Hedgehogs and Roadkill: Why Small Populations Face Greater Risk

Hedgehogs and roads have always been a poor combination. Across Britain, vehicles kill vast numbers each year, and these losses have long been linked to the species’ nationwide decline.

Yet simply counting deaths tells only part of the tale. To establish whether traffic is driving a local population towards collapse, researchers must first know how many hedgehogs are present.

A research team headed by Dr Lauren Moore at Nottingham Trent University aimed to assess both measures simultaneously.

Their findings challenge a widely held belief: a higher number of road deaths does not necessarily indicate greater danger. In some cases, the reverse may be true.

Counting hedgehogs and road deaths

The researchers selected four locations in Nottinghamshire towns and villages, representing different road networks and habitats.

During 18 months, the team covered 838 km (521 miles) on night-time survey routes, capturing and marking hedgehogs along the way.

They also conducted roadkill searches by car, travelling the routes before dawn on every second day.

By comparing these two datasets, the team found that traffic killed between 15 and 29 percent of every local population annually.

Roads accounted for 38 to 41 percent of all recorded deaths, making traffic the leading cause of mortality at each location.

Large populations can withstand losses

The unexpected result emerged when the sites were compared. The biggest populations experienced the highest number of vehicle deaths, but remained stable because reproduction and survival offset the losses.

The researchers used a computer model, Vortex, to project each population 50 years into the future. At existing mortality levels, the two largest populations had more than an 88 percent likelihood of surviving for that period.

“In the past, research has assumed that high roadkill rates have a large and negative impact on hedgehog populations and that a small amount does little harm,” said Dr. Moore.

“This does not tell us the full picture and in fact populations vary a lot in terms of size, reproductive rates, other causes of death and food availability.”

Small hedgehog populations cannot rebound

The smaller populations showed the contrasting pattern. Groups containing fewer than 15 hedgehogs were unable to replace even a small number of road deaths.

At these sites, the loss of only one or two animals annually could trigger a decline. The model indicated that the two smallest populations would halve within several years before becoming locally extinct.

At one village, present-day traffic advanced the projected extinction date by 18 years. In the smallest population, the model predicted extinction within 11 years regardless of the mortality rate.

“Hedgehogs are declining in the UK and we have found how even a small amount of roadkill – perhaps just one or two individuals a year – could have devastating effects on an already small population,” said Dr. Moore.

Male deaths alter the outcome

The outcome also varies according to which hedgehogs are killed. At two of the four locations, males accounted for more road deaths than females.

This is significant because hedgehogs do not form pairs. Populations can lose males while continuing to breed successfully, as a small number of surviving males can father numerous litters.

For the larger populations, this lessened the effect of high roadkill levels. It offered little safeguard to smaller groups, however, because losing several females could hinder breeding.

Habitat is the key factor

Supporting a vulnerable population requires attention beyond the roads. Larger areas of suitable habitat can sustain more hedgehogs, building the numbers that smaller populations require.

Keeping an untidy, wild area in a garden can provide both food and nesting sites.

Creating 13 cm square openings in fences and walls, often known as hedgehog highways, enables hedgehogs to travel between gardens without having to cross additional roads.

“We need to better support hedgehog populations until there are tried and tested ways to reduce collisions happening in the first place,” noted Dr. Moore.

Identifying the underlying causes

Fay Vass serves as Chief Executive of the British Hedgehog Preservation Society.

“This unique study not only looks at the number of hedgehogs being killed on roads, but examines the wider picture to help us understand how this can impact hedgehog populations now and into the future,” said Vass.

She said that hedgehog populations throughout the UK are declining, and that research of this kind is vital for identifying the root causes so conservation action can help turn the trend around.

Even the largest and healthiest populations have a threshold. When the team modelled an annual mortality rate of 40 percent, previously stable groups started to decline.

At 50 percent, even the biggest populations ultimately became extinct. This possibility is not implausible, as UK traffic is forecast to continue increasing.

The study was also partly carried out during COVID-19 lockdowns, when traffic dropped by as much as 49 percent. Under normal conditions, the true outlook may therefore be worse than these figures indicate.

Beyond roads and roadkill

Dr Silviu Petrovan placed the findings in a broader context. He oversees conservation strategy and research at the People’s Trust for Endangered Species.

He also has a research position in the Zoology Department at the University of Cambridge.

“These findings show that the decline in hedgehog populations in rural areas is likely to be exacerbated by roadkill, where many threats relating to land management have already made large areas of our countryside less suitable for one of our nation’s favorite mammals,” said Dr. Silviu Petrovan.

“Effective prevention of road mortality for hedgehogs is likely to be difficult given the vast network of existing roads, meaning that we need to consider widespread conservation measures to enable our rural hedgehogs to become more resilient to this important additional threat.”