COMPARATIVE BIODIVERSITY ALONG A GRADIENT OF AGRICULTURAL LANDSCAPES

The aim of this study is to compare biodiversity in contrasted landsca pe units within a st-nail region. In western France agricultural inten sification leads to changes in landscape structure: permanent grasslan ds are ploughed, fields enlarged and surrounding hedgerows removed or deteriorated, brooks are straightened and cleaned. South of Mount Sain t Michel Bay, four landscape units have been identified along an inten sification gradient. Several taxonomic groups (small mammals, birds, i nsects and plants) have been used to evaluate the characteristics of b iodiversity along this gradient. The hypothesis that intensification o f agricultural practices lead to changes in biodiversity has been rest ed. Biodiversity is measured by the species richness. Shannon's divers ity index. equitability and similarity indexes. Our results show that intensification of agriculture does not always lead to a decrease in s pecies richness, but to several functional responses according to taxo nomic groups. either no modification, or stability by replacement of s pecies, or loss of species. For most of the studied taxonomic groups s pecies richness does not vary greatly alone the gradient. Depending on the landscape structure and farming systems this gradient is probably truncated and does not allow to show major changes in species richnes s. An alternative hypothesis is that used indexes are not sensitive en ough to reveal changes in biodiversity. Nevertheless, similarity index es reveal that sensitivity to changes varies, invertebrates being more likely to perceive the dynamics of the landscapes studied than verteb rates or plants. These points have to be taken into consideration when elaborating policies for sustainable agriculture or nature conservati on (C) Elsevier, Paris.