What impacts do roads have on hedgehogs?


The most obvious impact of roads on hedgehogs is direct mortality. Hedgehogs, like many animals, are susceptible to being runover and a recent estimate suggests as many as 335,000 are killed on Britain's road network each year. In the past decade or so the number of hedgehogs seen dead on roads in the UK has declined, but this is believed to be a reflection of a falling population and not a reduced susceptibility to traffic. In addition, roads can act as barriers to hedgehog dispersal, essentially fragmenting habitats, while pollution from motor vehicles may impact the hedgehogs directly (e.g. air/water pollution) or indirectly (reduction in or contamination of soil-dwelling prey).

The details

The strategic road network (SRN) in England. - Credit: Department for Transport (DfT)

In 2015, the UK government estimated that we have 360 thousand kilometres (about 246 thousand miles) of road; an increase of 2.4%, or 93 thousand kilometres (58 thousand miles), in the two decades since 1995. The majority of these, about 87%, dividing up Britain’s towns and cities and cutting through our villages and countryside bringing cars into contact with hedgehogs. Moreover, traffic volumes on Britain’s road network seems ever-increasing. During 2016, the Department for Transport statistics show motor vehicle traffic was at a record high of 320.5 billion vehicle miles; up 1.2% from last year, which itself was up by 1.6% on 2014’s volume. In addition to people doing more journeys, the number of cars on Britain’s roads is also increasing as the Campaign for Better Transport reported that bus use had declined by two-thirds in the last six years. In January 2016, Philip Gomm of the RAC told the BBC News website:

Over the past 20 years, the rise in the number of cars on the road in Britain has been relentless, going up from 21 million in 1995 to 31 million in 2015.

Similarly, large road networks and traffic volumes are found in western Europe and the European Commission’s ‘eurostat’ website states:

Although motorways constitute only a small part of the entire road network, their length has more than tripled over the last 30 years.”

Roads and the traffic they carry has a significant potential to impact wildlife populations. Perhaps the most obvious impact roads can have on wildlife is as a source of mortality; most of us will have seen animals dead by the side of roads. Impacts can, nevertheless, be more subtle, too. Habitat is lost as new roads are built, and the roads themselves, particularly major roads such as motorways, may act as a physical barrier to prevent animals moving from one area to another and, hence, populations from mixing. Finally, the pollution from road run-off and vehicle emissions may have less conspicuous effects; reducing food and causing low-level poisoning. Road traffic collisions are the impact we have the most data for, so we’ll begin with these.

Death under the wheels

Tracking studies have shown considerable variation in the susceptibility of hedgehogs to traffic. In The New Hedgehog Book, Pat Morris’ described one of his studies, during which he tracked 80 hedgehogs and found only a couple were killed on the very busy local roads – Morris noted a similar study in New Zealand that observed only 4% of tracked individuals dying on the roads. During his Ph.D. thesis, conducted on a London golf course, however, Nigel Reeve found that traffic accidents accounted for about 18% of known deaths. Tracking of hogs released into Oxford’s Wytham Woods by Patrick Doncaster found 13% were killed on the roads; representing one-third of the known deaths and, in their 1991 paper to The Veterinary Record Ian Keymer and his colleagues reported that 35 of the 74 (47%) dead hedgehogs they autopsied had been killed by cars. (Caution should be applied to the statistic from Keymer et al., however, because most hedgehogs provided to them had been hit by cars.) Han Kristiansson studied a hedgehog population in southern Sweden between 1972 and 1979 and found collision with cars was the main cause of death during the summer months; accounting for about 80% (32 of 40 hogs) of known mortality.

Traffic represents a significant danger to hedgehogs, with roads adding to an already fragmented habitat. - Credit: Marc Baldwin

In terms of the number of hedgehogs seen on a per kilometre driven basis, the figures show huge variation according to location and habitat type. A study by Pat and Mary Morris published during 1988 looked at hedgehog road casualties in New Zealand and the UK. The researchers found that the average fatality for the UK was 0.04 hogs per kilometre, i.e. one hedgehog for every 24 kilometres or so driven. The dataset varied from 0.03 to 0.06 according to the type of road, traffic volume and peripheral habitat. The Morris’ estimates were substantially lower than the average mortality rate observed by Keymer and his colleagues, who observed 1.28 hogs per km (ranging from a maximum of two hogs per km to one every two km) on a stretch of “fairly busy, class B road in north Norfolk”, but higher than those recorded by Amy Haigh and her colleagues on Irish roads, where the fatality rate was much lower at around 0.003 per km (one hog per 379km driven). In their study on the hedgehogs brought into some rescues centres in the UK and Netherlands, published in the journal Lutra in 1999, Nigel Reeve and Marcel Huijser reported that none of the road casualties originated from Jersey, which they attribute to the smaller road network, fewer cars and lower speed limit. Overall, this serves to illustrate how variable road casualty rates are within the UK.

Similar variation in road can be seen across continental Europe, where annual mortality rates are higher. In Germany, for example, it has been estimated that in the west of the country alone, as many as one million hogs may die on the roads annually. A study in south-east Germany, the results of which were published in Zeitschrift für Säugetierkunde during 1981, found more than five hedgehogs per kilometre in small villages, although the overall average was one per km. Studies from southern Sweden and northern Spain have yielded slightly higher average casualty rates of 1.67 and 1.71 hogs per km, or about one hog per 600 metres or so. In their 1998 review of the road casualty rate in The Netherlands, where it is estimated that as many as 340,000 hogs may be killed on the roads annually, Marcel Huijser and colleagues report that the minimum estimates for hedgehog road casualties are between 0.3 and almost three per kilometre. So, across Western Europe as a whole hedgehog road casualty rates are highly variable, although the average annual rate of hedgehog road death may be closer to two individuals per kilometre of road.

In New Zealand, the Morris’ found that the overall average was 0.064 hedgehogs per kilometre; i.e. one dead hedgehog for every 15 kilometres (9 miles) driven. When these New Zealand data are compared with the equivalent British data collected by the Morrises, it is clear that more hedgehogs were killed on the roads in New Zealand than in Britain, and this is despite New Zealand having a lower traffic density. An obvious explanation for this might be that the hedgehog population in New Zealand was higher than in Britain, but habitat was an important factor. The highest casualty rates (0.11 per km, or one hog per nine km driven) were observed in suburban and horticultural areas. Thus, if New Zealand has more of its road network running through such habitat than Britain, we might expect to see more road casualties. A team led by Amy Haigh observed something similar in Ireland; almost 60% of hedgehog casualties were found on roads running through pasture.

Patterns in the data

Driver surveys suggested a peak in road deaths during June, July and August, a finding confirmed by scientific surveys that have also shown a distinct sexual skewing of casualties through the year. Kristiansson noted a tendency for males to be more at risk of road traffic accidents early in the year and females later in the year, as did Nigel Reeve and Marcel Huijser in their review of wildlife rescue centre data from Britain and The Netherlands, published in 1999. Neither of these studies reported more males than females in total; the separation was only in the timing. Haigh and her colleagues observed a similar trend but also found that overall significantly more males than females were collected from Irish roads during their study between April 2008 and November 2010.

A hedgehog killed on an urban street. One recent study suggested as many as 335,000 hedgehogs may be killed on Britain's road network every year. - Credit: Marc Baldwin

Several authors have pointed out that males often arouse from hibernation early to start feeding up before they go looking for mates – in doing so they frequently roam further than females. Ergo, the fact that males are most active during the spring and summer months, which also represents the peak of the breeding season, probably accounts for the observation that as many as 70% of hedgehog traffic victims during this time are males. Conversely, it is the females that tend to be more active during the late summer and autumn – the pressures of raising hoglets may mean that she cannot devote sufficient time to feeding during the summer, so she must continue to feed later into the year. This feeding dichotomy may explain why more females are killed on the roads later in the year, when many males may already have settled into hibernation.

In terms of susceptibility to traffic, it may be more than habitat, traffic volume and sex that make a difference – age may also have a role to play. Kristiansson, for example, found that most of the hedgehogs run over during his study were adult animals. It is possible this simply reflects a behavioural trait of youngsters ranging less widely than adults; but, an intriguing study in Europe suggests it may also result from a difference in size and thus clearance under a car. At the Fourth International Hedgehog Workshop held at the University of Lund in Sweden during January 2000, Marcel Huijser presented the results of his study on whether cars could drive over hedgehogs without hitting them. Huijser found that about one-quarter of the passenger vehicles he tested had insufficient clearance to prevent hitting a walking sub-adult hedgehog; almost 98% could clear a juvenile of two or three months old.

While it may be argued that driver surveys are perhaps not the most rigorous method of assessing population changes, it cannot be denied that they do provide valuable information on the distribution of animal populations. One interesting point that can be seen in almost all driver survey data is that more hedgehogs are found dead on roads in and around villages, towns and cities than those running through rural locations. For example, between June 2001 and August 2003, Grzegorz Orlowski and Lech Nowak at the Agricultural University of Warclaw studied nearly 50km (34 mi.) of road in the agricultural landscape of the Lower Silesia region of south-west Poland. The biologists report that 75 (about 20%) of the 383 mammal carcasses they found were hedgehogs and 70 of them (93%) were found in built-up areas; the remaining four were killed in what the authors described as “open countryside”.

We might expect that urban roads have higher traffic volumes and traffic may be more constant (i.e. day and night) and therefore the chance of a hedgehog encountering a car in an urban area is higher than in more rural locations. This is, perhaps, part of the story but the data available for hedgehog population distribution suggests something else. We may see more hedgehogs dead on urban roads because it is in urban settlements that they find refuge. It has been suggested that factors including changing farming practices and increasing badger numbers have led to much of our modern countryside being unsuitable for hedgehog habitation; rather ironically, it is thought that it’s the diversity of habitats offered by our gardens that provides them with the resources they need, especially since the shift towards organic and wildlife gardening.

Pick a number

So, what does this mean in terms of total hedgehogs run over each year? In his Complete Hedgehog, the late St. Tiggywinkles founder Les Stocker estimated that somewhere in the region of ten thousand hedgehogs are killed on these roads each year. Pat Morris arrived at a similar figure in his New Hedgehog Book. Based on the combined Mammals on Roads surveys between 2001 and 2004, which found 6,411 hedgehogs during 270,000 miles of car journeys between July and August, and scaling up while accounting for six months of inactivity, Morris arrived at a figure of between 12,000 and 15,000 hedgehogs killed annually on our roads – it is ordinarily the higher of these figures that is quoted by animal welfare charities, although a more recent analysis suggests both may be considerable underestimates.

A road sign in Wellington, Somerset, warning drivers to watch out for hedgehogs on the roads. - Credit: Amanda Burton

In a paper to the Mammal Society’s journal Mammal Communications, published in 2016, David Wembridge, Martin Newman, Paul Bright and Pat Morris review data from four surveys, conducted between 1952 and 2004, and attempt to re-estimate the number of hedgehogs killed on Britain’s roads every year. Wembridge and his colleagues factored in aspects such as the road width, which varies with road class and is strongly associated with the number of casualties. Perhaps more importantly, the researchers also took into consideration how long the bodies of hedgehogs remain identifiable on the road, which varies from about a day to 20 days or more (depending on the road, weather, traffic volume, activity of predators, etc.), and how likely dead hedgehogs are to be spotted by surveyors. For the purposes of their analysis, they calculated estimates based on a range of these persistence times and detection rates. Persistence times of 4.5, nine and 18 days were considered and estimates calculated assuming a detection rate of 0.4 and 0.8 (i.e. 60% and 20% of carcasses, respectively, are missed by surveyors) for each persistence time – the result being six estimates from worst case to best case.

When the existing survey data were fed into this model, the final output was a range between 83,700 hedgehogs killed if the carcasses persisted for 18 days and 80% were found, to 669,000 if they persisted for 4.5 days and only 40% were found. Wembridge and his co-workers considered, however, that a detection rate of 80% was most realistic and settled on persistence times of between 4.5 and nine days, based on findings from other surveys. The result is a final estimate that between 167,000 and 335,000 hedgehogs die on roads in Britain every year.

Taking the worst-case scenario from the analysis presented by Wembridge and his team, this means about 20% of the population, or one in five hogs, will be run over in any given year. Similar figures have been estimated in Europe, although it remains unknown whether such losses represent a significant strain on hedgehog populations. In a 2000 paper to Biological Conservation, Marcel Huijser and Piet Bergers suggested that traffic could reduce hedgehog populations in The Netherlands by about 30% and that this is probably sufficient to affect the survival of local populations.

By contrast, in his paper to the Journal of Zoology in 1990, Hans Kristiansson argued that, although road kills have been regarded as a threat to the hedgehog population in Sweden, the concern is exaggerated; he suggests that only populations with a small number of breeding females are likely to be at significant risk. Kristiansson’s data suggest that breeding females are more likely to be involved in road accidents late in the year, after they’ve bred and weaned their hoglets. The jury is still out, however, and we cannot say with any certainty whether 20% is a large enough loss to affect hedgehog populations in Britain. Traffic is only one of a myriad of sources of mortality and it seems likely it will have different impacts depending on what other pressures a given population is under.

Cattle grids

Death under the wheels of vehicles is the most obvious form of road-related death; however, cattle grids can also present a big problem for hedgehogs. In The New Hedgehog Book, Pat Morris writes of one particular grid that he heard about in which 52 hedgehogs had died. Cattle grids consist of a pit, of varying depths, stretching the width of the road with bars running its width – the idea is that the bars are spaced sufficiently far apart that the legs of any animal trying to cross it can slip between them, but that the wheels of vehicles cannot. Unfortunately, most hedgehogs are also small enough to fit between the bars and hedgehogs in general are pretty good at falling into things! Once in the pit, there is no escape from its sheer sides.

Many cattle grids are now built with ramps on one side to allow the escape of any small animals, particularly hedgehogs and frogs, that accidentally fall in. - Credit: Phil Baldwin

The plight of hedgehogs in cattle grids was first publicised by Major Adrian Coles MBE, who set up the British Hedgehog Preservation Society in April 1982 after finding a hedgehog that had fallen into a grid on his land. Major Coles used his influence as a councillor of Shropshire County Council to persuade them to install escape ramps in all the grids in their authority. In 1982, the first escape ramp was installed in a cattle grid on the A117, near Ludlow in Shropshire. Currently, in the UK, the design of cattle grids is stipulated by the British Standards 4008 and BD37/88; hedgehog ramps are a recommended inclusion. Section 7.1.6 of the Highways Agency’s Design Manual for Roads and Bridges (vol. 6, sect. 3, pt 3/57-87, chap. 7) states:

It is recommended that a sloping ramp 150mm [approx. 6 inches] wide not steeper than 1 in 3 [18 degrees from horizontal] be constructed leading from the floor to the edge of the pit to allow for the escape of small animals, such as hedgehogs, which would otherwise be trapped.”

In his Complete Hedgehog, Les Stocker presents different dimensions for the ramp, writing that it should be at least 200mm (8 inches) wide and not steeper than 30 degrees from horizontal (equivalent to a gradient of just less than 1 in 2).

Fatal attraction?

Before we look at the impact that roads can have on wildlife populations, it is worth taking a moment to consider why hedgehogs may be found near roads in the first place. In his discussion on hedgehog conservation, published in the German journal Natur und Landschaft during 1971, biologist Walter Poduschka as among the first to suggest that hedgehogs may be attracted to roads in search of food; road surfaces are generally warmer than the surrounding land and this may be attractive to insects and, following rain, earthworms. Similarly, roadside verges can represent good foraging sites as they’re generally allowed to ‘grow wild’; the mix of wild flowers and shrubs is attractive to insects that are fed upon by hedgehogs, while the taller grasses and shrubs themselves may provide cover and therefore be a draw for hedgehogs.

The promise of food is probably one factor that draws hedgehogs to roads, but when you take into account the data we have on their habitat preferences, it seems that the link between hedgehogs and roads may be more intrinsically linked. Hedgehogs seem to prefer edge habitat to that of more closed woodland; they tend to stick close to and follow linear features such as path edges, hedgerows and treelines. In a 2001 paper to the Journal of Animal Ecology, for example, Patrick Doncaster, Carlo Rondinini and Paul Johnson examined how hedgehogs reacted to novel or unfavourable terrain by radio-tracking hedgehogs released in areas of varying habitat favourability. The results of this study showed hedgehogs exhibited a strong attraction to habitat edges and the biologists suggest that such edges may act as dispersal/foraging corridors; the biologists also observed that more animals stayed close to roads and urban areas (cf. arable pasture) than would be expected by chance.

Hedgehogs may be attracted to roadside verges where short grass and/or the presence of wild flowers provides increased feeding opportunities. - Credit: Marc Baldwin

So, if hedgehogs take advantage of edge habitat and linear features, does the placement of such habitats relative to roads impact the number of traffic victims? The answer appears to be yes. In a brief but fascinating note to the winter 1999 issue of The Road RIPorter (the quarterly newsletter of Wildlands CPR, a US-based charity that campaigns to restore areas of habitat to their natural state by limiting off-road vehicle use) Marcel Huijser presented the preliminary results of a survey on hedgehog habitat use and road mortality – the final data analysis was published as part of Huijser’s thesis.

Huijser wrote that these data corroborate previous tracking studies, finding that hedgehogs spend much of their time in or close to hedgerows and forest edges, using closed forests infrequently; but also imply that where hedgerows or woodland edges are orientated perpendicular (i.e. at right-angles) to a road, we can expect as many as 25% more traffic victims compared with hedgerows situated parallel to roads. In other words, perpendicular hedges may guide hedgehogs straight onto roads. Certainly, during the tracking studies Amy Haigh carried out in Ireland as part of her Ph.D. thesis, individual hedgehogs (adults and juveniles) appeared to enter fields and cross roads at specific spots. Haigh doesn’t mention the position of hedgerows relative to these crossing points in her 2014 paper to Wildlife Biology, but she does suggest that it might be possible to use these data to target mitigation activities, wildlife tunnels for example, more effectively.

Unfortunately, despite Britain being one of the first countries to introduce wildlife underpasses, with badger tunnels built as early as 1958, we have very few projects aimed at reducing wildlife road kill. Spain and the United States, for example, have multiple wildlife tunnels and bridges to allow wildlife to cross busy main roads, particularly close to national parks, while the Danish government spends some US$5 million (£4 million or €4.6 million) per year trying to reduce animal traffic accidents.

The intersection of linear features with roads may help explain why some areas seem particularly bad for hedgehog road deaths; so-called hedgehog ‘black spots’. In his New Hedgehog Book, however, Pat Morris implies that reports of the numbers of hedgehog road deaths may be exaggerated, because hedgehog skin is tougher than that of many of the other species commonly falling foul of traffic (e.g. rabbits) and stands up to the battering of tyres – this is the persistence time we’ve already mentioned. Consequently, one hedgehog carcass could persist along a road for several weeks giving the appearance of black spots. While this seems a very plausible problem for the driver surveys, not all authors agree. In a 1998 paper to the Dutch zoology journal Zoogdier, Marcel Huijser and Piet Bergers report that as many as 65% of hedgehog corpses disappear from the road within 24 hours, suggesting that numbers may actually be underestimated.

Carving up the country: habitat loss and fragmentation

Roads are widely implicated in the fragmentation of habitat, which may reduce or even prevent mixing of adjacent populations. It stands to reason that serious fragmentation may lead to the isolation and eventual extinction of populations. If populations become isolated from their neighbours they tend to suffer reduced genetic diversity as a result of inbreeding (i.e. mating with their relatives), which reduces the available gene pool and can make the population less resistant to changing conditions. Furthermore, isolation decreases the ability to recolonize after a catastrophic event or even simply replace individuals lost to road traffic accidents. We have already seen that, in some rural areas, hedgehogs may be attracted to roads to feed; but this does not necessarily mean they’re also willing to cross road. (Much like being attracted to a river to drink doesn’t necessarily indicate a willingness to swim.) Indeed, tracking studies of hedgehogs in urban settings have shown that roads may have a ‘barrier effect’, with animals appearing to avoid some roads altogether.

Urban sprawl is an increasing phenomenon in Britain and concerns have been raised that it is fragmenting habitat and isolating small mammal population. - Credit: Matt

While tracking hedgehogs in two areas of Southampton, Thorndean and Redbridge, Carlo Rondinini and Patrick Doncaster observed that eight of their 16 subjects didn’t cross any roads, despite the simulation models they created suggesting all 16 would be expected to cross at least one. Of the 76 tracking trajectories (routes) collected, only 18 crossed roads at some point. Interestingly, there was some individual variation because, although more than half of the hedgehogs in Thorndean failed to cross any roads, a few individuals crossed frequently. Overall, when the biologists looked at the habitat frequented by the hedgehogs and ranked it in order of preference, they found that road was the least preferred habitat at both sites. Writing in the journal Functional Ecology during 2002, Doncaster and Rondinini conclude:

Our field test has shown an overall significant tendency for both sexes alike to avoid crossing roads, with avoidance increasing in proportion to road verges compared with playing fields and gardens.”

Where Doncaster and Rondinini observed hedgehogs with a preference for roadside verges, they found that these animals tended to use them as day resting sites. Finally, the biologists noticed that if their subjects were going to cross, they tended to run. Doncaster and Rondinini witnessed hedgehogs running with their legs extended and their belly raised above the ground; this was in contrast to the low posture adopted while foraging. Similar behaviour is reported by Nigel Reeve in Hedgehogs; by Fabio Bontadina in his thesis at the University of Zurich in 1998; and by Jaap Mulder in his 1999 abstract to the journal Lutra about the behaviour of hedgehogs on roads. Doncaster and Rondinini suggest that hedgehogs may run across roads because they dislike the feel of the synthetic surface and because crossing roads exposes them to danger and, frequently, bright lighting.

During a study along a disused stretch of road in Windsor Great Park, a deer park on the Berkshire-Surrey border in England, researchers caught hedgehogs from the surrounding area and placed them in boxes on the grass verge, with the open end facing on to the road. The hedgehogs’ behaviour—in terms of walking, running, pausing, freezing and hunching—was recorded from the time the animal left the box to the time at which it either crossed the road or moved away from it. The biologists then drove a car along the road at a steady 20 mph (32 kmph) once the hedgehogs had left the boxes.

Roads, particularly large fast ones such as motorways and dual carriageways, can be significant sources of mortality for wildlife and act as barriers to movement, resulting in habitat fragmentation. - Credit: Marc Baldwin

The results from the experiments show that the hedgehogs were just as likely to cross the road as they were to wander along the verge; however, their behaviour on the road was different to that observed while on short grass. Hedgehogs moved twice as fast on roads and animals paused at the road edge and raised their body posture when running across the road – this, the authors suggest, may represent an aversion to the synthetic surface. When the car was used, hedgehogs would either freeze or run away; this reaction was seen at relatively close range. Although 80% of the animals reacted to the engine starting 50m (164 ft.) away, the average distance for most to respond to the approaching car was 17m (56 ft.) and those that chose to run started to do so when only about 8m (26 ft.) from the vehicle. The authors point out that the short reaction distance suggests that running is unlikely to be any more beneficial than freezing; it may even be worse, because the hedgehog’s profile is taller while running than it is when freezing, so a car is less likely to clear the animal.

If hedgehogs actively avoid roads in certain situations, or at the very least are prone to higher mortality when crossing, there is the very real potential for roads to act as physical barriers to hedgehog movement. We know from modelling studies, such as that published in the journal Ecological Modelling during 2005 by Jochen Jaeger and colleagues, that populations living in areas surrounded by roads are less likely to receive immigrants from other areas, and this puts them in a precarious position as we’ve already mentioned. Moreover, other well-known results of habitat fragmentation include changes in the vegetation composition (more weeds and invasive species) that consequently affects the type and quality of the vegetation and, ultimately, the flow of energy and nutrients within the system. In addition, fragmentation can alter the availability of cover in the habitat, resulting in so-called 'edge effects' that can bring species together that might not otherwise interact, increasing predation. Finally, it can affect the movement of prey species. In his paper to Biological Conservation in 1984, for example, Hans-Joachim Mader at the Institut für Naturschutz und Tierökologie in Bonn described how even a single road cutting through a forest in his study area near Heidelberg in west Germany seriously restricted Carabid activity, effectively dividing the population in two either side of the road:

During a two-year mark-release-recapture experiment only a single beetle out of 742 recaptures was caught after crossing the highway at this study area.”

Tracking studies suggest hedgehogs move faster across tarmacked surfaces, suggesting they don't like travelling across them. - Credit: Marc Baldwin

Habitat fragmentation is a particular concern of hedgehog biologist Hugh Warwick and, in an article to the December 2016 issue of British Wildlife, he points to tracking studies demonstrating how hedgehogs may travel a couple of kilometres (just over a mile) in a single night while foraging and this may be seriously hampered by roads. Referring to an analysis carried out by University of Oxford biologist Tom Moorhouse (see main hedgehog article), Warwick explains:

A Minimum Viable Population (MVP) analysis has shown that for the very best habitat, which is not far off suburbia for a Hedgehog, the minimum estimate for a sustainable population would be 32 individuals in 90ha of contiguous land. That is nearly 1km2 of good-quality, connected land. The MVP estimate for a rural landscape is considerably larger, with a minimum of 120 individuals in at least 3.8km2.”

The suggestion is that an increasing road network makes maintaining suitably large and connected areas of habitat more and more difficult, particularly when thinking about motorways and dual carriageways. Indeed, Warwick mentions work currently underway at the University of Reading to look at whether hedgehog populations either side of large motorways, such as the M4, are genetically distinguishable.

Through a glass darkly

In conjunction with the direct impact of mortality, roads can affect wildlife in ways that are perhaps more subtle. Generally, it is considered that roads serve to intensify toxic contamination, most notably from exhaust emissions, among roadside populations. Since the late 1970s, we have had data showing that concentrations of certain heavy metals increase in invertebrates and small mammals with increasing proximity to motorways and with increasing traffic volumes; lead in earthworms, an important prey species of the hedgehog, for example. Asphalt run-off contains high concentrations of lead, cadmium, copper and zinc, as well as de-icing salts, rubber fragments and polycyclic aromatic hydrocarbons (PAHs). Indeed, a study in Shanghai, China looking at the rainwater run-off from asphalt roads found that PAHs were present in substantial quantities, up to 4,023 ng/L, with almost half originating from vehicle emissions.

Pollution from vehicle exhausts is an under-studied impact of roads on hedgehog populations. - Credit: Marc Baldwin

Similarly, in a 1999 paper to the journal Water Research, Katerina Perdikaki and a former lecturer of mine at the University of Essex, Chris Mason, presented data on invertebrate samples (waterlice, river shrimp and alderfly) from nine East Anglian rivers, collected above and below crossings of the A12 and A14 trunk roads. Perdikaki and Mason found that concentrations of lead were significantly higher in both sediments and invertebrates downstream of the road crossings than upstream of them.

Many of the heavy metals and other pollutants that may be present along the roadside are lipophilic (i.e. fat soluble, which could potentially be released during arousal from hibernation) and may persist for many years, impacting reproduction and possibly leading to the death of the animal. I must emphasise that I am not aware of any significant studies demonstrating this in hedgehogs. I suspect, however, that this more likely reflects a lack of study in this area and there seems no good reason to assume that the hedgehog should be immune to such things, while other small mammals (typically rodents) are not.

Cause for concern?

So, with all the potential problems that roads can cause hedgehogs, do they pose a significant threat to the hedgehog population as a whole? Morris points out in The New Hedgehog Book, that if his estimate of 15,000 hedgehogs killed on Britain’s roads per year is correct, it represents only about 1% of the total estimated population and is unlikely to have a substantial impact. The more recent data suggest is may actually be 10-20% and David Wembridge and his colleagues, who included Pat Morris, suggest:

If road mortality in Britain is appreciable, as suggested here, populations isolated as a result of habitat fragmentation may face a greater extinction risk than considered previously.”

In his Road-RIPorter article, however, Huijser argues that although local populations may, indeed probably have, become extinct because of traffic, the net balances of human influences (i.e. green spaces, playing fields, gardens etc.) seem to be positive. In addition, the tracking data we have suggests that while hedgehogs may avoid roads, many do still cross and roadside verges may offer potential nesting and feeding sites for them. Haigh’s work in Ireland also suggests that it may be possible to target activities to reduce road deaths or help crossings. Finally, Kristiansson and Nigel Reeve have both suggested that although many females are killed in the autumn, they have probably already bred by this time and, when compared to factors such as climate and food availability, road mortality is unlikely to be a major influence on hedgehog populations. Conversely, however, the high mortality of males happens early in the year, perhaps before many have had the opportunity to breed and it remains unknown whether this affects the mating probability for females.

With increasing habitat fragmentation caused by urban development and road-building, gardening for wildlife is increasingly important for the continued survival of species such as hedgehogs. - Credit: Marc Baldwin

I don’t think there are many biologists who think that we’ll lose our hedgehog population purely from road traffic accidents; this is but one nail in the metaphorical coffin. The more pressing question is how road deaths, combined with other factors (e.g. predators, garden hazards, climate change, pollution, pesticides etc.), add up to influence the population. There is some concern that roads may be the final straw…