on the forest ecosystem. Even if possums are kept at very low numbers, ongoing browsing of seedlings by deer or goats on the forest floor can prevent the reestablishment of many species, thus disrupting the normal patterns of forest dynamics45. In addition, birds are important for the dispersal of forest fruits and seeds. Predation on native birds by rodents, stoats and ferrets reduces bird numbers, which potentially reduces the dispersal of many forest species. The direct and secondary targeting of these predators by 1080 operations benefits forest regeneration as well as assisting bird recovery.
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