By Alison Ballance
“Introduced predators and competitors have been identified as factors in the decline of New Zealand bat species, so we know that without control of those mammal pests the bats will decline.”
Gillian Dennis, Massey University
Lesser short-tailed bats, or pekapeka-tou-poto, are one of just two endemic bat species found in New Zealand. They’re very vulnerable to predation by rats, so the Department of Conservation (DoC) uses poison bait to manage the rodent problem. However, six years ago then-DoC ranger Gillian Dennis found herself facing a quandary: what about the risk to the bats from the toxins that are meant to protect them from predation? Gillian began a PhD at Massey University to look at the issue, and she tells Alison Ballance that it has been a slow process teasing out the details of this conservation conundrum. However, using a combination of Gillian’s research and on-going monitoring by its own staff, the Department of Conservation has concluded that the benefit to the bats in Pureora Forest from using poison baits to minimise the threat of predation by rats far outweighs the small risk to the bats.
To protect breeding birds and bats by knocking back rat numbers, DoC regularly uses first-generation anti-coagulant poisons, such as diphacinone and pindone, which act to disrupt the blood clotting mechanisms of vertebrates. Standard practice is that the toxin is delivered in hard cereal-based baits that are placed on the ground in bait stations throughout the forest.
It had been suspected that New Zealand bats might be susceptible to toxins but only a single bat had ever been found dead, during the 1980s on the West Coast, next to a cyanide bait used to kill possums.
There are several aspects of the biology of short-tailed bats that might make them especially vulnerable to ground-based poisons. They are opportunistic feeders with a broad eclectic diet, eating everything from insects to fruit and nectar and pollen, such as that from parasitic Dactylanthus flowers, which they help pollinate. They also spend a lot of time foraging on the ground, and are regarded as the most terrestrial of all bat species. They are able to tightly furl their wing membranes out of the way, to ensure they won’t get damaged, and they run around on the ground at night as well as flying.
Gillian also points out that in South America diphacinone is used to control vampire bats, which carry rabies and are considered a health threat. The vampire bats are very sensitive to the toxin and just a small amount is toxic; it’s thought that short-tailed bats could also be as sensitive.
Then, in 2009, Gillian found a number of dead bats next to a roost tree in Pureora Forest, and post-mortems revealed the presence of anti-coagulant toxin in their bodies. DoC immediately stopped rat baiting and the deaths stopped. However, not controlling the rat populations is a conservation conundrum, as bats do much better when there are no rats around.
Gillian says that the story of the greater short-tailed bat highlights the vulnerability of our bat species to rodents. By the early 1960s this species had already been wiped out on mainland New Zealand, most likely due to predation by rats as well as loss of habitat. It survived only on rat-free Great South Cape Island near Stewart Island. When rats were accidentally introduced to the island the bat and two species of endemic native birds were made extinct.
One of the prime questions facing Gillian was do the bats eat the baits directly or are they getting it indirectly through eating insects, such as weta, that have consumed small amounts of bait? She investigated this question firstly with a colony of captive bats at Auckland Zoo, seeing if they approached or ate non-toxic baits. Then she looked at wild bats, filming at non-toxic baits to see what animals, if any, approached or ate the baits. The bats showed almost no interest in the baits and never ate them, but Gillian recorded numbers of weta and other invertebrates – which bats eat – eating the bait. The conclusion was that secondary poisoning rather than direct poisoning is affecting the bats.
The rat control operation carried out at Puroera in the year the bats died used diphacinone presented as a paste, nailed to trees in biodegradable plastic bags. This is not the usual method of presenting toxin, and it was thought it may have resulted in bats or insects having more access than usual to the bait. So the following year the Department of Conservation followed the survival of a well-studied population of short-tailed bats during a more standard rat control operation in Fiordland. The toxin pindone was made up into hard cereal pellets that were contained in bait stations. There was a very high survivorship of bats that year in Fiordland so the following year DoC decided to use the same baiting regime at Pureora.
Gillian was still concerned, however, as earlier research had showed that invertebrates do eat hard cereal baits. So it was still possible for bats to be consuming small quantities of toxin that might not kill them but might still be affecting their health.
To see if these sub-lethal effects were occurring Gillian measured a number of factors in the Pureora bats. Anti-coagulants prolong blood clotting time, so she measured pro-thrombin time which is an early indicator of anti-coagulant poisoning. She assessed the body condition of the bats, and gave them a visual check for bleeding and anaemia. She was particularly interested in pregnant females, as the toxin can cause abortions or birth deformities. As a comparison she also collected all this information from the Fiordland bat population in a year when no rat control was being carried out. While she didn’t find any measurable health effects, tests on bat guano showed that the bats were still ingesting small doses of toxin.
"For now the use of poisons is the best option we have for broad-scale rodent control in native forest,” says Gillian. “And while this might present a risk to bats, we can minimise that risk by delivering baits in bait stations when pest control is done within bat habitat. Other studies that have been done with bats comparing survival in years where there has been pest control to years when there hasn’t been pest control, have shown that bats definitely benefit from having rodent control.”
DoC continues to use poison to control rat populations and closely monitors the survival of the Pureora Forest bat population.
There are three recognised subspecies of short-tailed bats, and a number of distinct populations that are found from Northland right down to Codfish Island/Whenua Hou near Stewart Island. The sub-species each have a different threat status, with the central North island sub-species, which includes the Pureora Forest bats, is listed as ‘At risk – declining’. The southern lesser short-tailed bat is classified as Nationally Endangered while the northern lesser short-tailed bat is in the threat category of Nationally Vulnerable.
Pureora Forest is also home to New Zealand’s second bat species, the long-tailed bat. In the South Island this species is classified as ‘Threatened: Nationally Critical’, while in the North Island its threat risk is ‘Threatened: Nationally Vulnerable.’ Long-tailed bats are considered to be less at risk of poisoning during rodent control operations because they forage on insects on the wing, and they generally forage above the canopy or along forest edges. Gillian says that although long-tailed bats would be less likely to eat invertebrates that had fed on baits laid on or near the ground, further investigation is needed to properly assess the chances of these bats being exposed to poisons.