On the road with the beetle generation A study has revealed the resilience of smaller species in withstanding and expl oiting natural disturbances. Malcolm Smith reports

Beetles are everywhere. Just one family of them - the Carabids, or ground beetles - has a staggering 25,000 species worldwide. Well over 300 are to be found in Britain, in woodland, in pastures, on our hills and moors. And in our gardens.

No ecologist or beetle enthusiast would be surprised to know that some species confine themselves to particular habitats and are never found elsewhere. Others are more ubiquitous, living in a range of habitats. What is much more surprising is that where you find them depends on their size. The largest species are in woodland and moorland, medium-sized ones live in rough grassland, and the very smallest are in grassland managed intensively for farming.

This unusual finding is the result of a study funded by the Scottish Office and carried out by Shona Blake and Dr Garth Foster of the Scottish Agricultural College, with help from Drs Mick Eyre and Martin Luff of Newcastle University. They looked at beetles in 50 grasslands, 30 woods and nine moorland sites, comparing the sizes of those they found. Their results are about to be published in the journal Pedobiologia.

The big boys, such as Carabus glabratus, black in colour (like so many ground beetles) and up to 30mm in length, were common in moorland and woodland. Few of them ventured into grassland. At the other extreme, beetles such as the little brown and black Nebria brevicollis (12mm) and the black and orange Calathus melanocephalus (7.4mm) dominated grassland where there was regular use of fertiliser and heavy livestock grazing. Other, even smaller, beetles included a couple of species no more than 4mm long, found especially in pastures cut for silage. These little chaps never ventured into the moors and woods.

Overall, one or more species of ground beetle 24mm or over were found on 32 of the 39 woodland sites but in only 24 out of the 50 grasslands. The more intensively managed the grassland, the smaller the species of beetle. So in places with few grazing animals and little fertiliser use, the average ground beetle was more than 15mm long. On more managed grassland, the average was 11mm.

Ms Blake and Dr Foster have had a go at explaining this little and large beetle drive.

After analysing a variety of factors, they concluded that the main correlation was with disturbance. In intensively managed grasslands there are many more animals moving about, grazing on the productive turf. Frequent doses of fertiliser add to the amount of disturbance in the system.

By contrast, moorlands have very few sheep or cattle grazing on them and little human activity (with the exception, perhaps, of grouse shooting on some). For disturbance, little-managed grasslands - which tend to be richer in flowers - equate more with moors than with intensively managed pastures.

So why does disturbance favour little beetles rather than large ones? Ms Blake and Dr Foster relate it to fluctuations in energy values. In heavily managed (disturbed) grassland, fertiliser doses are large and sudden. Frequent cutting for silage is another significant and very sudden activity, this time removing energy from the system. Such fluctuations, according to accepted ecological wisdom, are best exploited by species that are widespread, able to disperse quickly and able to sustain big changes intheir numbers.

Such species are likely to be small ones. This is because a larger body size implies a longer development time for the creature to garner enough food to grow. Most large ground beetles, many of which are flightless, breed in the autumn and have over-wintering, non-growing larvae. Smaller species often breed in the spring and have fast-growing larvae.

A steady supply of resources over a prolonged period is what is needed for a long life cycle. In intensively managed grassland, life is more hectic, the sudden dollops of energy favouring fast growth. It suits the scurries of little ground beetles down to the ground.

If conditions suddenly become unfavourable, small ground beetles can disperse by flying off to a better spot - an option denied to the (usually) flightless larger species. After all, if your home is being ploughed up, legging it is less likely to get youout of trouble.

But this story of the virtues of small stature does not end here. For wading birds such as the curlew or oystercatcher - which, with many others, make a meal of ground beetles - the size of the prey is vital. Catching larger beetles makes more sense thancatching smaller ones; you get a bigger meal for your effort. And there is proportionately less of the hard, indigestible outer skeleton on larger beetles. Meanwhile, the larger the beetle, the easier it is to spot - unless the more rank vegetation, which larger beetles have made their own, gives them much better hiding places.

Perhaps that is why you often see flocks of lapwing or curlew feeding in the short-cropped turf of intensively managed pastures. They have given up searching the rank tussocks. There could be a virtue in being large after all.