NBMP annual report 2024: second year of concern over short-term trends

Long-term monitoring shows that population trends for five bat species in Great Britain have increased and six remain stable since 1999. 

However, recent short-term trends suggest fewer increases and some emerging declines, particularly in specific regions. These shifts may indicate new pressures, highlighting the need for continued monitoring and research to guide conservation efforts. 

As indicator species, bats help reflect the broader health of our ecosystems — meaning changes in their populations can signal wider environmental shifts. 

Since monitoring began in 1999, five bat species in Great Britain — greater horseshoe bat, lesser horseshoe bat, Natterer’s bat, common pipistrelle and soprano pipistrelle — have shown long-term increases in population. 

Six others, including Daubenton’s bat, whiskered/Brandt’s bat, serotine, noctule and brown long-eared bat, appear to have remained stable. No species have shown a significant long-term decline at the national level. 

Short-term trends over the past five years present a more mixed picture. 

While long-term data from multiple surveys continue to show increases for five species, more recent short-term trends are less consistent. 

Brown long-eared bat shows a statistically significant decline from the Roost Count at the GB level over the last five years. Only the greater and lesser horseshoe bats show a continued increase in the most recent hibernation surveys, with other surveys indicating no significant change. 

At the country level, regional differences are emerging. 

Out of 26 combinations of species and survey types, only two trends — hibernation surveys of greater and lesser horseshoe bats in Wales — show significant increases in both the long- and short-term. Eleven trends are stable across both timeframes. In contrast, 12 cases show a slowing or reversal in trend when comparing short-term with long-term results. In most of these, a previously increasing population now appears stable, but three cases show a significant decline in the short term. 

While these short-term changes may reflect natural population fluctuations, the fact that these less positive results are widespread across species could indicate emerging pressures on bat populations. 

Continued monitoring and targeted research are essential to understand what is driving these changes and to inform future conservation efforts for these slow-reproducing and ecologically important species. 

Bat populations are thought to have been in decline since at least the early 20th century, well before 1999 — the baseline year for most population trends. For some species, genetic studies suggest these declines may have begun much earlier. 

Factors linked to these declines include agricultural intensification, habitat loss, persecution, pesticide use, water quality issues, reduced insect prey, development, land-use change, and climate change. 

Since the 1990s, some populations have shown positive trends, coinciding with conservation measures and protective legislation. However, more recent short-term trends suggest emerging concerns that may need further investigation.