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

Declines in insect abundance are a cause for concern, with potential downstream impacts on ecosystem function. Insects are key food resources for insectivorous vertebrates, with evidence that reductions in prey availability may influence population dynamics. Yet quantifying direct effects at large spatial scales is difficult, both due to data limitations and because correlations may reflect shared responses to environmental change rather than trophic links. We assess the role of insect abundance in the dynamics of 10 insectivores (five birds, five bats) in the United Kingdom by pairing insect and vertebrate abundance data at three spatial resolutions (100, 50, 10 km) from multiple citizen science programmes. To address the challenges of inference and combining heterogeneous data, we adopt a multiple-specification approach spanning: (1) association, (2) prediction, and (3) causal inference. We found evidence of overall declines for all bird species evaluated and for most indices of insect abundance, though none of the bat species tested showed consistent declines. Despite indices of both insect and bird abundance declining, declines did not always co-occur spatially. We also detected only moderate evidence of links between insect change and insectivore population change, notably blue tit and great tit with moth abundance, and grey partridge with Diptera abundance. Across taxa, spatial scales, and analytical approaches, we did not find consistent evidence that changes in insect abundance are a dominant driver of insectivore declines. Both insects and insectivores may instead be responding to multiple, and sometimes non-overlapping, pressures associated with environmental change. We emphasise that absence of strong evidence does not imply absence of effect, and highlight the difficulties of drawing direct conclusions from heterogeneous observational monitoring data.