Population control of unwanted species is a very complex subject. The impact on the welfare of animals (including both target and non-target species) as well as the impact on the environment need to be included in any population management plan.
Unwanted species control needs to be scientifically sound and justified by evidence and have measurable outcome-based objectives that are clear, achievable, monitored, and adaptive.1 Many of the current methods used don’t fit these criteria.
Plans also need to be cost-effective. The current Predator Free 2050 5-year Action Plan2 aims to eradicate ship rats, mice, stoats and possums. This plan is set to cost $1,200 per hectare (and wouldn’t necessarily result in success). This equates to ~$32 billion!3
There is a lot of attention on the invasive population control methods used in NZ, including poisons and deadly traps, but we need to prioritise non-invasive methods that don’t involve harming animals.
Below are some examples of alternative research methods for unwanted species control in NZ.
While non-invasive research isn’t the status quo, there are some promising cases of NZ researchers investigating ways of controlling unwanted species by targeting their ability to reproduce.
This involves research in the development of genetic biocontrol technologies, including research into altering specific genes to bias sex ratios (i.e., making a species predominately female to decrease mating and unwanted offspring) and disrupting the genes required for reproduction or development (i.e., making that species infertile).
Some institutes in NZ have been exploring this type of research:
Predator Free 2050 Ltd is conducting a wide range of research, one project that sounds promising is aimed at trying to improve the effectiveness of oral baiting to make target species permanently infertile.4
Together with the University of Otago, they are conducting the following projects:
Landcare Research is conducting research aimed at understanding Stoat Breeding: This project is exploring the recently constructed stoat genome to better understand their breeding genetics, and how these could be used for control. Stoats are a challenging predator to eradicate due to their behaviour and reproductive rates, and ability to range over large distances. Findings may enable future stoat-specific fertility control, with the potential for weasels and ferrets.4
Genomics Aotearoa is coordinating collaborative research investigating whether recent overseas advances in mice producing only male offspring can be adapted for rat eradication. There, a naturally occurring gene determining the sex is replicated onto another chromosome, eventually leading to even genetically female mice having a male phenotype (so can’t bear offspring).5 Sociological assessment of attitudes to genetic technologies, with particular emphasis on understanding relevant mātauranga Māori, are being undertaken.
Genomics Aotearoa completed the genomes for ship rats and possums in 2021. The genome assembly of the kiore/ the Polynesian rat (in collaboration with Hokotehi Moriori Trust) is also nearing completion. Further work is planned in a new project with Predator Free 2050 to develop genomic datasets that allow an understanding of gene flow in Norway and ship rat populations. This will help determine how best to introduce single-sex offspring suppression rats for rat population control.6
Research into fertility has some challenges. The biggest is turning theory into proof-of-concept in the lab into a field-ready tool. Ultimately, a successful infertility tool has to work towards eradication, or you end up stuck in reducing a population size without removing the impacts as infertile predators still eat prey.
In the past, NZ research has investigated using infectious agents to spread infertility through pest populations, but the social license for such approaches is too low, according to the information we have access to.
Current infertility oral baits are quite expensive, have some but limited efficacy (not as good as injections), have quite short-lived effects (so frequent re-application would be needed for effective control) and are not species-specific so could also impact non-target species.7
The solution here isn’t black and white, and we still have a long way to go. What’s clear is that we won’t get further with non-invasive methods of controlling unwanted species if we don’t conduct and fund more of this type of research.