Exploring the welfare benefits of wildlife contraception
September 24, 2024
Wild Animal Initiative scientists published a paper this month discussing the potential for contraceptives to alleviate resource competition among wild animals, and the negative welfare effects associated with it. In this Q&A, we hear from Researcher Simon Eckerström Liedholm, who led the research, about the significance of the new paper and the promise of contraceptives for improving wild animal welfare.
Your paper presents wildlife contraceptives as a potentially more humane alternative to conventional population management practices like poisoning and culling, and as a tool that could be used with the primary intention of improving wild animal welfare. How are those two things different, and what is the significance of that distinction for researchers and practitioners?
SEL: This distinction is subtle but important. In the near term, we might be posed with questions like whether to continue using a slow-acting anticoagulant poison or a type of wildlife contraceptive in order to control an invasive species. In that scenario, the use of the contraceptive is very likely to be preferable from an animal welfare perspective. But as our understanding of wild animal welfare and our ability to care for wild animals increase over time, we will be able to responsibly help animals live better lives — even in cases where humans are not directly impairing their welfare. It’s possible that contraceptives would also have beneficial effects on wild animal welfare if the choice is between minimal or no management and the use of wildlife contraceptives.
What is the relationship between population density and wild animal welfare?
SEL: High population density relative to carrying capacity is expected to affect welfare negatively, primarily because it causes resource competition. To illustrate this, let’s say a species of mammal is able to colonize an island just off the coast of its natural range. When the first few founders reach the island, there are plenty of resources like edible plants, water, and shelter. These benign conditions allow the founders to survive as adults and reproduce for a relatively long period of time, for the offspring to survive a potentially risky juvenile phase, and for the parents to raise larger litters — all contributing to a growing population.
But as the population grows, resources are progressively depleted. Perhaps edible plants become harder to find, and the best spots for avoiding inclement weather are taken. When animals lack access to sufficient resources, they may die of things like starvation, exposure, or aggression from members of their own species, among other things, and the population growth rate slows down. Importantly, the factors that cause the growth of the population to slow down are often unpleasant or painful for the affected individuals. For example, severe hunger is associated with negative welfare, and so are the ailments that may arise from malnutrition.
The effects of population density on welfare seem to depend on the carrying capacity of the environment. A dense population living in a lush environment with plenty of resources may have a higher carrying capacity than a less dense population living in an arid environment where resources are scarce. So we don’t necessarily expect lower average welfare in the population living in the lush environment just because their population is denser.
How have wildlife management practitioners historically addressed high population densities?
SEL: Wildlife managers have controlled population densities using different tools depending on the species in question. Poisons are typically used for smaller animals like rodents and insects. Larger animals are often shot, or trapped and then shot. Sometimes fences, nets, or spikes are used to keep animals from accessing or crossing between areas — for instance, you can read about the State Barrier Fence of Western Australia, or the use of spikes on infrastructure to deter pigeons and other birds. In some cases, the population density is objectively large compared to other populations or species that people manage, but there are no conflicts with people. In these cases, the populations are usually not managed at all.
What are the potential benefits of wildlife contraceptives, and where are we most likely to see them?
SEL: Using wildlife contraceptives to maintain the population at a lower level by simply reducing the birth rate should allow established members of the population to have better access to resources. For instance, we might see juveniles become more likely to survive to adulthood because their parents will have fewer offspring to take care of, and they’ll have fewer conspecific adults to compete with for food. For these reasons, some studies have found wildlife contraceptives to have positive effects on body condition and survival, which may indicate positive welfare effects.
We’re most likely to see benefits in cases where high population densities mainly affect survival, rather than reproductive effort. In a scenario where resource competition increases, causing adults to reproduce less to conserve energy, the population is essentially experiencing “self-contraception.” In this scenario, average welfare is expected to be roughly the same irrespective of population density, so wildlife contraceptives aren’t as likely to improve welfare. There may be other contextual factors that affect the benefits, too, such as to what extent resources freed up by contraception are used in ways that benefit welfare. For example, if some bird species invest the extra resources in producing longer tail feathers to attract mates rather than improving fat reserves, it’s less clear what the welfare effects would be.
“The potential for the use of wildlife contraceptives to provide population-wide welfare benefits by reducing population density has scarcely been explored in the academic literature. Our paper is one of the first to clearly draw the connection between these factors, and to suggest a path forward for exploring this connection further.”
Could wildlife contraceptives have any negative or unexpected effects on welfare or ecosystem health — productivity, resilience or biodiversity?
SEL: It’s possible, which is why we call for thorough research on any kind of intervention before it’s implemented in the wild. A few studies have looked at potential side effects of different kinds of contraception — Gray and Cameron provide a summary in their 2010 paper. One takeaway is that surgical castration or spaying can cause some of the treated animals to die while under anesthesia, and is also likely associated with pain, discomfort, and fear — both when the animals are being put to sleep and when they wake up. Some methods may also cause changes to reproductive behaviors that might or might not negatively affect the welfare of the treated animals or other members of their populations.
Researchers and ecosystem managers have explored wildlife contraception as a tool to counteract processes that threaten aspects of ecosystem health, such as to control the common brushtail possum in New Zealand, or feral horses in the United States. Like any type of wildlife management, contraception should be preceded by an assessment of the risks and benefits to both human interests and animal welfare, to make sure the intended benefits are achieved and there are no unexpected outcomes.
Why do we need more research on wildlife contraceptives, and what are the next steps?
SEL: Just like the health and happiness of humans and domesticated animals are important, the welfare of wild animals is morally important as well. But we know so little about what wild animals experience in the wild and how we can help them effectively.
The potential for the use of wildlife contraceptives to provide population-wide welfare benefits by reducing population density has scarcely been explored in the academic literature. Our paper is one of the first to clearly draw the connection between these factors, and to suggest a path forward for exploring this connection further.
Consequently, there are a lot of open questions, such as: What taxa and traits are likely to be associated with the largest benefits, and why? What are the welfare effects of the use of wildlife contraceptives for highly numerous commensal species such as pigeons and rodents? These and many other related questions can — and should — be answered by both empirical and theoretical studies, so that we can better understand and ultimately improve the lives of wild animals.
I hope this research will serve as inspiration and a jumping-off point for future research into population-level welfare effects, as well as research into the relationship between population dynamics and wild animal welfare. Few previous studies have looked at how demographic factors and changes to survival rates affect welfare — which my colleague Luke Hecht, Grants Manager & Researcher at Wild Animal Initiative, explored in a 2021 paper.
What sparked your interest in wildlife contraception, and why is it important to you?
SEL: My interest in wildlife contraception began around 2020. It was partly inspired by a well-researched report by Ozy Brennan, and partly by conversations with others at WAI, including Luke. The effects of resource constraints and other negative effects on welfare that may arise at high population densities — such as increased disease transmission — stood out to me as one important and fairly universal factor that might determine welfare across many different species of wild animals. Wildlife contraception seems like a tool that could help populations remain at levels conducive to better welfare, while also potentially helping to address welfare issues associated with high juvenile mortality.
