The New Naturalist Library: Hedgehog

HOW MANY HEDGEHOGS ARE THERE and what controls their numbers? Why aren’t there more of them? Is it true that there are fewer these days? These are important and legitimate questions that are frequently asked and in recent years there has been some progress in addressing them, albeit with the usual problem of uncertainty due to the limited number of studies.

Bearing in mind that animals add offspring to their population each year, why don’t we end up with too many of them or indeed too few given that animals also die, sometimes in large numbers. The natural mechanism by which populations are regulated was at one time a hot topic for discussion by academic ecologists. It was suggested that there were two basic ways in which animal populations could be held in check, by density-dependent factors and density-independent ones. The latter include extraneous influences that are there anyway, irrespective of how many animals are present. That would include the weather, for example, ‘good’ and ‘bad’ years would cancel each other out and maintain numbers that fluctuated around a normal, long-term average population density. In the case of the hedgehog, a prolonged winter might mean fewer animals surviving and a reduced population until this was offset by a warm, moist summer with plenty of food about and enhanced survival of the young, allowing numbers to pick up again. During his studies of hedgehogs in the Outer Hebrides, Digger Jackson gathered data which showed that the weather did indeed have a strong effect on breeding success in the local hedgehogs. The population density was strongly correlated with temperatures during the previous winter and preceding summer, with warmer conditions boosting both breeding success and survival rates (Jackson, 2007).

Another density-independent factor affecting hedgehogs might be the availability of nest sites. This is certainly something that limits the local abundance of hole-nesting birds, whose numbers can be boosted by providing tree-hole substitutes in the form of nest boxes. The same applies to dormice and putting up nest boxes, as a substitute for tree holes increases their population density. But hedgehogs are not constrained by having to nest in only one type of site, and it is hard to think of another factor that might limit their numbers in a similar way. However, they do need special facilities for winter nesting which will not be available out in open fields, for example, but there is little to stop them congregating in locations that do have the necessary attributes. In my study of hedgehog hibernacula in Bushy Park (see Chapter 6), there could be a dozen hedgehog nests within a 100 metres or so on some occasions, with plenty of room for more if there had been greater demand.

Density-dependent factors are those that act directly in response to the number of animals already present. Too many animals eating worms or grazing a field will mean that any additional feeders will reduce the food available for everyone and cause some individuals not to thrive. This way the population is prevented from becoming too large. If it is reduced for some reason, there will be fewer animals competing for the food and the population increases in response to greater availability of food, perhaps through enhanced breeding success or better survival of the young. The increase continues until the habitat reaches ‘carrying capacity’, beyond which density-dependent control will step in to keep the numbers in check.

This is obviously a simplistic description and in reality the regulation of hedgehog numbers is bound to be brought about by a complex mixture of different factors. The academic issue need not concern us very much except that it became a serious financial and public relations matter when action was taken to deal with the hedgehogs that had been introduced to South Uist (see Chapter 12). A policy of killing the animals was adopted, assuming that translocating them to the mainland would be harmful if the numbers there were density-dependent. The logic was that if the mainland of Scotland could support more hedgehogs, then their numbers would already have increased until carrying capacity was reached. In other words it was assumed that the mainland was already ‘full up’. Taking hedgehogs from the Uists to the mainland would then displace others already there, to the likely detriment of their welfare. It was therefore deemed appropriate to kill the Uist hedgehogs rather than remove them for release elsewhere. The logic is clear, but there is no actual evidence to support the assumption that the size of the hedgehog population is density-dependent. Like the Emperor who had no clothes, the policy lacked a sound foundation. In fact one of the few hedgehog population studies available at that time explicitly said, ‘It is argued that the population size appears to be more influenced by environmental factors such as food availability, winter nest sites and winter climate than by density-dependent factors’ (Kristiansson, 1990). Ignoring, or overlooking, that study cost the lives of over a thousand hedgehogs before the policy was changed to one of translocation rather than killing.

Actually it is hard to see how the presence of too many hedgehogs would result in a natural suppression of their numbers. They do not appear to have social behavioural mechanisms that might achieve some sort of population control. For example, they do not defend exclusive territories or feeding areas and they will often feed together at a food bowl or in a good feeding area with little sign of one animal attempting to monopolise the resource, even in times of food shortage. Running short of food because of excessive numbers might cause more to starve and thus reduce the population, except that hedgehogs are highly omnivorous. If they were short of worms, for instance, the evidence suggests that they would switch to something else. Any situation where all the many and varied potential food types became dangerously scarce as a result of hedgehogs eating themselves out of house and home would hit a lot of other creatures too, such as blackbirds, weasels, polecats and frogs. A situation like that might arise as a result of prolonged drought or ongoing use of pesticides, but not because hedgehog numbers had doubled.

In practice, of course, it turns out that population regulation mechanisms are highly complex, not just a matter of one type of control or another. In the case of hedgehogs, it is likely that factors such as weather and the duration of winter will play a key part in some years, but not in others. Density-dependent factors might become important in localised areas, especially in dry weather, but predation could also be an increasingly significant issue. Patrick Doncaster artificially increased the local hedgehog population density by importing some from elsewhere. Subsequent dispersal, especially by males, and predation by badgers reduced the population close to its original level within one month. After that, the number of hedgehogs remained relatively stable for a further two months of the study (Doncaster, 1994). As for the many threats posed by pesticides described in Chapter 9, while these will undoubtedly reduce numbers of hedgehogs, they can hardly be said to control them because they do not act to support or build up the population, only reduce it. Similarly, when gamekeepers seek to control hedgehogs, they are aiming at a population size of zero rather than an optimum level.

FIG 203. Hedgehog numbers vary between habitats and from year to year, probably in response to a wide variety of influences.

Another topic that has excited interest among academic ecologists is the occurrence and possible causes of population cycles. There is a possibility that regular fluctuations in hedgehog numbers will be reflected in the vermin bag records kept by gamekeepers on large estates. This prompted a small study which looked at numbers of hedgehogs that had been killed across two decades, between 1943 and 1965, on an estate in Hertfordshire and another in Hampshire (Jefferies & Pendlebury, 1968). There were major differences in the numbers killed per 1,000 acres on the two estates, but no regular pattern in the numbers taken each year that might reflect a cyclic population. The problem with this study and other analyses of vermin records, including amounts paid out by churchwardens (see Chapter 12), is that the number killed is not simply a reflection of population size. It also depends on the amount of effort (e.g. the number of active gamekeepers) put into killing the hedgehogs. There is no way of measuring the effort that was spent catching them each year long ago. If it was not constant year to year, any apparent fluctuation in numbers of hedgehogs might simply be due to the greater or lesser number of people or traps being used to catch them. Similarly one cannot compare one place with another unless the catching effort was the same at both. As a result, there is no reliable evidence that hedgehog populations are cyclic like those of certain rodent species such as lemmings. However, numbers do appear to fluctuate from year to year, probably as a result of variations in breeding success and food availability, but not in the form of regular cycles of abundance.

Repeat surveys using the same methods and amount of effort have certainly shown that a hedgehog population can collapse. Hedgehogs were introduced to the island of North Ronaldsay in 1972 and by 1984 they were said to number 1,000 or more. A survey in 1987 by Hugh Warwick estimated the population at roughly 514, which appeared to have fallen to barely 100 when he surveyed the same sites using the same methods a few years later. More recently, the numbers are said to have increased again (Hugh Warwick, pers comm). Other populations established on islands have flourished for a while, but then dwindled to extinction (see Chapter 2). In New Zealand, Bob Brockie counted the number of hedgehogs killed on the roads that he drove regularly and found a severe reduction in 30 years, but no evidence of cyclic fluctuations.

With large animals that live out in the open, a telescope or aerial photographs might be used to count all those seen in a given area and thus calculate a population density in terms of numbers per hectare or square kilometre. Obviously that cannot work with small mammals such as mice and voles and for them we rely on a procedure known as ‘capture-mark-recapture’ (CMR). The standard technique is to set out a grid of traps that catch a sample of the animals alive overnight. These would then be marked in some way and released for the traps to catch another sample on the next night. The proportion of marked animals in the second sample (60 per cent, for example) is deemed to be the same proportion as the full second sample forms of the population as a whole. There are more sophisticated variants on this procedure (which is known as the ‘Lincoln Index’) that use multiple sampling to increase the accuracy of the estimate. CMR works well for species that live at high densities, such as insects or voles, but with hedgehogs many more traps would be needed, spread over an impractically large area, in order to obtain sufficient animals for the calculations to generate consistent results. In practice, traps are not used and instead most hedgehog studies have been based on walking about finding the animals by torchlight. Suppose you catch five hedgehogs on the first night and the second sample is only four, of which one is marked. That would imply a population size of 16 animals. But if the second sample was five, with one marked, the estimate would be 25. In other words, with a low-density population a single extra animal (or one less) makes a big difference to the calculated population size. Stumbling upon a mother with her three offspring, just once, would generate a completely different population estimate. I calculated that the population density during one of my studies lasting about three weeks (Morris, 1988) was 1 per 4.5 ha (0.22 per ha), although shortly afterwards four newly weaned young appeared, making it about 1 per 3 ha (0.33 per ha).

More reliable perhaps are population figures based on catching and marking all the animals present, although this will take some time and may miss a few. This is known as a Minimum Number Alive (MNA) estimate. Using this approach, Nigel Reeve had about 50 different hedgehogs present each summer on the golf course he studied, an area of 40 ha (equivalent to 1.25 hedgehogs per ha) in a whole season. But this included animals seen only once which, arguably, were not residents there. In a three-week period (to compare the estimate with mine above), many of these transient animals would not have been seen and the estimated population density would have been lower. On a golf course in New Zealand, Bob Brockie marked 207 hedgehogs on 56 ha in two years and 150 on 16 hectares of farmland in 17 months. At any one time, the farm population density was never above 2.5 per ha or above 1.75 per ha on the golf course. But such estimates are strongly affected by hedgehogs clustering around good places to feed. Pitfall traps around a dotterel colony in New Zealand caught 400 hedgehogs in two years.

FIG 204. Population estimates have almost all been based on searching for hedgehogs by torchlight in open grassland, but many hedgehogs live in areas of scrub or gardens, where they remain hidden. Estimates are likely to be inaccurate, variable and not applicable everywhere. In turn, this adds to the problem of estimating the total British population of hedgehogs.

FIG 205. Hedgehogs may cluster in gardens or other favoured feeding areas, making a very high population density locally, which creates a misleading impression of abundance when scaled up to take account of a wider area.

To translate a ‘population figure’ into an actual density as numbers per ha, it is necessary to know how large was the area being sampled. With grid trapping for small mammals, that is easy because the traps can be set out to cover a known area of say 0.5 ha. Hedgehogs sampled by torchlight are normally assumed to be living in the area searched, usually open grassland (Fig. 204).

Actually many of them will be nesting somewhere else, but where else? In other words it is hard to know how large is the total area being used by the estimated number of hedgehogs. The problem is particularly clear when hedgehogs are marked as they visit a food bowl in a garden (Fig. 205). Over a few weeks a dozen or more may visit a garden of 100 m2, but they don’t all live there. Our radio-tracking revealed that such a garden might not have any resident hedgehogs at all! So the population density is not 12 in 100 m2 (equals 120 per ha). It is 12 in who knows how many hectares. This may be an extreme example, but it illustrates the principle that the population density estimate is affected by the size of the sampling area. The smaller the sampling area (e.g. a garden), the greater the number of animal ‘passers-by’ that will be seen only once or twice. This creates an impression that the hedgehog population includes many nomadic animals, which may actually be true. According to Bob Brockie, about 20 per cent of the population in a given area may be ‘transients’; Nigel Reeve found a similar proportion on a west London golf course (Reeve, 1981). A similar effect is seen with hedgehogs that come to gardens, where a significant proportion of marked animals are seen once and do not reappear. If these transient animals are included in CMR calculations, it results in improbably high population estimates. Despite all of these drawbacks, most of the published estimates of hedgehog population density have relied upon CMR methods often using different analytical methods, making direct comparisons problematic.

Using powerful lamps, a team of searchers and a sophisticated version of CMR, Digger Jackson estimated that there were about 57 adult hedgehogs per square kilometre (0.57 per ha) in the machair grasslands of South Uist (Jackson & Green, 2000). No formal surveys were conducted in other habitats, but he considered that hedgehogs were less abundant on the peaty ‘blacklands’ and scarcer still on the upland areas of moorland, although they were still present there. Searching with lamps, a comparison of hedgehogs found on pasture and amenity grassland in four different English counties and two different seasons found an average of 0.47 hedgehogs per ha in the latter, but barely a tenth of that in pastureland (Parrott et al., 2014). In such studies, it is assumed that the size of the area used by the hedgehogs is the same as the area being searched by the observers, even though some of the hedgehogs will be living some of the time somewhere else.

‘Distance Sampling’ (DS) is a different way of estimating numbers that has been devised to overcome the problem of not knowing the size of the area being sampled. It involves walking along a transect route, counting animals seen on either side, together with their distance from the path being followed by the observer. The subsequent calculations will define retrospectively the area that has been sampled and express the population in terms of numbers per hectare (see, for example, Buckland et al., 1993; Hoodless & Morris, 1993). It is a method ideally suited to counting hedgehogs by torchlight, but suffers when too few are seen. That results in such variable population estimates that they become statistically unacceptable. Longer transects would find more animals and reduce the problem, but it is hard to find areas that are big enough to accommodate these within a fairly uniform habitat. It also means more hours need to be spent walking and searching.

One of my students, Alison Tutt, tried using both the DS method and various types of CMR on the hedgehogs of Alderney in the Channel Islands (Fig. 205). Transects were searched in areas of grassy habitat in order to estimate numbers of hedgehogs there and compare the results obtained by different methods. Eleven transects were chosen ranging from 175–2,750 m in length and sampled for up to 17 nights each. All of the hedgehogs were marked and data collected for analysis using CMR and DS methods. Averaged over several nights, the DS technique gave population estimates ranging from 0.42 to 0.63 hedgehogs per ha (a close match to the average reported elsewhere by Parrott et al., 2014), but the two shortest transects generated figures of 1.34 and 4.37 per ha (the latter being on the cricket pitch), highlighting the problem of variable results, depending on how the study is conducted, and the need to use the longest possible transects. Using five different versions of the basic CMR techniques generated density estimates in grassland areas ranging from 0.64 to 1.83 hedgehogs per ha. Again the cricket pitch, site of the shortest transect, generated much higher figures of 3.6 to 17.38 hedgehogs per ha, depending upon which CMR method was used.

FIG 205. Alison Tutt compared population density estimates using different methods among the blonde hedgehogs of Alderney. There were plenty of open areas to search at night, with few obstructions and little disturbance. Even the island’s airstrip was available as a sampling site.

Sampling the worms on the different transect sites showed that the cricket pitch had more than three times the density of worms (172 per m2) than on the other grassy habitats, with hardly any of them available (6 per m2) in an area dominated by bracken. That explains why the hedgehogs were actually preferentially feeding on the cricket pitch, aggregating at a higher density there and seriously skewing the results of the study. Another sampling study in France (Hubert et al., 2011) used distance sampling (43 transects, 127 hedgehogs) to estimate population densities in the Ardennes countryside at 4.4 per km2 (0.044 per ha) and 36.5 (0.36 per ha) in an adjacent urban area. The latter figure is similar to many of those reported in the literature for good hedgehog habitat in Britain; the former may be nearer what we might expect in intensively managed farmland. That study compared hedgehog numbers with availability of typical hedgehog food and found 14 per cent of their hedgehogs were on pastureland where there is plenty of food, with no hedgehogs in forest areas, despite an abundance arthropods and worms. But hedgehogs are harder to see in woodland than in pastureland.

TABLE 25. Population densities per ha in various grassland sites, estimated by different methods (based on Tutt, 1993).

In spite of all the drawbacks involved in population estimation, it is obvious that there are not hundreds of hedgehogs per hectare, and most of the published figures offer numbers that seem reasonable (Table 27). However, they are almost all based on ‘good hedgehog habitat’ suitable for doing research projects where you need to be able to find plenty of animals. This means mostly short grassy turf where hedgehogs are reasonably easy to spot and few will escape detection. We have little data on the numbers of hedgehogs that inhabit woodland areas and population densities in many rural habitats probably vary a lot according to what type of farming predominates locally.

TABLE 26. Comparative numbers of hedgehogs found in rural and urban habitats in France (based on Hubert et al., 2011).

TABLE 27. Some published estimates of population density. Despite being based on various methods, and for studies lasting varying lengths of time, there appears to be a consensus that hedgehogs generally occur at a density of less than one per hectare, with more in small and particularly favoured areas where there is abundant food.

At the time of writing (2017), the Random Encounter Method is being trialled, not used on hedgehogs hitherto. It aims to identify where hedgehogs are present but also estimate their abundance. It is based on the frequency of occurrence of animals, in footprint tunnels in this case (see Appendix), but results are not yet available.

Whatever method is used, hedgehogs may congregate in a small garden or good feeding patch, and create a misleading appearance of abundance. That becomes a serious problem when we try to estimate how many hedgehogs there are in Britain as a whole. This is a somewhat futile exercise, but journalists in particular always seem to want to know ‘How many are there?’ The first attempt seems to have used a density figure of one per acre (i.e. 2.4 per ha), suggested by Maurice Burton (Burton, 1969). When I asked him about this, he told me that it was based on seeing about ten hedgehogs in ten acres during an evening’s stroll in Kew Gardens. Scaling that up to take account of the total size of Great Britain would put the population at some 50 million hedgehogs, but most of Britain is not like Kew Gardens. We could eliminate mountainous areas and make it 30 million, then reduce numbers for arable land and so on. Employing this approach, a group of us (Harris et al., 1995) used different density figures for the various broad habitat types (for example, 1 per 2.5 ha for woodlands and unimproved grasslands, 1 per 10 ha for built-up areas and 1 per 20 ha for coniferous woodland, bracken and arable land), and estimated the pre-breeding population of the hedgehog to be about 1,550,000 in Britain (1.1 million in England, 310,000 in Scotland and 145,000 in Wales). This estimate also took into account the fact that most published population density estimates had been based on studies made during the summer and therefore included juveniles. The adult pre-breeding population would be somewhat smaller than any number based upon density figures obtained later in the summer (when most published studies were undertaken).

A more recent attempt to estimate the size of the national hedgehog population suggests a figure that lies between 731,546 and about 12 million! (Croft et al, 2017). But despite the highly sophisticated modelling used, the estimate still relies on the few published estimates of population density from the small sample of sites where relevant fieldwork has been carried out, sometimes long ago. The enormous range in that estimate is itself an indication of just how much we need better data and how ineffectual modelling is without it. We need a more structured approach to studying population densities. For example, the models predict high hedgehog numbers in arable habitats, but that is probably because published studies have (of necessity) focused on the animals that live around the margins of arable fields. As the authors point out, ‘the population density over most of an arable landscape may be close to zero’. Yet studies of population density in hedgehogs (or other species that live across a wide range of landscapes in comparatively low numbers) are extremely challenging for practical reasons and also not a fashionable subject for research funding. It’s all rather speculative and hard to defend whatever figure is offered as the total number of hedgehogs in Britain. It is so imprecise that it will be no use for monitoring population trends. For this we must fall back on using indices of abundance, which can be more reliably calculated.

One obvious indication of hedgehog abundance is the frequency with which we see them dead on the road. We can count hedgehog road casualties more easily than for many species because they are so conspicuous and also easily recognisable. For most people, the majority of hedgehogs they see are dead and flat. To ignore these seems a waste of an opportunity to learn something about hedgehog numbers, but what do the numbers actually mean? Some have argued, as they have with badger roadkills, that larger numbers dead on the road is an indication of a declining population because so many are being killed, leaving fewer alive (Fig. 206). Others argue the opposite, that more dead ones must indicate a large and healthy population in order to supply so many victims. Yet another line of argument says that the number of animals seen dead on the road is simply a reflection of traffic density and tells us little about the animals themselves (except that they are dead!).

FIG 206. Road casualties are the way we see most hedgehogs most often. They offer an obvious way of counting numbers, but what do those numbers actually mean? Do more roadkills mean more hedgehogs or do they imply fewer because so many have been killed?

Counting dead hedgehogs is relatively easy and likely to be accurate owing to their distinctive appearance, even when squashed. But people kept insisting it was a waste of time because ‘everyone knows’ that the number killed on roads simply reflects the number of vehicles passing by. It was ‘obvious’ that counting dead hedgehogs was just a crude way of measuring traffic density. This turns out not to be true. The number of hedgehogs I saw killed per 100 miles driving about in New Zealand in 1987 was much higher than in Britain, despite the far lower traffic densities there (Morris & Morris, 1988).

There are good reasons to believe that the numbers of other mammals that are killed on roads reflect their local abundance in the adjacent habitats. For example, the frequency of badger roadkills that I have seen in Britain has increased enormously in the past 30 years (sometimes I now see more dead badgers than hedgehogs) and is matched by independent estimates of an increasing badger population based on counts of setts, suggesting a link between population size and numbers killed on the roads. A survey of roadkill rabbits showed that the number seen dead was directly proportional to the numbers seen alive nearby (George et al., 2011). In other words, counting animals seen dead on the road could serve as an index of abundance in the living population. It does not say how many animals are alive locally, but it does reflect local population density. The number of hedgehogs killed on the roads does not simply reflect traffic density (Bright et al., 2015), a fact also confirmed by studies carried out in the Netherlands (Huijser & Bergers, 2000). So the scepticism has been refuted and we now have confirmation that counting hedgehog roadkill is a meaningful activity and worth doing.

FIG 207. J. L. Davies was the first to suggest monitoring hedgehog numbers by regular roadkill surveys and used numbers killed per 100 miles driven as an index of abundance to compare seasonal mortality between two years in Hampshire. It was 40 years before we tried that idea nationally.

But it is not a new idea. Sixty years ago, J. L. Davies published details of the number of hedgehogs he had seen killed on roads in Hampshire and the distances that he had driven (Davies, 1957). He suggested that this could form the basis for an index of abundance, the number of casualties per 100 miles. This indicated a greater abundance of hedgehogs in the second year of his observations (1953–54) compared to the previous year (Fig. 207 & Table 28). He further suggested that this form of monitoring could be taken up by the Mammal Society and extended nationwide to monitor the hedgehog population. I deeply regret not pursuing that idea in the 1960s, but everyone said that it wouldn’t work and counting road casualties was a waste of time. Anyway I could not see how to muster enough volunteers to obtain sufficient data for a robust analysis. In 1990, I nevertheless decided to try this idea and used a radio programme to recruit volunteer counters. Scepticism remained and caused one angry biologist to send a written complaint to the BBC accusing them of promoting pseudoscience. But the roadkill surveys of 1991–94 inclusive turned out to be very instructive and, in retrospect, an important milestone for revealing changes in the hedgehog population.

TABLE 28. A roadkill survey from the 1950s by J. L. Davies in Hampshire (Davies, 1957).

However, if comparisons are to be valid, the data must be collected in a standardised way. A protocol was therefore established that ensured all my volunteers operated according to a set of rules that included instructions to record the mileage driven on all journeys greater than 20 miles, whether a hedgehog was seen or not. Counts would not be done in the rain, on wet roads, at night or any other time when visibility was impaired. Journeys on motorways and in major cities would be excluded, also the opposite lane on dual carriageways. Observers were urged not to include counts on the same stretch of road if driven a second time within the same month and to count only definite hedgehogs not ‘probables’ or ‘possibles’. The count was limited to three summer months (July–September inclusive) in order to maximise the numbers seen, and reduce the demands made on volunteers. It also avoided the problem of bad weather in the spring or autumn resulting in the effective population size being reduced by some animals being in hibernation and therefore not available to be counted.

For five years (1990–94) observers scanned about 30,000 miles of road annually. The numbers of hedgehogs counted were fairly consistent between the years, with an average of about one per 30 miles driven. The same three regions of the country recorded the highest numbers in all five years (East Anglia, East Midlands and North East England), ranging from 3.3 to 9.75 hedgehogs per 100 miles (Table 30). The lowest counts were also in the same three regions in all five years (South West and South East England, West Midlands). If roadkills were just random numbers, you would not expect such consistency year to year. Nor would you expect this pattern if it were simply a crude measure of traffic density. Despite the greater number of miles driven by observers in the south west, fewer hedgehogs were counted there per 100 miles than in the south east. The southeast consistently scored poorly, despite its very high traffic density. Statistical analyses suggest that there were no major `differences between observers or years, so data from all five years could be combined to create a snapshot of relative abundance for comparison with similar surveys sometime in the future. Overall, observers travelled 162,043 miles and counted a total of 5,321 hedgehogs, giving an abundance index of 3.28 per 100 miles for a sample of five years in the 1990s.

A decade later, the PTES began regular monitoring of mammals killed on British roads, using an almost identical protocol to allow comparisons to be made with the surveys made in the 1990s. In the intervening years, great strides had been made with involving volunteers in ‘Citizen Science’ projects, especially using the internet (see Chapter 13). In the first four years of the Mammals on Roads (MoR) project, hundreds of volunteers surveyed 433,000 km (269,054 miles) of roads, covering most parts of mainland Britain. They counted 6,411 hedgehogs. That equates to a crude average abundance index of 2.38 per 100 miles, a decrease of nearly a third since the 1990s (Fig. 208). Even within the period 2001–04, the index declined consistently year on year for four years, especially in eastern England (Fig. 209). The regional rank order of the abundance index remained much the same, with hedgehogs apparently more abundant in the east and northeast and least numerous in southwest England. They did seem to have increased somewhat in Scotland since the 1990s. Evidence of a consistent downward trend also helps finally to nail the argument about traffic density. If more traffic results in more roadkill, then the trend over time in the number of dead hedgehogs counted should have been upwards as numbers of vehicles increased, not the reverse.

TABLE 30. Average roadkill index for 1990–94 inclusive, showing regional differences in relative abundance of hedgehogs.

FIG 208. Structured roadkill counts in 2001 and a decade earlier, showing consistency in rank order and also a general decline in hedgehog abundance.

FIG 209. Roadkill counts per 100 km driven in 2001–04 show a similar rank order of regions and a continuing overall decline, even within four years.

The MoR surveys recruited many more observers than in the 1990s, generating very large numbers of survey journeys, leading to results that are more robust than earlier. Statistical analysis indicates that enough data exist from MoR to generate consistent values for the abundance index and also to predict future trends with considerable confidence. These roadkill counts have formed a basis for answering the question of whether or not the hedgehog population is in decline. Yes it is, and numbers appear to have fallen by about a third in a decade. The roadkill counts mainly survey rural areas and numbers may have held up in urban habitats that were excluded from the counts, suggesting that additional survey methods are needed, but see below.

The counts made by Davies in the 1950s during the months of July, August and September can be compared with the more recent figures. He recorded 56 hedgehogs on Hampshire roads in 3,575 miles, equivalent to 1.57 per 100 miles. In 1952–53 it was about 1.33 per 100 miles and 1.9 in 1953–4. These are lower values than recent surveys have found (2.31 for this region) and it is hard to see why, except that he covered only a very limited sample of roads. I did try some counts myself in the 1960s, recording dead hedgehogs each week on my three different routes to work, and found 13 hedgehogs in 400 miles. Using only the numbers for July, August and September to be comparable with later surveys gives an abundance index of 3.25 per 100 miles, a figure comparable with those obtained nationally in the 1990s, but well above what would be expected now on those same routes. Indeed, some sample journeys I made along the same old roads (largely unchanged) 40 years later frequently found no hedgehogs at all.

Roadkill counts in other countries usually do not follow a standard protocol similar to that used in Britain, so direct comparisons are perhaps inappropriate. However, in Ireland 133 casualties were counted in a total of 50,430 km, giving a crude abundance index (i.e. not restricted to July–September) of 0.264 per 100 km, equivalent to 0.42 per 100 miles (Haigh et al., 2014a). In New Zealand’s North Island, Bob Brockie’s average abundance index was 40.23 hedgehogs in 100 miles (25 per 100 km) in the 1950s, falling to 17.7 (11 per 100 km) in the 1980s and 2.7 per 100 miles (1.7 per 100 km) in 2005, a drastic decline in the same half-century, paralleling that which has occurred in Britain (Brockie et al., 2009). In 1987, my wife and I counted hedgehogs on about 4,600 miles of roads in New Zealand and found an overall average of 17.8 per 100 miles (11.1 per 100 km) in North Island, a comparable figure with Brockie’s, and about double the figure for South Island. Dead hedgehogs were also several times more abundant in New Zealand than in England when we counted using the same protocol later that same year.

Badgers are absent from New Zealand and farming there is probably less intensive, so it is hard to account for the changed status of hedgehogs in that country. Brockie suggested that a simultaneous decline in both Britain and New Zealand might indicate a disease being responsible, but offered no evidence for this idea and nobody here has reported a sudden increase in deaths from a mysterious new illness. The one new thing, at least in Britain, has been the widespread release into the environment of seriously effective pesticides (see Chapter 9).

There is nothing new in the suggestion that hedgehogs are in decline. As long ago as 1957, Maurice Burton counted 20 freshly killed hedgehogs in 50 miles between London and Salisbury, plus a few older remains (a total of more than 40 hedgehogs per 100 miles!). During the 1950s and 1960s he gained the impression that there was a ‘marked falling off in the numbers of casualties. During that period also my impression is that the numbers of hedgehogs has fallen throughout the countryside’ (Burton, 1969). In 1990, I sent a questionnaire to members of the National Federation of Women’s Institutes and found that more than a third of the 1,200 respondents felt that there were fewer hedgehogs than in the recent past. Only a minority (25 per cent) thought that hedgehogs had become more numerous. The greatest pessimism was expressed by those living in the southeast and southwest of England, the same regions that also had the lowest numbers of hedgehogs killed on the roads. Another survey, launched by the PTES in 2005, resulted in 20,000 people submitting information, nearly half (47 per cent) thought that hedgehogs had been declining over the previous five years, especially in the Greater London area. In 2016, Gardeners’ World magazine sought the opinions of its readers and 51 per cent of the respondents said they had not seen a hedgehog in the past 12 months, up from 48 per cent the previous year.

The PTES initiated an annual questionnaire-based survey (‘Living with Mammals’), adding to its Mammals on Roads surveys, and also a special one specifically directed at gathering observations of hedgehogs (‘HogWatch’). Meanwhile, both the RSPB and British Trust for Ornithology (BTO) have been gathering information about hedgehog sightings from more than 2,500 sites as an adjunct to three of their regular surveys of bird abundance. Thus, by 2010 there were several different lines of enquiry generating independent sets of survey data that might reflect changes in hedgehog numbers, including urban areas. All of them seemed to indicate a declining population, but varied in their methods and offered differing estimates of severity. Moreover, some were ‘anecdotal’, based on impressions rather than actual counts, so were they fully reliable?

FIG 210. Indices of abundance derived from the PTES ‘Mammals on Roads’ data and from direct observations of hedgehogs noted in the BTO’s annual ‘Breeding Bird Survey’. The indices are standardised against the figure for 2000 (MoR) and 2002 (BBS: which began collecting data on hedgehogs a year later) to reveal annual changes. Some years appear to be better than others, probably as a result of the ‘noise’ often found in data from independent observers, but the overall trend is consistently downwards, despite both sets of data being obtained in a completely different manner.

FIG 211. Mavis Righini operated a small hedgehog rescue centre in south east London. The number of hedgehogs brought to her declined steadily for eight years, perhaps another indication of a failing local hedgehog population. However, as with many of these graphs, we have to take account of human behaviour too. In this case, she began to limit her intake, taking only those from her own borough, because of the time and work involved in handling so many animals. But numbers continued to decline.

The PTES commissioned population statisticians at the BTO to review these various approaches to decide whether they confirmed any apparent trends and the extent to which they could be relied upon, given all the many drawbacks involved in gathering such information from so many people who might not all be following their instructions in exactly the same way (one of the drawbacks of using ‘Citizen Science’ data). The review concluded that all these different approaches clearly indicated a decline in hedgehog numbers. This was despite the surveys being in existence for only a comparatively short time (between 4 and 14 years, 1996–2010). Roadkill counts were the most consistent and reliable and also go back furthest in time. This and the information gathered via the BTO’s own ‘Garden Birdwatch’ survey were sufficiently robust to predict with 80 per cent confidence a population decline of 10 per cent in a decade. In other words, the apparent decline in hedgehog numbers is both real and based on reliable data (Roos et al., 2012). The actual rate of decline varied slightly between the surveys, but given the vagaries of weather, observer effort and other variables, it is unrealistic to expect exact figures or total agreement to the nearest decimal point. Further independent evidence comes from the Game Conservancy Trust, who record that the number of hedgehogs killed by gamekeepers has halved in 50 years (see Chapter 12). The central message is clear – we are losing our hedgehogs!

One slightly confusing factor is a study, by Anouschka Hof and Paul Bright, which used a completely different method of assessment and suggested the rate of loss was less than had been claimed. They compared distribution maps, past and present, recording where people had reported seeing hedgehogs. The idea was to see if there had been a change over time in the number of map squares where hedgehogs were reported to be present. They concluded that there had indeed been a decline: there was a 5–7 per cent reduction in the number of map grid squares where hedgehogs had been recorded between 1960 and 1975 and the distribution in 2000–2015 (Bright & Hof, 2016). This could imply that the hedgehog’s demise had been less drastic than previously thought and perhaps the earlier statements about hedgehog losses might be exaggerated and alarmist. However, this is a simplistic approach that takes no account of the actual number of hedgehogs present. A 10-km square on a map will be recorded as ‘occupied’ even if there is only one hedgehog left! The rest of the population may have gone already, but that will not be evident by counting the reduction in numbers of occupied squares on maps. The result is to overestimate abundance and mask actual decline in population size. What that study did show is that there has been a reduction in geographical coverage. It also adds to the series of investigations, all using different approaches, that combine to indicate a downward trend in the hedgehog’s fortunes. Despite the many drawbacks of Citizen Science, the weight of evidence all points in the same direction – hedgehogs are getting scarcer.

At the time of writing (2017), efforts are under way to assess both distribution and abundance of hedgehogs by using ‘tracking tunnels’. These record the presence of hedgehogs by registering their footprints within a plastic shelter instead of using much more expensive traps (See Appendix). The idea is to obtain information on hedgehog population density in different habitat types and also in relation to the proximity and local abundance of badgers. The same sampling protocols, using tracking tunnels, could be repeated annually or at longer intervals to gain an index of population density and quantify any apparent increase or decline in the future (Yarnell et al., 2014).

Demonstrating that hedgehogs are becoming fewer invites the obvious question as to what is causing this to happen. Many of the ‘likely suspects’ are identified in Chapter 9 and reviewed by Hugh Warwick (Warwick, 2016), but these relate to direct causes of death and may not be the reason for long-term losses at the population level. Increasing road traffic is an obvious factor to consider (Fig. 212). Roadkill is a very explicit form of death. We see the bodies and assume a negative effect on populations and it is likely that roadkill is contributing to a reduction in the total hedgehog population, at least to some extent. However, the greatest threat is probably the insidious expansion of both urbanisation and intensive farming. These rarely kill individual hedgehogs outright, but instead prevent them from living successfully. The effect is on the population, not just on the individual. Also we are not reminded of the threat by having regular sightings of dead hedgehogs, so the effects of farming and urbanisation may go unnoticed. One effect of changed farming practices is the increase in field sizes since the 1960s to allow the use of larger and more efficient machinery, especially in arable land. This creates very large areas of open habitat and several recent studies suggest that hedgehogs prefer to avoid wide-open spaces, where they are more vulnerable to predation. Another effect of farming is to reduce the availability of key prey items. Simply ploughing the soil causes the number of earthworms to decline sharply in comparison with land subjected to reduced tillage, an effect that has probably worsened over time, exacerbated by the application of chemical treatments specifically designed to minimise populations of other macroinvertebrates such as slugs and beetle larvae. This may account for the relatively high roadkill counts sometimes seen in landscapes dominated by arable farmland, roadsides being more productive places to feed than the neighbouring fields. But road verges are a dangerous place to be.

FIG 212. Road traffic is an obvious threat to hedgehogs, but its contribution to population decline is difficult to assess. We see the flattened victims – maybe other threats are more serious, but the effects are not so visible.

Chapter 9 outlined the dangers posed by cumulative organochlorine pesticides, which have now been removed from use and ecosystems should become progressively clear of them and their effects. Nevertheless their use in gardens and on farmland may well have had an impact on hedgehog numbers in the past, from which the species has not recovered. Since then, we have seen the introduction of neonicotinoid poisons, used specifically to eradicate insect larvae from lawns, football pitches and other forms of managed grassland. The emergence of these chemicals exactly coincides with the period of fastest decline in hedgehog numbers. So there is a strong possibility that amenity grasslands, and even many garden lawns that have been treated with these chemicals as part of their normal management, have been abruptly denuded of a substantial proportion of their soil macrofauna, including worms, leatherjackets and beetle larvae. Cockchafer larvae are 30 mm long and are, or at least were, both common and large enough to be useful food for hedgehogs, moles and many species of birds. Eliminating them must have hit some local animal populations very hard. My own house is adjacent to a well-managed football pitch.

FIG 213. Pastureland offers a good feeding habitat for hedgehogs, with plenty of worms and insects around the cowpats that nutritionally enhance the sward, but the dairy industry is in decline and many farmers have given up keeping cows or other livestock. Much now depends on the nature of future farming subsidies.

In early summer, we used to have lots of cockchafer beetles attracted to the lights of our house, but since the management of the football pitch was enhanced by the use of chafer grub control chemicals, we have had none at all. Elsewhere, hedgehogs are reported to have become scarce recently on apparently suitable grassland in urban parks where they had formerly been abundant. Even where pesticides are not used, increased trampling and soil compaction reduce the prospects of foraging successfully for soil invertebrates.

There may be lessons to be learned here, especially if football pitches, parks and recreation grounds turn out not to be useful hedgehog habitat any more. They comprise an important element of many green spaces in our urban areas, but they may have ceased to be supportive of hedgehogs in the recent past and their isolation prevents recolonisation. The problem now is that these chemical treatments were apparently intended to be used for two or three seasons specifically in order to break the insect life cycles. Once the insects are gone, they are likely to stay gone, especially for isolated urban sites, unless by some freak event a few beetles manage to recolonise and survive to breed and begin the slow process of rebuilding a viable population. It will take years and in many places may never happen. Even if the insect populations do recover in time, hedgehogs may be unable to recolonise due to the absence of a source population and difficulty of negotiating the many obstacles to dispersal in the urban environment

Those chemicals were not just for professional managers of sports turf, but were readily available in garden centres. So some people will have sterilised their lawns too. It is sad to think that kindly gardeners may have been encouraging hedgehogs by putting food out for them and making holes in their fences, but at the same time treating their lawn in such a way as to remove, perhaps permanently, the insect larvae that form one of the most important natural foods for hedgehogs. Removing chafers, leatherjackets, worms and who knows what else severely reduces natural food for hedgehogs and has the potential to disrupt soil ecosystems. The use of neonicotinoids has been banned as from late 2016, at least from use on flowering plants, but toxic chemicals will remain available for the management of amenity grassland unless the ban is extended following ongoing assessments. We may heave a sigh of relief once these dangerous chemicals are no longer in use, but the damage has been done and many small populations of hedgehogs may have been eliminated perhaps never to return.

There are safer substitutes becoming available that avoid chemicals altogether. Certain species of nematodes (‘roundworms’) that are natural parasites in Britain are being cultured and sold as a form of biological control. A packet of nematode larvae sufficient to treat 500 m2 of turf contains about 250 million worms, enough to boost the natural population sufficiently to wipe out the target pest, but they are minute and no use as hedgehog food. Heterorhabditis bacteriophora and Steinernema feltiae kill cockchafers and leatherjackets respectively without apparently doing any other damage. But this misses the point: removing large insect larvae might make a nice lawn, smart golf course or robust turf, but it is at the expense of depriving hedgehogs of vital fare. Much of the damage will already have been done and it is likely that many areas of previously suitable habitat will remain devoid of these large insects and the hedgehogs they support. Perhaps we have moved on from the research phase to one of needing to raise public awareness of what has happened and we now need to take some form of action to repair the damage (see Chapter 13). Meanwhile we can be confident in adding grass treatments using neonicotinoids to the list of probable explanations for the recently accelerated decline of hedgehog populations, along with the probable losses from secondary poisoning by widespread use of rodenticides.

FIG 214. Progressive expansion of cultivated farmland at the expense of pasture. Cultivation involves actions that restrict food availability for hedgehogs (e.g. ploughing and use of pesticides). Enlargement of fields to increase efficiency removes winter nesting places and materials. The result tallies with regional differences in hedgehog roadkill, fewer where arable land predominates and more where non-cultivated land predominates. Dark green = less than 20% of farmland cultivated; Pale green = 20–60% cultivated; Yellow = more than 60% of farmland cultivated. (Based on Tapper, 1992)

A similar situation occurs on farmland. The use of pesticides and the removal of hedgerows to enlarge field sizes for gains in productivity are just two of the more obvious things that will not have benefitted hedgehog numbers. The intensification of farming in response to subsidies and economic need has transformed the British landscape. Before the First World War there were millions of horses in the countryside, creating short turf and piles of dung that provided excellent foraging for hedgehogs. Horses were normally kept in small fields fringed with hedges where the hedgehogs could nest. The horses have been replaced with tractors and hedges removed to accommodate bigger machines and enhance their efficiency. The fields have been regularly ploughed and treated with multiple applications of fertilisers and pesticides. Those changes have greatly increased farmland productivity and given us the countryside that is familiar to us now. These same changes must have contributed to a significant reduction in numbers of hedgehogs, but because we do not see the dead bodies (unlike road casualties), we remain largely unconscious of the massive effects that these changes are likely to have had on hedgehogs at the population level. Radio-tracking studies have shown that large areas of farmland are now sub-optimal habitat for hedgehogs and questionnaire surveys have shown that hedgehogs are more abundant where farmers receive special payments to support wildlife by maintaining hedgerows and uncultivated areas. But those subsidies are at the mercy of political change. Policies that help support farmers are likely to change again with Britain’s withdrawal from the European Union, but each policy change, now and in the past, will have its effects on wildlife. Once hedgehogs are squeezed out, it may be a long time before they return, if they ever do, even though more benign agricultural policies might be pursued in the future.

According to the Royal Horticultural Society, the equivalent of 15 square miles of town front gardens have been lost nationally, often paved over, in the ten years between 2005 and 2015, and more than 5 million of our urban front gardens now have no vegetation at all. In London, half the front gardens have been paved, often to accommodate a parked car. We have lost a lot of back gardens too, especially the large ones surrounding big Victorian and Edwardian houses, where the whole site has been cleared to make way for several smaller modern dwellings. Often these do have a garden, but it is tiny and fenced off from its neighbours.

In some ways, fencing is even more of a threat than the more obvious loss of flower beds and shrubbery to patios and decking. Roads are another obstacle restricting hedgehog movements. But why does it matter? We know from the principles of island biogeography that two small islands will support fewer species than one large one covering the same total area. Similar principles apply on land, where our countryside is increasingly divided into smaller and smaller parcels by the seemingly relentless construction of roads, fences and other barriers to free movement, in addition to existing natural obstacles like rivers or lakes (Fig. 217). Each time a large area is divided into smaller parts, species that need plenty of space find themselves with less available habitat. Often the barriers are not absolute and can be crossed, but only with difficulty. Studies on mice (Apodemus spp.) and voles (Clethrionomys spp.) revealed that they would much rather stay on one side of a road, even a narrow country lane barely 5 m wide, than cross to the other side (MacPherson et al., 2011). A similar study with those species in Germany showed the same effect, even on a forest road with no traffic at night (Mader, 1984). Hedgehogs might be bolder and they will at least attempt to cross roads, but they are generally reluctant to do so, especially wide ones, and many end up dying in the attempt. Hedgehogs will often turn back rather than venture onto a hostile road.

FIG 215. Concrete barriers erected along the central reservation of our motorways now create barriers for many miles that no animals can cross. This divides animal populations into ever-smaller units, raising the risk of their local extinction.

Consequently, the ever-increasing numbers of roads and other barriers results in the national population of hedgehogs becoming divided into smaller and smaller groups. This situation is often termed a ‘metapopulation’, one in which the total number of animals alive is distributed among many smaller sub-groups whose members can use habitat links to disperse and exchange genes with members of other sub-groups. Gene flow is crucial if genetic diversity is to be maintained, but roads, canals, fences and suchlike all obstruct the process, leaving small groups of animals largely isolated and unable to benefit from the occasional arrival of new animals that reinforce a perilously small sub-population and bring a fresh input of genes.

In small populations, restricted availability of mates means that the animals are likely to experience inbreeding. This increases the probability of individuals acquiring two harmful recessive genes (one from each parent), perhaps directly affecting viability. Inbreeding also reduces the spectrum of genetic attributes, leaving the population less able to evolve in response to changing circumstances. These are theoretical problems, based on established principles of inheritance and supported by laboratory studies. But research on wild bird and mammal populations also suggests ‘inbreeding depression’ can significantly affect many aspects of an animal’s life, including birth weight, survival and resistance to disease. On the other hand, we see natural behaviour that suggests the issue may not be as dire as sometimes suggested. For example, the breeding and territorial behaviour of harem species like fur seals serves to restrict genetic diversity by limiting the gene pool to traits inherited from only a few dominant males. It is also the case that some species like the cheetah continue to survive despite having a very limited gene pool, having passed through some form of genetic bottleneck in the past. All the world’s pet gerbils and golden hamsters derive from just a few individuals. With all this conflicting information, it is hard to say definitively that inbreeding is a threat to hedgehog survival, only that it may well be.

There has been little research into the genetic composition of hedgehog populations or the effects of barriers that separate them. A study of hedgehog genetics in Oxfordshire took samples from 160 members of 8 isolated populations within a 15 km radius. Some of those populations may have been as small as 12 animals, others perhaps 25. Overall they showed a high degree of genetic differentiation, implying restricted gene flow between the groups. This was presumably due, at least in part, to natural and man-made barriers in the countryside (Becher & Griffiths, 1998). Other investigations are in progress elsewhere aimed at determining the effect of motorways and roads as barriers to gene flow. A study of wood mice (which can be obtained in larger samples than hedgehogs) has shown genetic richness and heterozygosity is greater in farmland than in urban sites, where there are more barriers to animal movements, resulting in a greater degree of inbreeding (Wilson et al., 2016). On the other hand, there are several robust hedgehog populations that have survived for decades on islands, despite having had only a handful of founder members and with inevitable inbreeding among the early generations in their new home.

FIG 216. Modern garden fencing like this is a barrier to almost everything. Hedgehogs are thereby excluded and their movements are constrained, reducing gene flow. Small groups of hedgehogs (and other species) are left isolated and facing almost inevitable decline.

FIG 217. Thanks to campaigns like ‘Hedgehog Street’, many gardeners and landowners are now creating special ‘hedgehog holes’ in their fencing to help reduce this problem. (Hedgehog Street)

Nevertheless, it is evident that population fragmentation caused by barriers and obstructions to free movement is at least as important as simple loss of areas of habitat. That is the principle behind campaigns to make holes in garden fences to facilitate dispersal by hedgehogs and other species (see Chapter 13). A study of habitat connectivity in Switzerland used predictive computer models to establish that there were key pinch points within the habitat that would compromise animal movements and that connectivity and habitat quality were two of the key variables which determine suitability for wildlife. Predictive models that take account of habitat connectivity are a new tool that may allow evaluation of the impact of future landscape changes or management scenarios and might assist in designing wildlife-friendly designs of new housing or factory estates, for example (Baaker et al., 2014).

Whatever the genetic implications of isolation, it is certainly the case that small populations are more vulnerable to random events, such as flooding or shortage of food in a drought. Small populations could also fluctuate strongly from year to year depending on minor variations in sex ratio, reproductive success or survival rates. Modelling the effect of these changes indicates that small, fluctuating populations suffer an enhanced risk of extinction (Soulé, 1987). It may be pertinent to note that small numbers were introduced to certain islands around the British coast (see Chapter 2), but in many cases the population collapsed, and sometimes died out, following a ‘boom and bust’ population increase. Even the much larger North Island of New Zealand appears to have suffered a collapse in its population of introduced hedgehogs. Small populations need to have links to other groups of the same species which can then stage a kind of rescue effect through the periodic arrival of animals dispersing from elsewhere.

The issue of population viability can be explored by using software such as ‘Vortex’, written by Robert Lacy of the Chicago Zoological Society and published in 1993. This simulates wildlife population dynamics and how they alter through time in response to changes in the environment or variations in birth and death rates. It is the most frequently used tool for population viability analysis (PVA) and allows predictions to be made about what happens if numbers decline below a critical level. In 2013, the PTES commissioned Tom Moorhouse at the University of Oxford to perform a hedgehog population viability analysis, using the available data on reproduction and mortality rates. The aim was to get some idea of how small a hedgehog population can be and still remain viable in the long term. How small is too small to have a realistic chance of long-term survival? Also how much space might that minimum viable population need? Answers to these questions would provide some kind of guidance regarding the future prospects of the hedgehog, particularly those living in isolated groups in urban areas. As always, these population models are heavily dependent on the data used, much of which is somewhat uncertain. In real life, there will also be variation in litter sizes and survival rates and unpredictable factors such as food abundance and weather conditions. The outcome of the study would depend very much on how those variables were estimated and incorporated. Multiple runs of the model, with many different combinations of data and chance factors, provided estimates of the minimum viable population that varied between 32 and 250 hedgehogs. This wide range of estimates itself serves to highlight the uncertainties involved and the need for caution in interpreting the predictions. To view this in a broader context, informed opinion elsewhere suggests that a mammal or bird population should not number fewer than 50 in the short term if it is to avoid inbreeding depression, and in the long term should not be below 500 in order to maintain evolutionary potential.

Nevertheless we can take 32 as an approximate minimum number of hedgehogs needed to ensure a reasonable probability of a population’s persistence and relate that to figures described earlier for population densities. This suggests that an isolated urban hedgehog population may require at least 90 ha (and perhaps over 200 ha) of suitable habitat. Rural populations may need even more because much of their habitat may be less supportive than urban gardens. This puts into perspective the plight of many town-dwelling hedgehogs that are increasingly hemmed in by roads, walls and garden fences that soon reduce the area available to them to well below 90 ha. They are living precariously on an ‘island’ that is too small.

We can view these findings in relation to actual hedgehog populations. Nigel Reeve’s study area, a golf course in west London, extends to only 40 ha, but it had an apparently thriving hedgehog population in the 1970s, perhaps because the adjacent gardens added a lot of excellent foraging habitat, creating a realistically large area for survival. Regent’s Park in London is more than the necessary 90 ha (actually more than 160 ha), but its effective size is reduced because radio-tracking shows that about a quarter of it is shunned by the hedgehogs, perhaps because large areas are too exposed or they offer poor feeding prospects. Hedgehogs have already gone from London’s Hyde Park, a slightly smaller area, and also from St James’ Park, perhaps also due to a shortage of nesting sites. A broader survey in London highlighted the importance of parks and other urban green spaces for hedgehog survival in urban areas (Hof & Bright, 2009). But many of them are likely to be less than 90 ha in extent, even allowing for access to adjacent gardens, allotments or cemeteries. Those green spaces may also include football pitches and other forms of amenity grassland subject to intensified management practices, highlighting the precarious situation in which those small populations of urban hedgehogs exist, assuming they still do. Hedgehogs might still be present in many town parks and green spaces, at least for now, but maybe not forever unless active attention is paid to free movement between the widest possible range of sites and habitats. Access is crucial; barriers are a threat. Badgers are also a threat (see Chapter 9), especially in small or isolated habitat units where there are few places to hide from them.

FIG 218. Heavy use of amenity grassland reduces its ability to supply hedgehogs with invertebrate prey, even where pesticides are not used.

As a hibernating species, the hedgehog is likely to be sensitive to possible future climatic changes, but much depends on what actual changes take place. For example, an all-round warming, as seems to be happening now, could extend the active season and allow earlier breeding and enable more of the late-born young to survive. Hedgehogs might then enjoy the sort of success they have had in the North Island of New Zealand, at least in the past. If climate change gives us more sunshine, this will warm the soil and enable beetle larvae and caterpillars to grow faster and probably fatter, making better and more abundant food for hedgehogs. But, if more sunny days also means less rain, we may be confronted with long periods of summer drought, severely restricting the availability of important hedgehog prey. Warmer winters may be shorter, but will also increase metabolic fat consumption during hibernation, requiring hedgehogs to become active sooner or die. If climate change means more torrential rain and less drizzle, that could be a problem too, and if it means less rain altogether, that will certainly create difficulties by making worms harder to get. Climate and weather patterns are bound to change, they always have, but it’s hard to predict what the effect will be on hedgehog numbers. Meanwhile, other factors may be even more pressing and take effect sooner.

The possible effects of climate change come on top of where we are already. The ‘State of Nature’ report published by a coalition of 52 wildlife-oriented groups in 2016 quantified losses and gains among more than 400 British species. This long tale of woe detailed major losses: 54 per cent among farmland birds since 1970, for example. It identified intensification of farming as the greatest single driver of population declines in the countryside and the hedgehog is likely to have been harmfully affected. Departure from the European Union offers opportunities to escape from the restrictions of the Common Agricultural Policy, but cannot free us from reality. Energy from the sun, via plants, becomes our food on the table. The more that wildlife intercepts it on the way, the less there is for humans and the less income there is for farmers. So, to feed our increasing human population with only a limited supply of land, farming has to become more efficient. That means removing weeds (competitors for space and soil nutrients) and also limiting losses due to pests. That’s what agricultural intensification is all about. Horrendously effective agricultural and horticultural chemicals may become a thing of the past, but something must take their place. The more we use pesticides or other methods to focus food production on human needs by killing beetles, caterpillars, slugs and the rest, the less there will be for hedgehogs and other wildlife that effectively live as passengers on our farming systems. The larger the farmer’s fields, the more efficiently they can be farmed, but the less habitat there is for hedgehogs to nest. Even in our own gardens, pathetically small by standards of the past, areas of decking and elegant paving reduce usable habitat for the hedgehog. The actions we take to manage neat lawns or weed-free flowerbeds and eliminate caterpillars from our vegetable patch will do nothing to improve the prospects for hedgehog survival. More than half of the people surveyed in 2016 had not seen one at all in their garden and only 12 per cent reported them still to be regular visitors.

Viewing this gloomy scenario and the graphs showing population declines has led the popular press to predict the hedgehog’s extinction, perhaps within as little as 15 years. This is nonsense, of course. Hedgehogs could not be exterminated in 15 years on the Uists, despite determined efforts to eradicate them and a limited land area to deal with. Hedgehogs will remain a British species for a long time yet, especially in areas of the country not under pressure from housing developments or intensive farming, and there is also much that we can do to help them to survive and prosper where adequate space and resources remain, including in our own private and public gardens (see Chapter 13).

Practical difficulties and the limited number of studies mean that we still do not have good figures for hedgehog population densities. Many of those that have been published may now be obsolete owing to changes in land use and farming practices that have occurred since the studies were conducted upon which those estimates were made. There is consequently little confidence in our estimate of the total size of the British hedgehog population. However many hedgehogs there may be in this country, structured counts of hedgehog road casualties suggest that the total population size may have fallen by up to a third during the last quarter of a century and the decline continues. Other methods of monitoring hedgehogs also all point to long-term diminishing numbers. The main causes are likely to be associated with changes in land use, particularly intensification of agriculture, rather than the direct causes of mortality to individuals such as predation or poisoning, although these and other threats such as road traffic will have contributed to reducing the number of hedgehogs and continue to do so. There is little sign of improvement in the hedgehog’s abundance and chronic decline is likely, a prospect that it faces along with many other species. Its future will depend a lot on how much help well-meaning people provide and also on the sensitivity of future land use policies to the needs of wildlife in general.

CHAPTER 10

Hedgehog Numbers and Population Biology