Key points
- The New Forest is of great conservation significance. Despite this, populations of ground nesting birds are in decline.
- Foxes are generalist predators found throughout the UK. They can predate nests and so are linked to declines in native wildlife such as wading birds.
- Researchers studied the contents of fox stomachs, collected from across the New Forest.
- The analysis showed that foxes have a varied diet and a large proportion of that was human food.
- Simulation models showed human food is supporting fox populations, likely undermining fox control efforts.
- Introducing wheelie bins could reduce the food available to foxes, but more research is needed to investigate this.
Background
The New Forest covers 566km² of diverse habitat in Hampshire including woodland, heathland and coastline. Home to 34,000 people, the Forest is sandwiched between large urban areas including Southampton and Bournemouth.
Resident breeding waders such as Eurasian curlew are struggling with poor breeding success. Nest predation is also a big issue for curlew in the New Forest, with foxes causing 56% of monitored nest predation events between 2021 and 2022.
To help improve curlew breeding success by reducing predation, the Forestry Commission is lethally controlling foxes. Yet, human-provided food may be supporting fox populations. This likely undermines culling efforts and puts additional pressure on ground nesting birds.
Fox diets include a range of different food from mammals, fish and plant-based food. Human-sourced food is attractive to foxes as it offers lots of energy, without a fox needing to spend much time or effort finding and eating it.
Many studies in the UK have studied fox scats, but few have looked directly at stomach contents.
What they did
Foxes are elusive and mostly nocturnal, which can make it difficult to study their feeding habits. So, between April 2021 and July 2022, wildlife managers collected culled foxes in curlew breeding areas across the New Forest. The scientists then examined the contents within 447 of these stomachs.
The stomach contents were sorted into broad food categories, which were: small mammals; rabbits and hares; other mammals (deer and grey squirrel); fish; non-gamebirds; gamebirds; reptiles and amphibians; plant material; invertebrates; human-sourced food; indigestible material (plastic, stones, mud, intestinal worms); and unidentified.
The researchers measured the weight of the stomach contents and how much space each food category took up. Some foxes had been baited with dog biscuits, meaning their stomachs were mostly full of biscuits. These were excluded from further analysis.
Different seasons align with key stages of the fox life cycle such as breeding and raising cubs, as well as shifts in natural food availability. Human infrastructure typically relates to the availability of human food, which may alter fox diet choices. Therefore, time of year and proximity to human infrastructure were analysed for their impact on the fox diet.
Simulation models were developed to estimate the number of foxes in the New Forest that are supported by human-sourced food. The researchers aimed to understand how human food might support fox populations and affect conservation efforts.
Recent adult fox population density data for the New Forest were unavailable, but the most recent estimate (from 1974) was 0.75 adult foxes per km². Culling may also bias the collected fox stomachs toward young, inexperienced foxes, who might focus on easier-to-obtain food items. However, this study provides insight into how human food could be supporting fox populations, potentially undermining culling efforts aimed at protecting waders during their crucial breeding season.
What they found
Fox stomach content varied a lot and was not dominated by any single food category. Human food accounted for 14% of fox diet on average. The most commonly found categories were: small mammals; rabbits and hares; non-gamebirds; invertebrates and human food; followed by plant materials and other mammals. Amphibians and reptiles, gamebirds, fish and indigestible material were relatively uncommon, and 22.6% could not be assigned to a category. Less than 20% of viable stomachs contained four or more categories.
Overall, human food was significantly more likely to be found in fox stomachs sampled closer to human infrastructure, like buildings and car parks. However, foxes are known to travel outside their territories to access food provided by humans. Because of this, it was not possible to determine if foxes with territories closer to human settlements consumed more human food than foxes with territories in more rural areas.
In summer, fox stomachs contained a slightly wider variety of food, but overall diet diversity remained unchanged between the two periods. Non-gamebirds were more commonly eaten during this summer breeding period when nestlings and fledglings are present. Rabbits and hares were more commonly found in fox stomachs during the winter when nights were longer, and foxes have more time to hunt in darkness.
The scientists used these data to model three potential scenarios based on: adult fox density estimates, breeding productivity data, foxes’ annual food requirements, and fox body weight. These models were used to estimate how many foxes are supported by human-provided food accounting for uncertainties in the data inputs.
Results showed that human food is supporting between 29.5% and 287.7% worth of foxes that are culled each year. Due to the uncertainties in factors such as adult fox density, the results of the simulations varied widely. Despite this, all simulations showed the human food would be undermining culling efforts to some extent.
What does this mean?
Foxes in the New Forest eat a varied diet, with human food being an important part of this. Human-sourced food is more prevalent in the diet, throughout the year, when foxes are nearer to human infrastructure. Human food might be more important for foxes living closer to built-up areas, but more information is needed to accurately assess fox territories.
This presents some ethical issues, as human food can contain harmful substances such as artificial ingredients and foxes are likely to ingest plastics when foraging. Foxes supported by human food could also be adding more pressure to vulnerable ground-nesting birds and their chicks, undermining ongoing conservation efforts.
Natural food is still the main part of fox diets, but this study suggests that human food is boosting fox population densities in rural areas and towns. In large cities, human food makes up such a large part of the diet that it appears to divert foxes from hunting natural prey and increases the population density in these areas.
In the New Forest, human food could be worsening the impact of foxes on breeding waders. This is because the extra food appears to boost fox populations rather than altering their diet. Diversionary feeding in rural areas requires careful strategies and thorough monitoring. Unfortunately, unregulated food sources are unlikely to prevent the increase in fox populations.
The data simulations suggest the number of foxes supported by human-sourced food is substantial. The exact proportion varies, but ultimately the foxes supported by human food undermine the effectiveness of culling efforts.
The amount of human food eaten did not change with the seasons, despite increased tourism in the New Forest during summer. Household waste is left for collection in bags on the street across the New Forest, making food waste very accessible to foxes.
Our results indicate that foxes are exploiting human infrastructure to access additional food. Improved food sanitation could help to reduce fox densities and predation pressure, helping to preserve the remaining breeding wader populations in the New Forest. It remains unclear what role this is playing in terms of influencing the local fox population dynamics and their prey species, but work is ongoing.
One study has shown that removing human food leads to a rapid drop in fox survival rates. Future research could explore the impact of introducing wheely bins on human food availability for wildlife in the New Forest. Additionally, given recent findings that foxes often consume dog poo (another form of human-sourced food) it would be valuable to investigate whether reliance on human-sourced food is higher than currently estimated.
Read the original paper
Williams, N. F., Porteus, T. A., Hardouin, E., Case, J., Rivers, E., Andreou, D., Hoodless, A. N., Stillman, R. A. and Short, M. J., 2024. Evidence of anthropogenic subsidisation of red foxes in a national park important for breeding wading birds. Mammal Research.