Tuesday, 9 February 2021

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!

Thursday, 2 January 2020

Changing recorder behaviours

When I started extracting data from the internet, almost all records were of single species from a locality on the same day. The value of the internet as a medium for recording was only starting to become apparent and it did not really gain much traction until the advent of iSpot in 2009. That was quite a 'game changer'. The next big leap came in 2013 when the UK Hoverflies Facebook group was established. The effects of these two events can be seen in figure 1.
Figure 1. Numbers of full and partial records of hoverflies from 2002 to 2019 extracted from internet sources. Numbers started to rise markedly in 2010 with a much bigger jump in 2014 which was the first full year after the establishment of the UK Hoverflies Facebook group. I only really started recording partial identifications in 2015; hence the low data for preceding years.

The numbers of active recorders is probably a better test of the popularity of social media amongst the photographic recording community. This is illustrated in Figure 2.
Figure 2. Numbers of recorders contributing to the data presented in Figure 1. The decline since 2013 reflects several factors; principally a reduction in the time I spend extracting data from Flickr and other sites, together with a gradual shift of recorders using spreadsheets or iRecord to submit data. The big drop in 2018 is almost certainly a combination of people shifting to the use of spreadsheets plus a big hit from the heatwave of June and July in which hoverfly numbers crashed.

One of the big problems for recording schemes is that recorders can be extremely selective over what they submit. Many would only take an interest in the rare or unusual; a situation that probably continues in many other schemes. The HRS has actively encouraged the submission of full lists on all occasions, and I think this process has been very effective as can be seen in Figure 3. This shows the benefits of schemes directly interacting with recorders in order to develop new ways of working. In many ways, it mirrors BirdTrack which only uses full lists in its interactive feedback.
Figure 3. The average numbers of records per recorder since 2009. This graph shows how there was a gradual rise in numbers until 2013 but in the following two years there was a paradigm shift in recorder behaviour. Part of this shift would have come from the reduction in my scanning of Flickr for occasional records, but even so, it does not wholly explain the results. It is noticeable how the average plummeted in 2018 but has rebounded very strongly in 2019.
I think this is a good indication of the way forward for other schemes and for other recording media. The important issue is to shift thinking away from the idea that recording is about dot maps and towards the idea that total lists may help to provide the detail deened to track at least some of the critical environmental influences on Britain's wildlife.

Saturday, 13 July 2019

Picking up signals from the data. Some examples

I have spent a sizeable part of the day trying to make something of the data that are emerging this year. My feeling is that there are probably quite a few species that have suffered but there will be a few that have benefitted. The same species may have lost out in one part of the country and gained elsewhere. I have started by looking at five examples:
  • Episyrphus balteatus, which seems to be having a good year and was far more abundant than expected in the winter.
  • Eristalis pertinax, which has appeared to be poorly represented in recent data.
  • Eristalis tenax, which appears to have been more dominant in recent data.
  • Helophilus pendulus, which has generated regular comments about its apparent scarcity.
  • Rhingia campestris, which has been notable by its absence even though it is a highly distinctive species.
To do this, I have compared the proportions of records rather than the absolute numbers of records. This approach is necessary because the numbers of records and recorders vary from year to year; as does the geographic spread of recording. So, context remains an important part of the analysis. Each species is compared against the data for the previous six years (2013 to 2018) extracted from Facebook and other social media.

Episyrphus balteatus

This is an aphidophagous species that occurs throughout the year but in particular dominates the records at certain times. In mid-summer, it vastly outnumbers all other hoverflies but peaks at differing times.
Figure 1. Episyrphus balteatus
As the data has grown, it has been clear that E. balteatus has been far more dominant than in other years. The precise reasons are unclear, but it seems likely that the warm winter allowed dults to be far more active and for the larvae to get started earlier in the year.

Eristalis pertinax

The larvae of Eristalis (and Helophilus) are aquatic. E. pertinax usually has two peaks, one in the spring and the other in late-summer. Numbers should start to rise in July but as yet that has not been detected.Late autumn abundance is very variable, depending upon both temperature and sunshine. It usually starts to disappear in late October but can be abundant into December during warm autumns.

Figure 2. Eristalis pertinax
Current evidence suggests that tthe spring generationwas smaller than the long-term average. It is probably too early to be sure that the autumn generation will be smaller than normal, but a recent dip in numbers may be indicative of this species having experienced a poor year.

Eristalis tenax

Adults usually hibernate, but in recent years this has been considerably less clear from the data. Winter 2018-19 was relatively warm but the numbers of E. tenax recorded seem to have been lower than the longer-term trend.
Figure 3. Eristalis tenax
Despite comparatively low numbers of winter records, it seems that E. tenax is experiencing a good year, with numbers noticeably higher than the longer-term trend.

Helophilus pendulus

Monitoring the Facebook group has pointed to this species having not been as numerous in previous years. The phenology for 2019 is somewhat odd, with a much shorter spring generation.

Figure 4. Helophilus pendulus
I wondered whether there might be a regional reason for this difference so split the data into five regions as shown.
Figure 5. Helophilus pendulus phenology in England & Wales
Figure 6. Helophilus pendulus in Scotland
The data present a very confusing picture that is difficult to interpret. If anything, numbers in Scotland appear to be marginally up on the longer-term trend, and South-East England seems t have somewhat elevated numbers but a shorter emergence season. Elsewhere, numbers seem to be lower than recent trends.

Rhingia campestris

This is a highly distinctive species whose larvae develop mainly in cow dung, although its distribution suggests that a wider range of dung is probably utilised. It is a species that has, historically, been considered to respond to hot summers and for some generations to almost completely fail. This was detected in 2018 and the response in 2019 is therefore of interest.

What emerges is a response that seems to be completely different in Scotland. Whilst there seems to have been a uniform crash in England and Wales, numbers in Scotland appear to be robust and possibly higher than in recent years. I suspect that the data for northern England may ultimately show a similar pattern to Scotland once records from various very active recorders are absorbed into the dataset (at the end of the year). Those from more southerly regions seem to suggest that numbers are exceptionally low; which is consistent with the failure of the summer generation in 2018 and thus a failure of breeding success to create the spring generation.

Figure 7. Rhingia campestris in England and Wales
Figure 8. Rhingia campestris in Scotland
I suspect that the elevated numbers in Scotland can be explained by the higher temperatures in 2018, which allowed greater breeding success in a region where higher temperatures would have allowed more rapid population build-up in late summer and consequently more larvae available to create the spring generation.


Wednesday, 10 July 2019

Getting started as a serious Dipterist

There was a recent post on the UK Hoverflies Facebook Group concerning 'starter kits' for new recruits. The query did not specifically seek funding but a response showed how some potential young Dipterists might be constrained by funds.

My days might be numbered, so I need to think about how best to do something positive, and one way of doing so is to help the next generation. I am therefore thinking about how best I might set up a fund to help young Dipterists. What do they need?

I have always believed that people should help themselves but I know that the costs of getting started are steep. So, if I was to set up a fund through one of the major societies what should I be thinking about? No promises!

What should a starter pack comprise? Who should qualify? How should such a fund be administered? What should its objectives be? There are numerous questions to answer before I do anything formal but in the meantime I need feedback to understand what would work?

Thoughts please? (sensible ones)

Thursday, 20 June 2019

Common species matter

I return to that perennial chestnut – whether there is any value in records of common species? New recruits to recording schemes often wonder whether their very limited repertoire has any value, whilst more experienced recorders are perhaps less excited by common species and therefore ignore them. I think it is essential that scheme organisers continue to emphasise the benefits of recording commoner and more easily identified species.

I am reminded of the fate of the House Sparrow. Ringers tell me that when they started they were actively discouraged from ringing house sparrows. So, when these cheeky little chaps started to decline nobody had any data! Even now, I’ll bet the data are incomplete but at least these days BirdTrack asks for full lists and the way it is designed makes you feel obliged to try to compile a full list. So, an opportunity was missed but future issues are now addressed by a hugely successful data-capture system.

With invertebrates we have an even bigger problem. There have been several stages in the decline of our wonderful insect populations. Firstly we had the development of more intensive agriculture post WW2. By the 1950s farmers were being actively encouraged to drain and plough and ‘improve’ previously fallow land. We lost vast areas of grassland and wet pasture. No more orchid fields, just a green monoculture of grass and cereals. That must have had a devastating effect on invertebrates but we don’t have any data to consider it.

Then came the pesticide revolution. DDT and a wider range of Organo-phosphates laid waste to our insect populations in the 1960s and 1970s. I can still remember a field in France in 1970 that had been treated – it was awash with the corpses of so many lovely animals. The shock has stayed with me for the following 50 years (I was 11 or 12 at the time). So, by the mid-1970s a lot of damage had been done, but still we had no decent recording system.

Then came the onset of recording schemes. The HRS started in 1976 – just in time for the first devastating heatwave and drought. Again, we have precious little data! There is a theme developing here. So, today, we have had a sequence of extreme heatwaves and droughts but can we analyse their effects? Probably not. The data are just too patchy. Yet, if every field naturalist had simply noted all the ‘common’ and readily identifiable species for the past 50 years we would have a fighting chance of picking up some signals.

So, the issue of insect decline is not going to go away! Indeed this spring we have witnessed some pretty worrying shortages of all manner of insects. It is time we addressed the data shortfall and valued records of ‘common’ species. After all, they could become the modern equivalent of the passenger pigeon!

So, if you have just joined the recording process, don't worry if your records are of a narrow suite of species. The data are useful. Similarly, if you are experienced please submit full lists. They really do count.