Where have the Hedgehogs gone?

I'm drafting an article for the 'Friends of Mitcham Common' that I thought might be of interest to a wider readership:

Christine Dore’s great article on hedgehogs on the Common (https://drive.google.com/file/d/1JSpVYrEanMtodZCXkI80G1ZfPwn88aVO/view)  highlights the plight of an animal that has undergone widespread decline across the country. The statistics are troubling as this excerpt from the BTO analysis (https://www.bto.org/our-science/monitoring/hedgehogs) suggests:

Although there has never been a full national census of hedgehogs, indications are that British populations have been in decline for some time. Estimated roughly to number about 30 million in the 1950s, a recalculation in the 1990s suggested only 1.5 million. For the current study, researchers at BTO analysed five different long-term data sets (including three bird surveys run by the BTO where volunteers record hedgehogs) and concluded that the proportion of sites with hedgehogs may have declined as much as 40% over the past ten years, based on current annual rates of change.

This scale of decline falls in line with declines in numerous other animals and plants, such as farmland birds (e.g., Skylark and Meadow Pipit that used to breed on the Common). We can be pretty sure that the drivers of Hedgehog decline are not confined to Mitcham Common – they are national, or at least regional. We can also gain a bit of an insight into possible drivers from the problems that have been encountered on some of the islands of the Outer Hebrides where Hedgehogs were not native but well-meaning unlicenced releases have seen a population explosion and a corresponding decline in the breeding success (and populations) of ground nesting birds such as Corncrake and many waders. Although highly controversial, there has had to be a Hedgehog cull there to restore breeding bird populations.

A developing thesis

The Hebridean cases show how Hedgehogs are well-suited to open grassland and to damp climates where there are no natural predators and very few roads or traffic. If we then look at south-east England, we can see that the environment is diametrically different: roads criss-cross the landscape at such regular intervals that habitats are disconnected and despite their best efforts our prickly friends have no defence other than to roll up to avoid being squashed! In the wider countryside, agriculture has intensified and breeding and feeding sites have been lost, especially where hedgerows have been ripped out. There is also evidence that in some places (e.g., a study in Norfolk) Badger predation has increased and Hedgehog declines may be partially linked to their impact.

Apart from the roads, the other two factors seem unlikely to have affected Hedgehogs on Mitcham Common. So, can we blame the motor car alone? Is there something wrong with the management? Both may be a contributing factor, but I think that the reasons are more complex. My feeling is that on going back to first principles we can identify the main problems:

Populations of most organisms are dynamic and in natural systems there are regulators of the majority of species. However, in today’s world there are barely any places (except pristine Rainforest) where those natural brakes on species’ numbers have not been interrupted. Once such brakes are removed, some species undergo population explosions (as seen on the Outer Hebrides) or a species dies out (hence the ongoing and very dramatic Anthropocene extinction rate and the much-publicised loss of Pollinators).

Until very recently, most conservation thinking laid the blame for the loss of farmland birds (and Hedgehogs) simply on rates of human population growth and associated intensification of agriculture and urbanisation. Comparatively little thought was given to longer-term drivers such as climate change. Part of the reason for overlooking climate change is that it is extremely hard to link biological changes to as yet small changes in average temperatures. Some links can be made – for example the shift in distribution of Atlantic Cod (a coldwater species whose range is contacting northwards) and of Bass (a warmer-water species whose thermal envelope has shifted northwards and it is commoner around our shores than 30 years ago). But, the marine environment is arguably more responsive to change because the thermal load take a long time to dissipate.

A complicated picture

Hedgehog numbers have crashed, so we can be pretty sure that the numbers of surviving offspring are insufficient to replace adult mortality (all causes). So, there are two obvious possibilities for this imbalance: either adults are dying off earlier or juvenile mortality is higher. There is of course the possibility that both are involved, and I think a case can be developed to cover that angle too. What might be the causes of mortality? There are several possibilities:

• Disease
• Predation
• Road casualties
•  Fire
• Food availability
• Nefarious activities

We can probably rule out disease, as this does not seem to have been highlighted in national assessments. At one time, road casualties were commonplace and perhaps would have accounted for a high proportion of un-natural mortality. Today, Hedgehog numbers are so low that any casualties on the roads represent a significant loss of the remaining population.

We can rule out Badger predation – I’ve never seen any reliable evidence of Badgers on the Common in my 60+ years of wanderings! However, in the past 50 years Fox numbers have risen (quite dramatically). I would have doubted that a Fox could tackle a mature and healthy Hedgehog but there are published reports of limb-loss that may lead to death, and I have found dead hedgehogs that seemed to have been predated, so I think we might argue that a rise in Fox numbers might be a factor. I suspect Hoglets are far more susceptible and that if anything Fox predation is likely to affect population recruitment. Nevertheless, I would rule this single factor as solely responsible for the Hedgehog decline. There is one additional and curious predator: it has been reported that Magpies pull out Hedgehog spines so it is possible that this might account for a small percentage of animals weakened and more vulnerable to other morbidities. Magpie numbers have exploded but I doubt that they alone would account for the problem.

At one time, fires regularly swept across the Common. Mum (Janet Morris) recounted to me that in the early 1960s the entire woodland opposite Commonside East went up in flames, with Hawthorn trees exploding (it burned for two days). I can recall small events in various places in the late 1960s and 1970s that left their mark because the tree canopy is smaller and the species composition differs from adjoining areas. There were some huge events that swept across the then grasslands from Windmill Road to the Seven Islands Pond. Yet at that time Hedgehogs were thriving. Fire is a very rare today and is never on the scale of past events (I suspect there was a single arsonist at that time). That may actually be a factor but not for immediately obvious reasons.

Anybody out at dusk or just after dark might see people with powerful torches – lamping. Most of the time they are after Rabbits, but it is possible that hunting dogs also attack Hedgehogs. There is no evidence that I know of to make this link, but with a precarious Hedgehog population any dog-kill would be significant.

That leaves us with food availability. Hedgehogs have quite a catholic diet but it is mainly composed of slugs, snails, earthworms caterpillars, earwigs and millepedes – all ground-dwelling. Slugs and snails tend to be most abundant in open ground (on the Common) and rarely seen in numbers in woodland, so we might suspect that grasslands and nettle patches are the most suitable for foraging territory. Therefore, if the extent of grassland diminishes, the number of viable territories will also diminish. This problem could be serious, as there has been a massive loss of grassland since the fiery days of the 1960s and 70s. BUT, I don’t think it is the only reason.

Climate change – the hidden enemy of Hedgehogs?

There is developing evidence (Román-Palacios & Wiens, 2020 https://www.pnas.org/content/117/8/4211) that localised extinctions are linked to extreme thermal events (mainly heatwaves), and a conceptual model to explain the links can be developed. Work that Stuart Ball and I are doing with hoverflies is starting to tease out some of these links but it will take a lot more to prove anything definitively. However, we can translate some of this to the plight of Hedgehogs on the Common.

In recent years there have been several very serious droughts. In 2018 and 2020 we watched as the Common shrivelled up, the ponds became muddy holes and the vegetation turned to dust; conditions that are highly unfavourable to slugs, snails and earthworms. Such droughts have become much more frequent since the big one of 1976 that might be the start of the Hedgehog’s woes.

Food availability for Hedgehogs during these times would almost certainly have been significantly depressed and would impose immense stresses on adults, but more especially on the Hoglets. Whether such events actually caused mortality is uncertain, but where food becomes scarce the young and weak in most animals are the first casualties.  Adults might weather the drought but may end up with insufficient body weight to survive the winter. So, I suspect that an increasing frequency of droughts and heatwaves is a major factor in the demise of the Mitcham Common Hedgehogs. But there is a further twist. During extreme heatwaves, mammalian thermoregulation is put under extreme stress and weaker individuals may die (a spike in mortality in Britain was seen after the hottest day of 25 July 2018). If humans can be affected by extreme heat, so too can other animals (i.e., consistent with the work of Román-Palacios & Wiens, 2020).

In summary

We cannot be at all sure what has caused the demise of Hedgehogs on the Common, but my best guess is that a series of factors are responsible. Some kick-started the process (e.g., roadkill and the 1976 drought), but have been compounded by others; in particular loss of foraging range and food availability. We can do nothing about the extreme events that will only get worse, but we can do something about loss of foraging range.

 

I have avoided saying more, as there is considerable local controversy about recent tree felling and bramble clearance, with blame placed on such events for Hedgehog declines. I don't think such assertions are correct but it is for the Conservators to deal with that problem and to come up with their own answers to the controversy!