Assessing direct effects of insect change on insectivore populations in the United Kingdom

1. Declines in insect abundance are a cause for concern, with potential downstream impacts on the function of ecosystems. Insects are key food resources for insectivorous vertebrates, with evidence that declines in these species could be driven by changing insect abundance.

2. Quantifying the direct effect of insect abundance on vertebrate population dynamics is challenging especially at large spatial scales (e.g. regional-to-national scales) due to data limitations, and because correlations between population dynamics can result from shared responses to environmental variation.

3. We provide a comprehensive assessment of the role of insect abundance on the dynamics of 10 insectivores (five birds and five bats) within the United Kingdom, by assembling and pairing insect and vertebrate abundance data at three spatial resolutions (100, 50, 10km) utilising several citizen science monitoring programmes.

4. To address the challenges of quantifying direct effects, we use a multiple specification approach combining: 1) association, 2) prediction, and 3) causal inference.

5. We found evidence of overall declines for all bird species evaluated and for nearly all indices of insect food availability, though none of the bat species tested showed evidence of overall decline. Despite indices of both insect and bird abundance declining, declines did not always co-occur spatially. We also found limited decisive evidence that insect change was currently driving insectivore population change, identifying only moderate evidence of links between both blue tit and great tit and moth abundance, and grey partridge and Diptera abundance.

6. Our results suggest that for most insectivores assessed, reductions in insect food do not appear to be a primary cause of declines and that both insects and insectivores may be impacted by several (non-overlapping) factors associated with environmental change. However, we discuss the challenges and limitations of assessing direct impacts of insect declines from observational monitoring data.