New Zealand Sweet Stakes

Natural History, May, 2001 by Laura Sessions

A scientist at Landcare Research in Nelson, New Zealand, Jacqueline Beggs has spent the past decade studying the effects of wasps in honeydew forests. She believes that the wasp-induced shortage of honeydew could contribute to a decline of native birds over the long term. Beggs has shown that when wasps reduce the number and size of drops below a certain level--a threshold reached when the insects revisit drops every six and a half hours--kaka give up even trying to feed on honeydew. Similarly, Henrik Moller, now at the University of Otago in Dunedin, New Zealand, and other colleagues at Landcare Research have shown that honeyeaters are even more sensitive. If wasps visit drops at a rate of once every three hours, honeydew will be depleted to the point that bellbirds and tuis will switch to other food sources or be forced to conserve energy by spending less time on vital activities such as mating and nesting. Furthermore, when the bellbirds, tuis, kaka, and native insectivorous birds attempt to feed on native invertebrates, they again face competition from wasps, which devour spiders, caterpillars, ants, bees, and flies. Populations of these invertebrates have been decimated or even eradicated in areas where wasps are common.

Unfortunately, scientists do not know precisely how wasps may affect native species in the long run, partly because no one fully understands the organisms involved, especially the invertebrates, and partly because their interactions are so varied and complex. Land mammals are also a major problem. New Zealand has no native terrestrial mammals, but a raft of mammals has been introduced in the last 100 to 150 years. Cats, rats, stoats, possums, and ferrets have had drastic effects on native plants and bird species, many of which are flightless and have few defenses against the invaders. It is therefore difficult to separate the "bottom-up" effects of little things like beech scale insects and wasps from the "top-down" effects of larger predators.

Wasps and mammals may interact to cause more serious harm than either group would alone. Possums, for example, eat many of the same things (such as mistletoe) that honeyeaters do, so the birds often cannot switch to other food types when wasps deplete the honeydew stores. Other effects of immigrant species have proved difficult to tease apart. During a long-term research project on the South Island of New Zealand, Beggs and Peter Wilson, also of Landcare Research, noted that kaka changed how they foraged and fed in response to competition with wasps for honeydew. However, the scientists could not test the overall effect of food shortage on the kaka's ability to raise young, because stoats had raided most of the nests, killing more than 70 percent of the kaka chicks and, even more dire, four of the seven nesting females. Female kaka incubate their clutches of one to five eggs for about twenty-four days; fledging takes a further seventy days. During these three months, both mother and chicks are at high risk of a stoat attack. Moreover, the death of females on their nests leads to a serious imbalance in the population; many breeding-aged males are unable to find a mate. Beggs and Wilson estimated that the kaka population they studied for ten years suffered 7 percent mortality, a rate that could cause this South Island population to become extinct in less than thirty years.