Local and regional changes in taxonomic and functional diversity of arable weed communities in Burgundy (France) between the 1970s and the 2000s
Résumé
Human activities lead to a process of homogenization of biotas in which specialist species are increasingly replaced by common and widespread species. Using a 30-year diachronic record of arable weed communities, we tested this hypothesis by quantifying changes in a-and beta-diversity, using both taxonomic and functional diversity and by partitioning beta-diversity into species replacement and richness differences. Arable weed communities were sampled in the same 158 fields of the Cote-d'Or region (northeastern France) between the 1970s and the 2000s. For each period, each field was characterized by crop types, soil characteristics and a High Nature Value (HNV) farmland index based on agricultural intensification at the landscape level. At the field scale, we observed a loss of 46% and 38% in alpha-taxonomic and functional diversity, respectively, which was in accordance with the decrease in the HNV farmland index over the same period. At the regional scale, there was an increase of 15% and 21% in beta-taxonomic and functional diversity (across fields), respectively. Crop type and soil characteristics explained similar levels of variation in species replacement, and crop type explained much larger richness differences in the 2000s suggesting that crop and associated practices may exert a high filtering effect. Our results also highlighted a marked decline of common weeds; a process that is far from being counterbalanced by the few colonizing weeds. Rather than to biotic homogenization, this pattern of loss has led to a higher differentiation of arable weed communities. This could correspond to a fragmentation of suitable habitats for species that depend on weeds. This pattern was associated with a decrease of species richness per field; the loss of common species and their associated functions may be of greater significance for agroecosystem functioning.