THE EFFECT OF MOLLUSK - GRAZING ON SEEDLING RECRUITMENT IN ARTIFICIALLY CREATED GRASSLAND GAPS

Two experiments conducted in spring and autumn 1992 examined the effec t of mollusc grazing on seedling regeneration from natural grassland s eedbanks by creating artificial gaps in plots in a grassland sward. Mo lluscs were excluded from half the gaps by application of molluscicide . Mollusc grazing in both the spring and autumn experiment significant ly reduced seedling recruitment, though the intensity of grazing was g reatest in autumn. Recruitment of five species was markedly influenced by molluscicide application. In spring, plots from which molluscs wer e excluded contained significantly more seedlings of Chenopodium polys permum and Ranunculus acris. In the autumn, exclusion of molluscs resu lted in increased populations of R. acris, Stellaria graminea and Rume x acetosa. Cerastium holosteoides populations were greatest in autumn grazed plots. Other species, notably the grasses Holcus lanatus and Ag rostis capillaris and the legume Trifolium repens were unaffected by m olluscicide application. Species diversity was significantly decreased by molluscicide application in the autumn. Gap size significantly aff ected the recruitment of two species. Ranunculus acris populations wer e significantly higher in small gaps in both spring and summer, while Chenopodium recruitment in the spring was greater in small gaps. Gap s ize also significantly influenced the risk of mollusc attack on Ranunc ulus as molluscs appeared to show an aggregative feeding response in t he high seedling density small gaps. Selective grazing of vulnerable s eedlings by molluscs may influence the eventual relative proportions o f the species present and so provide a potent mechanism in shaping com munity composition in grasslands.