Roadkill: an unquantified impact on invertebrate populations

Many readers will remember the RSPB's 'splatometer', which was abandoned because there were concerns that it could obscure number plates. Nevertheless, before it was abandoned, a trial was undertaken and its results were surprising: 324,814 insects were recorded at an average rate of one splat every five miles. Those figures look to be rather low and I suspect results may have been different had the device been fixed to another part of the vehicle.

The splatometer was a response to the need to find a way of monitoring changing insect populations.. It might have generated very interesting results over time, but sadly we shall never know. One of the possible outcomes might have been the start of an equally important idea - that road kill itself might be having a detrimental impact on invertebrate populations. This is a subject that has been in my thoughts for many years and I think needs a lot more attention. I am aware of an Indian study that demonstrated potentially serious impacts on butterfly populations but as yet I have found little else and nothing from the UK. I have therefore been looking for a possible way of investigating the problem.

In early May 2016 I noted a dead Anthophora plumipes on the roadside near Collyweston Quarry that got me thinking. The stretch of road I had just walked along might just be a useful study site. From then on, I made a serious effort to record insect roadkill along a 1 mile section of this road, the results of which are presented here.

The study site

An approximate 1 mile section of the A43 at Easton on the Hill (Figure 1) was chosen because it lies on the route of one of my regular walks. It is far from ideal because there are many places where the footpath is separated from the road by a grassy verge. Thus, records tended to be clumped around sections of path and tarmac

Figure 1. Study site and sections used to separate records

Methods

The eastbound carriageway was surveyed on a regular basis for any dead insects that might be attributed to collisions with vehicles. Although the gutter was inspected, most attention was given to adjacent footpaths and lay-bys because the gutter itself seemed to yield relatively few casualties. Incapacitated animals were removed and logged.

For much of the summer, it was not possible to visit the road on a daily basis and in several instances a period of a week or more elapsed between visits. The location of the casualties was logged according to the sections of the road shown on the map (Figure 1).

As this was a trial rather than a full-blown research study, the objective was to test the potential for investigating roadkill, rather than making an in-depth study. The lessons learned can be applied in subsequent years, either by me or by others.

Results

Data are presented in Table 1. Most of the records involved animals lying dead or dying on adjacent tarmac rather than on the road or in the gutter. Some were at a distance of at least 2 metres from the kerb, but the majority were in a zone about 1 meter from the kerb. 

Solitary bees	Bumblebees	Honey bees	Social wasps	Diptera
5	115	70	190	21
Coleoptera	Lepidoptera	Hemiptera	Orthoptera
7	6	1	1

Table 1. Total roadkill recorded between May and October 2016

Most of the casualties recorded were Hymenoptera, principally bumblebees during the summer months but in September and October social wasps and honeybees predominated (Figure 2). Lepidoptera were remarkable for their absence, although a small number of individuals were found. The relative lack of Diptera possibly reflects their tendency to break up on impact with vehicles and consequently they may be under-represented.

 

Figure 2. Casualties by month

Lessons

The distance from the kerb of many casualties suggests that ideal study sites may be difficult to find. In this case, no more than 25% of the route was anywhere close to 'ideal'. The numbers of casualties located was sufficiently high to suggest that detailed studies might be possible if sufficient replicates could be generated. My instincts are that sections of 50 or 100 metres would be viable. More detailed recording is needed to determine the width of tarmac required to highlight the full distance over which casualties may be spread.

It would not be wise to extrapolate from these data because they were collected on a relatively ad-hoc basis. No confidence can be placed in casualties remaining in situ for more than a day or two after impact. Some will crawl away and die in sheltered locations, others may be eaten by birds or small mammals.

Detailed recording of nearby nectaring sites may also be useful. My instincts were that the numbers of honey bee and social wasp casualties were quite closely related to nearby ivy patches. If this is the case, then apiarists might want to think carefully about where they position hives, because honeybee casualties do seem to be very high in the autumn.