Significance of rabbits for the population regulation of Hieracium pilosella in Breckland

(1) A population of Hieracium pilosella from which rabbits had been excluded was compared with a grazed population in the adjacent grass heath. Rosettes were mapped on 100 occasions over a period of 4.5 years and their reproductive activity and clonal growth recorded. (2) Exclusion of rabbits had little effect on overall rosette density, except to ameliorate somewhat the effects of drought. The probability of a rosette initiating an inflorescence was negatively density-dependent; this probability was significantly lower in the absence of rabbits over the whole range of density examined. (3) Although most flower buds outside the exclosure were eaten by rabbits, all rosettes that had initiated an inflorescence proceded with the clonal production of new rosettes on stolons or from axillary buds. Seedling establishment was rare in the exclosure and rarer outside. (4) As rosettes are monocarpic, the turnover of rosettes was greatly reduced in the absence of rabbits; recruitment and mortality were reduced and there was a shift to an older age-structure, in comparison with the grazed population. Exclosure increased the population half-life from 1.4 to 2.3 years. (5) A mechanism is postulated for the modulation by rabbits of the relationship between flower initiation and the density of H. pilosella in a grass heath sward, through selective grazing of the dominant grasses, Festuca ovina and Koeleria macrantha.