What did we find out?
On this page, you will find an in-depth report on the results of our study. We hope that you will find it interesting if you are curious about the full details of how we carried out the study and what we found out using the data from our volunteer surveys.
Western European hedgehogs (Erinaceus europaeus) are small mammals that live in a range of habitats including hedgerows, woodland edges and farmland, as well as in residential areas and gardens. They are currently in decline across much of their native range, such as in the UK, and elsewhere, invasive hedgehog populations are growing rapidly, such as New Zealand.
There are lots of different reasons for hedgehogs declining in the UK. One reason seems to be habitat fragmentation, which can sometimes be caused by badgers (Meles meles). Badgers are predators and compete for some of the same food sources as hedgehogs so when their population increases, it can exclude hedgehogs from large areas of habitat. Loss of habitat because of human activity also plays a part.
In places where hedgehogs are not native, there are concerns about the impacts they can have on their new ecosystems. For example, in the Scottish Islands of North and South Uist there were major concerns about hedgehogs preying on the eggs of threatened wading birds.
Read more about the hedgehog’s decline in the UK here
Read more about invasive hedgehogs in the Uists here
Hedgehogs in Guernsey
Hedgehogs (or pourché d'fossaï in Guernsey French) are not native to Guernsey. Although there is no record of exactly when and why they were introduced, sources suggest they were first introduced in the early 1800s, sometime before 1850. As for the other islands in the Bailiwick, Alderney’s hedgehogs became extinct shortly before the First World War and have since been reintroduced. An article from the Daily Telegraph suggests that hedgehogs were introduced to Sark in 1984 “by the wife of the island’s feudal seigneur… in the hope of introducing them to the island”. Hedgehogs have never been introduced to Herm.
Guernsey’s ecology is distinctly different from the UK. Most important for hedgehogs is the complete lack of badgers. There is still a decline in a few important habitat types though, similarly to the UK, including some types of grassland and species-rich hedges.
There have been relatively few studies on Guernsey’s hedgehogs, however, one recent study by a local student was particularly useful. Charlotte Le Marquand confirmed that there were hedgehogs all over the island by using footprint tunnels and showed that citizen science hedgehog projects would appeal to a large number of residents, both of which were important for this study.
Why is it important to find out how many hedgehogs there are in Guernsey?
We wanted to find out how many hedgehogs there are in Guernsey so that we can track the change in their population size over time. This will show if they are struggling and need extra protection or if they might be doing too well and affecting other wildlife negatively.
We cannot assume that their population is in decline like England because our ecology is so different. Similarly, we cannot assume they are having a negative impact on wildlife like in the Uists because there are other places where research has shown that hedgehogs may not be doing as much damage to bird populations previously thought, like New Zealand. There are also a few different factors that are unique to Guernsey that may affect hedgehogs differently, including douits, abreuvoirs and our roads. These are being studied by local hedgehog rescue centres and other researchers.
What did the volunteers do?
Volunteers were asked to place out a bowl of hedgehog friendly food and a shallow dish of fresh water in their gardens at dusk, then to observe from a distance for at least one hour.
They were asked to record the date, number of hedgehogs seen, their size, distinctive markings, and the time the first hedgehog was seen. This was repeated one week later, when they were also asked to record whether they believed any of the hedgehogs were the same as last week and why. Results were collected through online and postal surveys and photos of hedgehogs were sent to email@example.com.
What did you do with the survey results?
The surveys that were sent in were filtered to remove things like accidental repeats and surveys where there was confusion over the instructions or the questions. I then put these answers into an excel spreadsheet and used a statistics program called R to carry out statistical tests on them.
This bit might get a little technical so feel free to skip to the results if you like.
We decided to carry out our study using a combination of capture mark recapture and citizen science.
What is Capture Mark Recapture?
Capture-mark-recapture (CMR) is a style of study that scientists use for estimating the total number of animals within a population without having to count every single one of them. They do this by placing out humane traps, tagging or marking the animals caught and releasing them. Sometime later, the traps are set again and this time they count the number of animals that do not have tags vs those that do have tags. They use the following equation to calculate the total population:
By using this method, the researchers are assuming that the number of animals within the population has stayed the same throughout the study, for example, there were no new animals born, none died and none migrated. They also assume that being caught once will not put an animal off entering a trap again.
What is Citizen Science?
Citizen science includes any scientific study involving members of the public, whether they are collecting or analysing data. Citizen science is useful for collecting a lot of data in a short space of time or for collecting long-term data over a large geographic area.
How can you combine CMR with Citizen Science?
This table outlines how the different parts of a traditional capture-mark-recapture study relate to this study.
Each round-2 survey was classified as “sure” or “unsure” based on whether the participant was confident about whether they saw the same individual hedgehogs in both rounds. 91% of participants were sure of their “recaptured” hedgehog identifications and 9% were unsure. The hedgehogs seen by unsure participants are referred to as “unsure hedgehogs”. We calculated four different estimates: discounting all of the unsure hedgehogs, assuming all unsure hedgehogs were previously unseen, assuming all unsure hedgehogs had been seen in round 1 and assuming half were new and half were repeats.
This is the equation used to calculate each population estimate:
The mean estimate for the number of hedgehogs in Guernsey’s grassland is 668 ±52.
If this is very simply scaled up to the entire area of Guernsey, the mean estimate is 3631 ±280.
Where it says ± a number, this refers to the standard error. The smaller the standard error, the more sure you can be of a number’s accuracy.
None of the surveys said that there were very small hedgehogs, which would have indicated new births, no participants said that they took hedgehogs to a rescue centre after round 1 and roadkill was presumably low because of the reduced road traffic during lockdown. This means that the assumption that the size of the population stayed the same throughout the study was true. Because we only observed the hedgehogs and did not handle or disturb them, we also assumed the likelihood of seeing an individual hedgehog in round 2 was not affected by seeing them in round 1.
What does this mean?
According to these estimates, there is a very high density of hedgehogs in Guernsey – nearly 10 times that of the UK! This means that it is very important to do more research so that we know how they are interacting with other species and the environment. Further research might show that they have assimilated well into the local environment or could highlight issues of predation on vulnerable species. There are other places where this has been the case and most recently, some Scottish islands have had great success in relocating their hedgehogs to mainland Scotland.
Due to the large amount of variation in the scaled-up island-wide population estimates, it should not be taken as a definitive estimate of the island’s population because it makes several critical assumptions. Firstly, it assumes that there are hedgehogs in all of the different types of habitat and secondly, that there are the same amount of hedgehogs per unit area in every habitat type.
What is your postcode?
It was important to check that the total number of gardens we collected data from was proportional to the total population of each parish using a test called the Spearman’s rank correlation. The idea is that if they are in proportion, it means that we sampled a similar proportion of gardens in each parish. If this were not the case, it could affect the accuracy of the population estimate, for example, if there was an unusually high number of hedgehogs in Torteval and we sampled an unusually large proportion of gardens in Torteval, our estimate will be higher than the actual population size. St Pierre du Bois and St Saviour were combined as they share a postcode prefix.
Participants were recruited from all 10 parishes in Guernsey (Figure 1) and the number of participants was strongly correlated with the population of each parish (Spearman’s rank correlation, rho= 0.73, S= 32.64, p= 0.026).
Figure 1 The location of participants who successfully completed two rounds of hedgehog watching, produced by Digimap.gg using a postcode centroid method, where each dot represents the centre of a postcode rather than an exact garden location. Click the map to go to an interactive website.
Even though the number of gardens sampled in each parish was proportional to the number of gardens in each parish, this does not mean that the number of hedgehogs in gardens is the same as in the rest of the grassland habitat or in other habitat types. There are some studies where researchers have only made observations in one type of location, like counting toads on roads, and the estimates produced have been just as good as those made by sampling lots of habitat types. However, there is evidence that hedgehog density varies in different types of habitat. It is difficult to make predictions based on this, however, because their density is affected more by other factors like roads and badgers than by habitat type. This means we would need to study hedgehog density across different habitats in Guernsey specifically to tell whether we can made accurate estimates by only observing hedgehogs in gardens.
We checked whether there was a significant difference between the number of hedgehogs spotted by people who used different types of food, using a Wilcoxon rank sum test. We also used this test to see if there was a correlation between feeding hedgehogs regularly and the number of hedgehogs seen. Finally, we used a Chi-squared test to see whether there was a relationship between feeding branded hedgehog food and feeding hedgehogs regularly.
Do you normally leave out food for hedgehogs in your garden on a regular basis?
63% of participants said they regularly offer hedgehogs food but 37% did not.
People who fed hedgehogs regularly saw significantly more hedgehogs (2.5) than those who did not (o.5) (Wilcoxon rank sum, W=4282.5, p<0.001) (Figure 2).
Figure 2 The median number of hedgehogs seen per session by participants who regularly feed hedgehogs and those who do not. Error bars show standard error.
What sort of food did you offer the hedgehogs?
In the category “Other, please specify”, there was a wide variety of food. Examples include birdseed, sunflower hearts, burgers, sausages, cooked chicken, raw eggs, sweet potato, banana, peanuts, apple and blueberries. There are not very many studies on which supplemental foods are safe to feed to hedgehogs, however, the BSAVA manual for wildlife rehabilitation recommends meaty pet food and biscuits. There have been no studies assessing the safety of offering hedgehogs most of these other types of food. In future studies, researchers could provide participants with hedgehog food to limit this variation.
People who used hedgehog and pet food did not see significantly more hedgehogs (2.5) than those who only used hedgehog food (2) (Wilcoxon rank sum, W=539, p=0.391).
People who used hedgehog and pet food did see significantly more hedgehogs (2.5) than those who only used pet food (1) (Wilcoxon rank sum, W=388, p= 0.00126).
People who who used just hedgehog food also saw significantly more hedgehogs (2) than those who used pet food (1) (Wilcoxon rank sum, W=2415, p=0.00180).
Figure 3 Median number of hedgehogs seen per hedgehog watching session by volunteers who offered pet food, hedgehog food or a mixture of the two. Error bars show standard error.
There was a significant relationship between the type of food offered (hedgehog food or pet food) and whether food was offered regularly (Chi-squared=29.98, df=1, p<0.001). 82% of those who used hedgehog food fed hedgehogs regularly, whereas, only 31% of those who used pet food already fed hedgehogs.
What does this mean?
Participants who regularly feed hedgehogs in their garden saw significantly more hedgehogs than those who did not. This might be causative, meaning that more hedgehogs are attracted to gardens where there is a guaranteed food source. Alternatively, the correlation may be incidental because if there are no hedgehogs in your garden, you are unlikely to put out hedgehog food in your garden once a week.
In addition, people who used hedgehog food saw more hedgehogs than those who fed pet food, even when the hedgehog food was mixed with pet food. Again, this could be causative, meaning that more hedgehogs are attracted to gardens where there is hedgehog food or it may just be that people with more hedgehogs in their gardens are more likely to notice them and then go out to buy hedgehog food for them. This second idea seems likely given that there was an 82% overlap between people who offered hedgehog food and those who fed hedgehogs regularly.
On which day did you carry out the hedgehog watching?
We compiled the dates on which participants did their hedgehog watching, the times that hedgehogs were first seen and in the cases where no hedgehogs were spotted, whether there was any evidence of hedgehog activity in the morning. These were used to check for two types of errors called pseudoreplication and false absences.
90% of round 1 data collection took place on 1st June and 85% of round 2 data collection took place on 8th June. The remainder of sessions took place within a week of the recommended dates.
When did you see the first hedgehog? Did you find any evidence of hedgehogs the following morning?
In the majority of data collection sessions, hedgehogs were first seen within an hour of dusk (Figure 4). In 15 sessions, hedgehogs were first seen between 17:00-19:00 and in 8 sessions, hedgehogs were sighted at midnight or later (Figure 4).
After the hedgehog watching sessions where no hedgehogs were seen, 6 participants found no evidence of hedgehog activity, 1 found hedgehog faeces, 9 found the food had been eaten and 12 found both faeces and eaten food.
Figure 4 The time that hedgehogs were first seen in each session where at least one hedgehog was seen. The shaded area indicates when dusk fell over the days when data collection occurred.
What does this mean?
By asking participants to carry out data collection on the same day (whenever possible) and to send in photos of the hedgehogs they saw, we hoped to avoid an issue called pseudoreplication. Basically, we hoped this would stop us counting the same hedgehog more than once if it is eating at multiple feeding stations. Hedgehogs can travel over 2km overnight so this was a very real possibility. Our efforts were fairly successful because over 80% of data collection happened on the recommended dates and most participants submitted photos.
Another issue that we tried to avoid was false absences. This was less successful than avoiding pseudoreplication. False absences are when there are actually hedgehogs in a garden but the study design means that they do not get counted and instead, it looks like there are no hedgehogs. Most participants sat and physically watched their hedgehog feeding stations for the allotted hour, however, a few made use of wildlife cameras and reviewed the whole night of footage. This showed that in some gardens, hedgehogs would not emerge until the early hours of the morning, meaning that they would not have been counted using the “sit-and-watch” technique. Similarly, the majority of times when no hedgehogs were seen during observations, there was still evidence of hedgehog activity found in the morning. While finding faeces or all the food being gone does not guarantee that hedgehogs were present (faeces can be misidentified and other wildlife or domestic animals may eat the food), it does give an indication that there were at least some cases where no hedgehogs were seen, even though there was hedgehog activity overnight.
What sorts of things might have affected the accuracy of the population estimates?
There were a few different pieces of information we collected from our volunteers that we hoped would highlight the sorts of things that affected the accuracy of the population estimates produced and some extra information about how we interact with our backyard hedgehogs as an island.
In your opinion, were any of the hedgehogs you saw the same individuals that you saw in round 1 of hedgehog watching? Why?
We classified the reasons that people gave for why they thought hedgehogs were or were not the same individuals in both rounds then calculated the percentage of participants who gave each response, which is shown in the pie chart below.
Figure 1. The percentage of participants who gave different reasons for believing the hedgehogs they saw in round 2 were the same or different from round 1
What does all of this mean for Guernsey’s hedgehogs?
This means that we need more research to understand how our hedgehogs are interacting with our environment. Further studies may show that hedgehogs have assimilated well into our ecosystems, acting as pest control in our gardens, or they may be causing damage to other species like ground-nesting birds. Even though there are examples of hedgehogs being beneficial and examples where they are highly detrimental, our island's ecology is unique so it is essential to understand our own hedgehog population thoroughly before drawing conclusions or taking any action.
My next step is preparing a scientific manuscript for publication in a peer reviewed journal. This is hopefully an important step for finding out more about Guernsey's hedgehogs. I hope that in the future we can repeat this study to carry out power analysis, which would indicate how many sites need to be sampled each year to detect different percentages of growth or decline in the hedgehog population. It would also be helpful to measure the home range and habitat use of hedgehogs in Guernsey. This would help us to make much more accurate population estimates in the future.
This study could not have taken place without all of the Guernsey Hedgehog Project’s volunteers. I would like to thank Prof Toby Knowles & Dr Nicola Rooney for their support as project supervisors and to Dr Matt Zeale for his advice. Thank you to Digimap.gg for providing the map and to Highland Titles Ltd for providing funding. Thank you to The Guernsey Press, The Bailiwick Express, IslandFM and BBC Radio Guernsey for publicising this project. I am grateful to the Priaulx and Guille Alles libraries for their help finding local documents and to the Guernsey Biological Records Centre for answering my questions. Thank you to Hannah Garland Illustration for designing our logo and finally, thanks to Chris and Eddie Kirby for stuffing envelopes, answering phone calls and supporting this project in many other ways.
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