09 May 2014

1080; A new part of the foodchain

This blog post was written by postgraduate student Sam Harvey as part of the course Research Methods in Ecology (Ecol608). Sam revisits a Lincoln University research area that examines the fate of toxins in dead target animals in natural ecosystems published in 2013.

This is not another rant about why we should or shouldn't use 1080. I would instead like to highlight the hazards through which 1080 poses a threat to non-target species via secondary poisoning and trophic transfer. If one was however looking for such an article that stated the issues and gave both arguments for and against the use of 1080 a useful read can be found here. For readers foreign to this topic, 1080 or sodium fluoroacetate is the poison of preference in New Zealand for control of the introduced brush-tail possum. Although an effective control option for this pest, poisoning of non-target animals such as native birds and domestic animals occurs. These animals are often not poisoned directly from bait ingestion but through ingesting the toxic offal of poisoned animals which is called secondary poisoning. This has led to 1080 falling into disrepute with some of the New Zealand public.

Boss and Ice winding
Photo: Sam Harvey
I am a mad keen pig hunter, and there isn't a lot in this world I love more than to watch the dogs work for a scent. Once they have the scent they take off into the scrub hot on the tail of some beast that could either fill my freezer or give the hounds a good hiding. Suddenly a bail bark erupts at the bottom of the gully, the dogs have found him, and I sprint toward the sound as fast as I can. As I get closer, the occasional squeal and grunt emerges over the chaos of barks; the dogs are holding him now so he's as good as mine. I close the final meters and grab the pig so I can finish him off with a knife (sorry to you animal lovers but he wasn't doing the ecosystem any favors). I sit down for a rest with the dogs and notice the pup is missing so go to work trying to track it down. I find it a little way down the gully looking a bit sick lying next to a half-savaged possum carcass and my heart drops. The thought of 1080 killing one of your mates is pretty tough.

I have never been unfortunate enough to experience this (largely because I respect the 1080 signage around control areas). However, pig hunters being pig hunters, a fence and a sign can mean little and the scenario I depicted above has happened to more than a few keen hunters. This is the same for walkers with their domestic pets who, thinking they know better, ignore signage and warnings just to take their dog for a scenic walk. I speak of dogs primarily due to their susceptibility to 1080. For dogs the LD50 (the dosage required to kill half of a test population) is only 0.07 mg/kg body weight which is relatively low when compared to other animals such as birds which have tolerances ranging about 10 mg/kg body weight. So your average 12 g cereal bait laced with 1080 can kill several dogs. This low tolerance in dogs means there is a high risk of secondary poisoning when toxic offal is consumed. 
A tasty feed for an unwary hound
Photo: travelwayoflife@flickr
1080 signage at a forest entrance
Photo: Sam Harvey
It's time to look at this from an operational manager's perspective. By law 1080 signage must be positioned at all normal points of entry to a control area and remain there until 6 months after the last application. How do we know when it is safe to take the signs down though? When is the danger of poisoning really over? Well it all depends on the environment, whether it's hot or cold, dry or wet, open or enclosed. All of these factors affect the rate at which 1080 degrades and toxic offal putrefies. 1080 degrades relatively slowly in possums, the study by Eason, Ross and Millar which this blog is based on found concentrations ten times the lethal dose for dogs being in possums 25 days after death. Lethal concentrations of 1080 were also found in sika deer stomach (5.66 mg/kg) forty days after death. So we know that persistence in soft tissue could lead to fatal secondary poisoning for some time. However, could there be another part of the carcass that poses a threat long after the soft tissue has degraded? 

Bones remain long after death and if sufficient concentrations of 1080 could be found in bone marrow then danger of poisoning in dogs may persist well after a carcass has degraded. 1080 was shown to persist in deer bone marrow for 213 days in Eason et al.'s study however speculation within this paper suggests that this could be even longer. The same study found concentrations of 1080 in deer bone marrow (0.59 and 2.3 mg/kg day 30 & 40) which suggest a dog would need to consume 500 g to 1 kg of toxic marrow for there to be a risk. It was then suggested that 1080 concentrations may be even higher in possum bone marrow. 

So if I were to speculate I would suggest that the current period of 6 months after 1080 application before dogs can be present without a leash is a realistic time frame (Dogs may be present earlier than that when on a leash as the owner can completely supervise their pet). Further research into possum bone marrow concentrations may suggest this period is increased. However, I can't see this period extending to anything more than 9 months especially with current research into reducing the size of 1080 cereal baits which will mean that individual dosages are reduced limiting the concentration of 1080 in toxic offal.

But wait, if a dog can be poisoned through eating toxic carcasses shouldn't that mean that other non-target species could be poisoned too? This could suggest non-target species, such as native birds and insects, may not only be at risk from direct consumption of 1080 pellets but through trophic transfer as 1080 enters the food web.
A dead tree weta with mandibles large enough to eat 1080 pellets
Photo: funkypancake@flickr
1080 has a relatively short half-life and as a result bioaccumulation is not a huge factor. However 1080 concentrations have been found in several insect species after 1080 application, including cave weta, tree weta and some cockroaches. This may mean that ingestion of 1080 either directly from baits or from toxic offal could cause insect mortality. However studies have shown that weta survive 1080 ingestion even at concentrations of 15 mg/kg after which all traces were eliminated after 6-10 days. Similar results have been produced for ants where 1080 traces were eliminated after several days although some ants consumed enough for a fatal dose.

If insectivores were to eat toxic insect material then trophic transfer could occur to animals feeding on those insects. The problem with this is that I see two potential limitations. One, the relatively short half life of 1080 would result in a small window of time where poisoning could occur and two, the insectivores that ate these individuals would primarily be birds which generally have a higher tolerance to 1080 anyway. Regretfully, some poisoning of non-target species is going to occur during a 1080 operation. The overwhelming evidence is that the number of these individuals is insignificant when weighed against the benefits that these species can gain through the removal of unwanted predators.

In conclusion I believe that secondary poisoning with 1080 is an issue in New Zealand due to emotional attachment to the affected species. The poisoning of dogs and other domestic animals occurs but is highly avoidable if signage is respected. Furthermore the poisoning of native fauna is evident but the death of these individuals is insignificant compared to the benefits gained from the species as a whole.

Possum picture provided by travelwayoflife@flickr

Tree weta picture provided by funkypancake@flickr

For more on 1080 and the current 'battle for our birds' look here.

Eason, C.T., Ross, J., Miller, A. (2013) Secondary poisoning risks from 1080-poisoned carcasses and risk of trophic transfer - a review. New Zealand Journal of Zoology 40(3): 217-225.

No comments: