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Citizen scientists find 50% fewer insects in Kent

Logo: Kent Wildlife Trust

Dr Paul Tinsley-Marshall, Conservation Evidence Manager, Kent Wildlife Trust

A growing body of evidence highlights population declines in insects and other invertebrates. Much of this evidence is summarised by the recent Action for Insects report commissioned by a consortium of Wildlife Trusts, and authored by Dave Goulson. The consequences of insect decline are potentially catastrophic. Kent Wildlife Trust is leading a National Lottery Heritage funded project Nature’s Sure Connected, which seeks to develop best practice in landscape-scale monitoring. Over 100 stakeholders from the professional conservation sector helped prioritise the most important themes to consider. One of these focused on ecosystem function, an incredibly broad area. Considerable conservation effort is targeted at invertebrates, and the focus of delivery of conservation action has shifted from site to landscape-scale. Practitioners must evidence landscape-scale outcomes of conservation action. We narrowed our focus to insect decline, in light of the critical role of insects in ecosystem function and the topical nature of the issue. All insect functional groups (herbivores, detritivores, parasitoids, predators and pollinators) are at risk, as are the functions they support. Dramatic declines in insectivorous birds for example (spotted flycatcher -93%, grey partridge -92%, nightingale -93%) are worrying trends. Evidencing patterns in invertebrate abundance at all scales is vital as a tool to drive positive action and engagement. Here we used an innovative invertebrate sampling technique, conducted by citizen scientists, to assess the difference in invertebrate abundance over a 15 year timeframe.

A splatometer in action (Kent Wildlife Trust)
A splatometer in action (Kent Wildlife Trust)

The Bugs Matter survey is based on the windscreen phenomenon, a term given to the anecdotal observation that people tend to find fewer insects squashed on the windscreens of their cars now than a decade or several decades ago. This effect has been ascribed to major global declines in insect abundance. Using a standardised sampling grid termed a ‘splatometer’, members of the public were asked to record the number of insects and other invertebrates squashed on the number plate of their car, having first cleaned the number plate before commencing a journey. A national survey using this methodology led by the RSPB took place in 2004, and by repeating the survey in Kent in 2019 we were able to compare the abundance of invertebrates at these points in time.

Figure 1		The difference in splat density recorded on vehicle journeys between a) 2004 (in South East England) and b) 2019 (in Kent). Between 2004 and 2019 there was a statistically significant difference in ‘splat density’ of the order of approximately 50%, from an average of 0.2 splats per mile to 0.1 splats per mile, in 2004 (n=3838) and in 2019 (n=667).
Figure 1 The difference in splat density recorded on vehicle journeys between a) 2004 (in South East England) and b) 2019 (in Kent). Between 2004 and 2019 there was a statistically significant difference in ‘splat density’ of the order of approximately 50%, from an average of 0.2 splats per mile to 0.1 splats per mile, in 2004 (n=3838) and in 2019 (n=667). Full size images can be seen by clicking on the graphics

Between 2004 and 2019 there was a statistically significant difference in ‘splat density’ of the order of approximately 50%. This difference mirrors the patterns of decline widely reported by others1,2. It should be noted that as this observation is based on data from two points in time, it does not constitute a trend and cannot be interpreted as a decline. Inter-annual variation in variables such as weather cannot be ruled out as factors influencing the observed pattern. More data over a number of years will be required to confirm the direction of any trend, however the pattern we have observed correlates strongly with many other examples of decline.

Figure 2 Model estimates and confidence limits predicted by a generalised linear model of the difference in splat density recorded on vehicle journeys in 2004 (in SE England) and 2019 (in Kent). Between 2004 and 2019 there was a statistically significant difference in ‘splat density’ of the order of approximately 50%, from an average of 0.2 splats per mile to 0.1 splats per mile, in 2004 (n=3838) and in 2019 (n=667).
Figure 2 Model estimates and confidence limits predicted by a generalised linear model of the difference in splat density recorded on vehicle journeys in 2004 (in SE England) and 2019 (in Kent). Between 2004 and 2019 there was a statistically significant difference in ‘splat density’ of the order of approximately 50%, from an average of 0.2 splats per mile to 0.1 splats per mile, in 2004 (n=3838) and in 2019 (n=667).

We addressed two limitations of the 2004 survey. Firstly, in 2004 participants were not provided guidance on journey length, and a large number of long journeys spanning several counties resulted in the data providing poor spatial resolution of variation. Only variation between regions was resolved in the 2004 data, but by encouraging participants to submit data from both short and long journeys, we were able to map spatial variation in splat density within Kent. The second limitation we addressed concerned a criticism levelled at the methodology in terms of the effect of vehicle design on the rate of invertebrate sampling. Modern cars are more aerodynamically designed than in the past, and changes over time may affect the numbers of insects getting squashed. We actively recruited classic car owners to take part in the survey, allowing us to collect data using cars ranging in age from 1957 to 2018. We found a small but statistically significant positive relationship between vehicle age and splat density, suggesting that modern cars squash more invertebrates that older cars. This suggests that the signal from the difference in insect abundance is strong enough to be apparent in spite of more efficient sampling by newer vehicles.

Figure 3 The relationship with confidence intervals, between splat density and age of vehicle predicted by a generalised linear model. Modern vehicles were more efficient at sampling insects. In spite of this, significantly fewer insects were sampled in 2019 than in 2004.
Figure 3 The relationship with confidence intervals, between splat density and age of vehicle predicted by a generalised linear model. Modern vehicles were more efficient at sampling insects. In spite of this, significantly fewer insects were sampled in 2019 than in 2004.

More data is required to establish robust trend data, control for inter-annual variation in factors such as weather and vehicle speed, and increase confidence in the observed pattern of the effect of vehicle age and design. There are significant opportunities to expand coverage to other counties, and more data requires greater participation by the public. Experience drawn from other citizen science projects shows that surveys must be as accessible as possible to maximise participation. We ran our pilot survey in 2019 using paper forms and spread sheets, which are not conducive to mass participation. To address this we have drawn up a specification for a mobile app to facilitate wider participation and are seeking funding for its development. Growing this study has the potential to add to the growing body of evidence for significant invertebrate declines in the UK, and demonstrates the valuable contribution made by citizen science.

Logo: Nature's Sure Connected - Heritage Fund

Footnotes

1   Hallmann, CA, Sorg, M, Jongejans, E, Siepel, H, Hofland, N, Schwan, H, Stenmans, W, Müller, A, Sumser, H, Hörren, T, Goulson, D and de Kroon, H. (2017). More than 75 percent decline over 27 years in total flying insect biomass in protected areas. PlosONE 12. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0185809

2   Fox, R., Parsons, M.S., Chapman, J.W., Woiwood, I.P. Warren, M.S. & Brooks, D.R. (2013). The state of Britain’s larger moths 2013. Butterfly Conservation & Rothamsted Research Wareham, Dorset. https://butterfly-conservation.org/sites/default/files/1state-of-britains-larger-moths-2013-report.pdf

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