Introduction
I’ve recently been doing a risk assessment for a seabird island rat eradication. While I rarely condone widescale use of toxins in conservationWhy is animal conservation important? Animal conservation is important, because animals are the only mechanism to create biodiversity, which is the mechanism that creates a habitable planet for humans. Without animals, the energy from today’s plants (algae, trees, flowers etc) will eventually reach the atmosphere and ocean, much of it as carbon. The quantity of this plant-based waste is so More, isolated application to remote islands, makes sense. The impact is short-term, the results permanent and recovery miraculous. Animals bounce back in greater numbers, more diversity and heavier biomassThe weight of living organisms. Biomass can be measured in relation to the amount of carbon, the dry weight (with all moisture removed) or living weight. In general it can be used to describe the volume of energy that is contained inside systems, as the size of animals relates to their metabolism and therefore, how much energy they contain and More. But these projects are heavily controlled and monitored. To assess the risk to animals other than rats, I had to a literature research to ask, how toxic is rat poison? Below is a summary of some of the findings, which you might like to read.
Why you should NEVER use rodenticide
Unlike their use in isolated places, we are continuously spreading ‘second generation’ rodenticide (rat poison) throughout the ecosystem. It’s spread around playgrounds, houses, gardens, forests, farmland … it enters our waterways, soils and unlike it’s use on islands, it is continuously added to the environment. Here is a 1kg box of these incredibly lethal poisons you can buy for $42 from Bunnings and use in your garden with little or no real understanding of the risks.
In island conservation, if a 21 day interval is used between bait applications, the risk of primary poisoning is expected to span a total period of up to 35 days (Griffiths, Miller et al. 2011). But the bait can last for weeks to months in soil. Even though its insoluble, it can enter food chains via less affected species such as reptiles but kill any bird that eats them in the mean time.
https://www.actforbirds.org/ratpoisonRead more and sign the petition on https://www.actforbirds.org/ratpoison
Impact on humans
As a precaution, all human consumption of anything near the water or on land, is halted for 3-6 months, while bait is applied. This is because there is some evidence it can harm people. Yet we use this in our gardens and homes.
The Australian government has condoned widescale use in mouse plagues, which is killing all the natural control measures and creating a vicious cycle of more frequent and worse mouse plagues – collapsing farming.
For birds of prey, even miniscule amounts are absolutely lethal.
Yet, we can buy these incredibly toxic poisons off the shelf, with no constraints on their use. The broadscale, continuous use of these poisons in our back gardens, parks and wetlands, is almost certainly going to lead to the wholesale destruction of ecosystemsHow ecosystems function An ecosystem is a community of lifeforms that interact in such an optimal way that how ecosystems function best, is when all components (including humans and other animals) can persist and live alongside each other for the longest time possible. Ecosystems are fuelled by the energy created by plants (primary producers) that convert the Sun's heat energy More if we don’t stop it now.
The risks of uncontrolled use of rat poison – a brief review
Second generation anticoagulants have been effective for rodent eradications on islands but present risks to other non-target wildlife, through both primary (bait consumption) and secondary (consumption of animals containing rodenticide residues) pathways of exposure (Castaño, Campbell et al. 2022).
The risk of using this toxicant to birds, reptiles, invertebrates and marine organisms is evaluated along with the risk of any potential impact using a food web modelThe process, either mathematically or in the human brain, of creating an internal version of something that we can refer to, to better understand how it functions and our place within. Scientific modelling is where we take the best knowledge we have and build a version of what will happen, if we assume certain parameters. For example, we might model More. Primary pathways, where identified, are classified as either direct or indirect (Figure 1). Secondary pathways are through the soil layer (via soil invertebrates) or the aquatic layer (via drinking, or consumption of aquatic invertebrates).
Threat to island soils
Based on island research (Griffiths, Miller et al. 2011), rodent bait is expected to disappear rapidly because of consumption by invertebrates such as crabs and ants and any bait not consumed will degrade rapidly as a consequence of the warm humid conditions.
Little brodifacoum was transferred from disintegrating baits to soil in a study completed on Palmyra Island (Alifano, Wegmann et al. 2012) although the authors considered invertebrate activity (ants) to have influenced this result.
Residues that were detected in soil disappeared within 28 days at all sampling sites (Alifano, Wegmann et al. 2012). Results from other studies (e.g. Ogilvie, Pierce et al. 1997, Craddock 2003, Fisher, Griffiths et al. 2011) have recorded a greater transfer(of nutrients) the thing that sets animals apart from plants, is that they can move. Some of the biggest migrations on Earth every day, are the movement of insects like caterpillars, from the stem of a plant to a leaf and back, before turning into butterflies and transferring the energy elsewhere. Large-scale migration of grazing animals and migratory songbirds moves More of brodifacoum into soil. In these studies, brodifacoum residues detected in soil have persisted at levels above the minimum limit of detection for between 1 and 6 months after bait application (Fisher and Fairweather 2006).
Brodifacoum is expected to persist for a longer period in soil and in organisms that have been sub-lethally exposed to either primary or secondary poisoning. Data suggest that residues will be eliminated from soil and invertebrates such as land crabs within six months but may persist for a longer period in reptiles and other vertebrates.
Threat to island marine and freshwater systems
Brodifacoum 25W rodent bait sinks and breaks apart over a relatively short period (<24hrs) once wet and exposed to wave action.
Dead mullet (Moolgarda engeli and Liza vaigiensis) were found in the lagoon of Palmyra Atoll following application of rodent bait (containing brodifacoum at 25ppm) at 85 and 75 kg/ha and assays confirmed the presence of brodifacoum residues (Pitt, Berentsen et al. 2012). Residues were also detected in 9 of 20 Blackspot sergeant fish (Abudefduf sordidus) collected on Palmyra for analysis.
On Wake Island where the same rodent bait was applied but at a lower rate (18 and 9 kg/ha) no fish mortality was recorded. However, brodifacoum residues were detected in all four black snapper (Macolor niger) tested and one of eight popio (Caranx melampygus) (Island Conservation, unpubl. data). Further sampling of mullet at Palmyra two years later detected no residues of brodifacoum (A. Wegmann pers. comm.).
Because of the low solubility of brodifacoum, soil or marine sediment contamination is localized and limited to directly beneath degraded baits. Microbiological breakdown of brodifacoum in soil is dependent on climate, particularly temperature and the presence of microbial species (Fisher and Fairweather 2006).
Brodifacoum is highly insoluble in water and has been shown to have no impact on water quality (Primus, Wright et al. 2005, Fisher, Griffiths et al. 2011). As discussed above, brodifacoum from degraded rodent bait once in the soil or marine sediment remains bound strongly to soil or sediment particles where it is broken down by microbial activity (Ogilvie, Pierce et al. 1997, Fisher, Griffiths et al. 2011).
Threat to reptiles
Based on previous projects (e.g. Desecheo, J-L Harrera pers. comm.; Round Island (Merton 1987); Pinzon, K. Campbell pers. comm.; Palmyra, (Pitt, Berentsen et al. 2012)), the risk of individual mortality within gecko and skink populations is considered low. Gecko mortality has not been recorded previously as a consequence of rodent eradications where rodent bait has been applied aerially and the risk of primary or secondary poisoning to both geckos and skinks is anticipated to be low.
Threat to birds (acutely toxic)
Studies have shown a 10x increase in mortality for New Zealand Dotterels(Pitt, Berentsen et al. 2015). The impact is likely to be greatest for rocky shore-dwelling species.
Birds of prey are likely to predate on some animals that have consumed bait. Kites and sea eagles are scavengers and may consume secondary bait which is acutely toxic.
Buckelew et al (2011)(Buckelew, Byrd et al. 2011) found Bald Eagles particularly susceptible ‘Numerous bird carcasses, including glaucous-winged gulls and bald eagles, were found following bait application and toxicology analysis confirmed that most mortalities were direct or indirect effects of consumption of the baits.’ Secondary poisoning was presumed to be to blame, from eagles eating the carcasses of dead rats and gulls (the latter were feeding directly on the rat carcasses).
References
Alifano, A., et al. (2012). Migration of Brodifacoum and Diphacinone from Bait Pellets into Topsoil at Palmyra Atoll National Wildlife Refuge. 25th Vertebrate Pest Conference, University of California, Davis.
Buckelew, S., et al. (2011). Preliminary ecosystem response following invasive Norway rat eradication on Rat Island, Aleutian Islands, Alaska.
Castaño, P., et al. (2022). “Managing non-target wildlife mortality whilst using rodenticides to eradicate invasive rodents on islands.” Biological Invasions 24: 1-18.
Craddock, P. (2003). Environmental breakdown of Pestoff poison bait (20 ppm) brodifacoum at Tawharanui Regional Park, north of Auckland. Unpublished report prepared for Northern Regional Parks, Auckland Regional Council, Entomologia Consulting, New Zealand.
Fisher, P. and A. Fairweather (2006). Brodifacoum, a review of current knowledge. D. o. Conservation, Pesticide Information Review Series No 6. Department of Conservation, Wellington, New Zealand.
Fisher, P., et al. (2011). Environmental monitoring for brodifacoum residues after aerial application of baits for rodent eradication. Island invasives: eradication and management. Proceedings of the international conference on Island invasives. IUCN, Gland, Switzerland and Auckland, New Zealand.
Griffiths, R., et al. (2011). “Addressing the impact of land crabs on rodent eradications on islands.” Pacific Conservation Biology 17(4): 347.
Merton, D. (1987). “Eradication of rabbits from Round Island Mauritius a conservation success story.” Dodo 24: 19-43.
Ogilvie, S., et al. (1997). “Brodifacoum residue analysis in water, soil, invertebrates, and birds after rat eradication on Lady Alice Island.” New Zealand Journal of Ecology 21(2): 195-197.
Pitt, W. C., et al. (2015). “Non-target species mortality and the measurement of brodifacoum rodenticide residues after a rat (Rattus rattus) eradication on Palmyra Atoll, tropical Pacific.” Biological Conservation 185: 36-46.
Pitt, W. C., et al. (2012). Palmyra Atoll Rainforest Restoration Project: Monitoring Results for the Application of Broadcast of Brodifacoum 25W: Conservation to Eradicate Rats. QA-1875 Final Report, USDA, APHIS, W, NWRC. Hilo, HI. 85 pp.
Primus, T., et al. (2005). “Accidental discharge of brodifacoum baits in a tidal marine environment: a case study.” Bulletin of Environmental Contamination and Toxicology 74: 913-919.
