ADDING lime to streams to combat the damaging effects of acid rain may do more harm than good.
Recent research comparing insect communities in natural, unlimed and experimentally-limed upland mires in Wales reveals that while some species of beetles, bugs and spiders increase at limed sites, others decline. The structure of the community is dramatically altered and some rarer species die out.
In a paper published in the journal Conservation Biology, biologists Sebastian Buckton and Steve Ormerod at the Catchment Research Group of the University of Wales argue that liming is no panacea and should be "applied with care".
Acid rain is caused by the burning of fossil fuels, releasing nitrogen and sulphur oxides into the air where they dissolve in atmospheric moisture and subsequently fall as acidic snow, rain or mist. Reducing emissions from major polluters such as power stations and cars is the key long-term solution but it is also costly and it will be some time before the conservation benefits are seen.
Spreading lime over the sources of streams, by helicopter, tractor or hand, is a popular short-term measure to counter the effects of acid rain. However, many of the upland bogs and mires that feed these streams are of high conservation value, some are rare habitats of international importance, and the effect of liming on the insect communities that these fragile, naturally acidic, systems support is virtually unknown.
Mr Buckton and Dr Ormerod studied the invertebrate communities in limed and unlimed mires using "pitfall traps" - small plastic cups sunk into the ground into which unsuspecting insects tumble. The traps caught about 5,400 bugs, beetles and spiders and revealed distinct differences between the incidence of various types of insect.
At limed sites, some families of beetle, like the ground beetle, were much less abundant, being "replaced" by higher numbers of rove beetles. Spiders followed a similar pattern; at limed sites wolf spiders were "replaced" by money spiders.
"These major changes cannot be unequivocally attributed to liming," said Mr Buckton. "But applying lime to bogs and mires often causes areas of sphagnum - the hummock-forming moss that dominates bog vegetation - to be replaced by bare ground and pools. As vegetation structure is very important for ground-dwelling invertebrates this could be why liming affects them."
The nine species of water beetle that live in these parts were found at limed sites and only one species was found at unlimed sites.
"The problem is one of priorities," said Dr Ormerod. "Do we protect the streams or the mires that feed them? At the moment we can't even be sure that liming will restore damage in acidified streams - it is certainly no quick fix and would probably require huge areas to be limed to have any effect - an operation that we now know may have added risks."
The work is part of a wider project that will help to identify sites where lime could be applied without damaging rare habitats. But, for now, says Mr Buckton, liming should certainly only "proceed with caution".
* How acid rain and liming erode diversity
If streams are left untreated acid rain harms many insects,crustaceans and snails.
The larvae of species such as mayflies and caddis flies are extremely sensitive to changes in acidity - if it rises theydisappear. A once rich, diverse stream community becomesdominated by one or two species, like stonefly and dragonflylarvae, that can tolerate more acid conditions.
Many of the species harmed by increasing acidity are vitalin the food chain. Fish such as trout and waterway birds such as the dipper, which inhabits the rushing upland streams of Britain, depend on them. Dippers feed entirely on aquatic prey. Mayfly and caddis fly larvae are important parts of their dietespecially when they are feeding their young.
A lack of prey means fewer dippers nest along acidic streams, and those that do appear unable to raise as many young.
Similarly upland bogs and mires are important breeding andfeeding sites for birds like golden plover, dunlin and curlew. Changes in insect populations, whether due to acid rain or liming, may have knock-on effects for these birds too.