26th February 2021

Drivers of bat declines

Hugh Clark

What are the main reasons for bat population losses and recent recoveries? New research says we still don’t know enough to answer these important questions, but a range of major threats has been flagged. The evidence calls for more efforts to understand bat population losses and identifies what works in helping populations to recover.

Five of the 11 species of bat monitored in Britain are now seeing population increases, but the drivers of past losses and continuing changes remain unclear. Significant historic bat declines over the last century were blamed on agricultural intensification, loss of habitat and roosts through development and exclusion from buildings.

Today’s main threats include land use practices, climate change, pollution, development and infrastructure, and human disturbance. The new paper, published in the Mammal Society journal, Mammal Review, says evidence gaps or conflicts present barriers to successful conservation. The authors call for more focus on the impacts of climate change, urbanisation, offshore wind turbines, water pollution and their combined effects.

Agricultural management and climate change are having big impacts on populations of many mammals, birds, amphibians, reptiles, insects and vascular plants. However, the impacts on bats appear to differ from other species.

Intensive agriculture has impacted at least six bat species, for example by reducing insect prey availability, foraging areas and habitat connectivity and through pesticide exposure. Whilst some bats still hunt over intensively managed pasture, including the serotine, Myotis and long eared bats, arable land is generally avoided by the serotine, Leisler’s and greater and lesser horseshoe bats.

Loss of hedgerows as green corridors for hunting and commuting is also a problem for bats. Less intensively managed farmland has a positive effects on bats. Organic farms are associated with larger hedgerows and higher bat abundance, species richness and foraging activity.

The success of efforts to improve farmed landscapes for biodiversity and bats remains unclear. Agri-environment scheme practices such as less frequently trimmed hedgerows and more hedgerow connectivity have helped pipistrelles, noctules, Leisler’s and greater and lesser horseshoe bats.

Bat population change is also caused by roost disturbance, deliberate persecution and roost exclusion. It reduces adult female survival and bat community diversity. Historically, chemical timber treatment caused large numbers of roosting bat deaths throughout Europe. Despite legislation protecting bats and their roosts, building renovations are still causing roost disturbance or exclusion, which can reduce successful breeding.

Climate change has been identified as the second most important driver of changes to UK biodiversity, but its impact has only been assessed in a few bat species.

Increases in new native forest cover has helped at least nine bat species, but old growth forests are vital for some bat species. Canopy thinning, loss of older woodland features and replacements with younger trees are thought to have been a major driver of historic bat population declines. European forest loss and fragmentation remain a concern for specialist woodland bat species for which there is less research, such as the Bechstein’s bat.

Onshore wind turbines remain a concern for at least 21 species of bat, especially migratory species and open habitat foragers. The impacts of offshore wind turbines are harder to monitor, but at least 12 migrating European bat species may be impacted. Mitigating the death risks posed by wind turbines to bats is urgent, yet methods for estimating mortalities and the risks to bat populations by wind turbines at local, national, and continental scales are inadequate, the study found.

High numbers of bats are killed on roads, which also cause habitat loss, barriers, pollution and disturbance from light and noise, with at least six bat species known to be impacted. Hunting time and prey capture is reduced within 60 metres of roads, largely due to noise interfering with bat echolocation.

Light pollution affects some bat species more severely than others. Pipistrelles are attracted to hunting around artificial lighting, but light-shy species such as horseshoe bats, Myotis, Leisler’s and long eared bats lose foraging areas, which may reduce populations.

Water pollution such as microplastics, eutrophication and heavy metals, is a poorly understood concern, with two thirds of Europe’s surface water estimated to be in poor ecological status in 2020.

Domestic cats are considered to be the most significant predators of bats, killing an estimated 250,000 bats per year in the UK; especially house-roosting species such as pipistrelles and brown long eared bats.

Six of the UK’s 17 breeding bat species could not be assessed for impacts, because of a lack of evidence. New monitoring tools can help in identifying the impacts of some of these drivers on bat populations.

More targeted conservation strategies are needed to aid the recovery of bat populations, following decades of declines throughout Europe, say the researchers. They noted the findings of the Convention of Biological Diversity in 2019, which said more ambitious actions are required and that, despite global commitments in the Convention to reduce or halt biodiversity declines, ‘targets are perpetually missed by governments’ adding, ‘Understanding not only how wildlife populations are faring, but also the factors driving population change, is essential to reversing negative trends.’

The monitoring of ‘bioindicator’ species such as bats is a means of assessing the wider health of the habitats and species they rely on. The researchers focused on European bat population trends, but say many of the drivers are relevant to bats in tropical and sub-tropical regions too, including urbanisation, agricultural intensification and deforestation.

The evidence review was carried out by Ella Browning, Kate Barlow, Fiona Burns, Charlotte Hawkins and Katherine Boughey. The report was dedicated to the memory of bat researcher Dr Kate Barlow, whose earlier work informed this review.

FULL PAPER: "Drivers of European bat population change: a review reveals evidence gaps" by Ella Browning, Kate E. Barlow, Fiona Burns, Charlotte Hawkins Katherine Boughey Mammal Review (2021) https://doi.org/10.1111/mam.12...