The management of lowland neutral grasslands in Britain: effects of agricultural practices on birds and their food resources

1. The effects of agricultural intensification on biodiversity in arable sy stems of western Europe have received a great deal of attention. However, t he recent transformation of grassland systems has been just as profound. 2. In Britain, the management of grassland has changed substantially in the second half of the 20th century. A high proportion of lowland grassland is managed intensively. The major changes include a doubling in the use of in organic nitrogen, a switch from hay to silage, and increased stocking densi ties, particularly of sheep. Structurally diverse and species-rich swards h ave been largely replaced by relatively dense, fast-growing and structurall y uniform swards, dominated by competitive species. 3. Most of these changes have reduced the suitability of grassland as feedi ng and breeding habitat for birds. 4. The most important direct effects have been deterioration of the sward a s nesting and wintering habitat, and loss of seed resources as food. Short uniform swards afford poor shelter and camouflage from predators, whereas i ncreased mowing intensities and trampling by stock will destroy nests and y oung. Increased frequency of sward defoliation reduces flowering and seed s et, and hence food availability for seed-eating birds. 5. The indirect effects of intensification of management on birds relate la rgely to changes in the abundance and availability of invertebrate prey. Th e effects of management vary with its type, timing and intensity, and with invertebrate ecology and phenology, but, in general, the abundance and dive rsity of invertebrates declines with reductions in sward diversity and stru ctural complexity. 6. Low input livestock systems are likely to be central to any future manag ement strategies designed to maintain and restore the ecological diversity of semi-natural lowland grasslands. Low additions of organic fertilizer ben efit some invertebrate prey species, and moderate levels of grazing encoura ge sward heterogeneity. 7. There is now a need to improve understanding of how grassland management affects bird population dynamics. Particularly important areas of research include: (i) the interaction between changes in food abundance, due to cha nges in fertilizer inputs, and food accessibility, due to changes in sward structure; (ii) the interaction between predation rates and management-rela ted changes in habitat; and (iii) the impact of alternative anti-helminithi c treatments for livestock on invertebrates and birds.