LIGHT POLLUTION AND THE DECLINE IN BAT POPULATIONS

Light pollution and the decline in bat populations

During this century bats populations have been decreasing at an alarming rate; in fact some species have fallen by over 50 per cent. As a result, the greater horseshoe bat, once found throughout southern England, has now become extremely very rare, along with sightings of the elusive Bechstein's and barbastelle bat. Sadly, the mouse-eared bat has been declared officially extinct since 1992.

There are a number of factors that are believed to be responsible for these dramatic population declines, and perhaps the most obvious is the destruction of their woodland habitat. This has reduced the availability of natural roosting sites, and even when bats have tried to establish roosts in more built up areas these too are being continually destroyed as old buildings are renovated or demolished.

Perhaps the most sinister problem is the ‘double-edged sword’ effect that light pollution is having on bat populations. Nocturnal insects are attracted to bright lights and become sucked out of the countryside – the bats natural feeding grounds - and into suburban areas. A German study back in 2003 concluded that a single street light would kill, on average, 150 insects a night. In direct relation to this Philipp Heck - president of Dark Sky Switzerland - has suggested that the 50,000 or so streetlights left on in Zurich would kill over one million insects every night. But the attraction of nocturnal insects to night luminescence isn't the biggest problem here as captivated insects seem unable to feed, drink or procreate, only to end up dead from exhaustion. Unfortunately not only are bats losing the main food source from their habitat, they positively shy away from lights as this makes then vulnerable to attack from nocturnal predators. In our modern world of 24 hour activity we have the insects moving in one direction while the bats are going off in the other. Eventually the bats are going to lose.

Unfortunately there are still more problems that bats populations have to endure. Many more colonies are poisoned by the toxic chemicals used in timber treatments, but more worryingly are the indiscriminate use of blanket insecticides that not only have depleted the food supply but have also contaminated the food chain.

What can be done?

It is a slow process changing the way our farmers grow our foods as insecticidal use is ingrained in their normal practice. However we can support those growers who produce their food crops organically and also stop using systemic insecticides in our own gardens.

Unfortunately, eradicating the world of night lighting will never be an acceptable solution, but there are steps that can be taken which will reduce its impact. At the very least we can give nocturnal insects a fighting chance to recover from the worst effects of intensive night lighting. Where appropriate, using timer switches and passive infrared (PIR) motion sensors on outdoor lighting can at least reduce the amount of time they stay on, reducing both insect death and electricity costs. Perhaps the most useful way the man in the street can help is to plant flowers that – unlike most plants - are nectar rich during the night. This way, exhausted insects have a chance to recover their vital energy needs by dawn. Such plant species would include Red valerian – Centranthus ruber, the common evening Primrose, Old Man's Beard – Clematis vitalba, summer jasmine – Jasminum officinale, the perennial sweet pea – Lathyrus sylvestris, Verbena bonariensis, white campion – Silene alba, honeysuckle and the night scented stocks.

Consider improving the insect friendly habitats in your garden by introducing a wildlife pond and/or logpiles. Both of these will help support a range of flying, nocturnal insects throughout their differing life cycles.

For related articles click onto the following links:
TURNING NIGHT INTO DAY: LIGHT POLLUTION’S IMPACTS ON WILDLIFE
WHAT ARE BATS?
WHICH PLANTS CAN ATTRACT BATS INTO THE GARDEN