The decline of arable weed diversity in agroecosystems
It is widely acknowledged that the intensification of agriculture has lead to a decline in agroecosystems’ biodiversity. However, few precise and measured data exist to document this process. In France, 158 arable fields distributed across Côte-d’Or (Bourgogne region) were surveyed, in 1968-1976 and then in 2005-2006. The objective of these surveys was to quantify the changes in weed species composition and richness since the 1970s.
The main finding of the study was the significant decline of arable weed diversity at the field scale. The number of species per field decreased by average from 16.6 to 9.3 (40%). Mean cover abundance also experienced a significant 67% decline from 61.5 to 20.2 plants/m². Field hedges maintained a higher diversity compared to the field core area and could be considered both as a refuge area for threatened arable weeds (e.g. Bunium bulbocastanum, Caucalis platycarpos) but also as a way of entry for species that were formerly only found on roadsides (e.g. Geranium spp., Sisymbrium officinale).
Four main trends were observed in the changes of species (new, increasing, stable or decreasing):
- The most meaningful result was the increase of nitrophilous species that can be directly linked to the 42% increase in the amount of fertilizer used since the 1970s. According to ecological indicator values, decreasing (Teucrium botrys, Legousia speculum-veneris) or extinct species (Saxifraga tridactylites, Lactuca perennis, Nigella arvensis) have low nitrogen requirements compared to stable (Poa annua, Taraxacum officinale), increasing (Rumex obtusifolia, Senecio vulgaris) or new species (Cirsium vulgare).
- The increase in oilseed rape crop since the 1970s (74% increase of landuse in the studied area) had a significant effect on weed species composition. The repeated use of the same active substances (e.g. trifluraline) favoured the presence of weed species that are not sensitive to herbicides used in oilseed rape crop: Anthriscus caucalis, Geranium spp., Scandix pecten-veneris, Sisymbrium officinale, etc.
- The decline in insect-pollinated species was also observed, while self- and wind-pollinated weed species remained stable. Moreover, not only the number of species in the community has decreased, but also the interactions between species.
- The study also showed that decreasing weed species had a specialized ecology, while generalist species (able to grow on a large range of environmental conditions) were the most stable. Generalist species are indeed able to cope with changes, such as farming practices.
The decline of vegetation diversity in cultivated fields has a non-negligible impact on the functioning of agroecosystems (e.g. decline of both insect-pollinated species and weeds used by bird species). A regular and extended monitoring of arable weed flora (e.g. network ‘Biovigilance Flore’; see EPPO RS 2008/110) would be of great value to survey and prevent the emergence of new weeds. Such monitoring would also allow to maintain a level of diversity compatible with the functioning of agroecosystems.
Sources
Contact;: Guillaume Fried, LNPV, FR or Xavier Reboud, INRA, FR. E-mail: fried@supagro.inra.fr ; reboud@dijon.inra.fr
Dessaint F, Fried G, Barralis G (2007) Déclin et changements au sein de la flore adventice : quelle évolution en 30 ans ? 20ème conférence du Columa. Journées internationales sur la lutte contre les mauvaises herbes. Dijon, FR, 2007-12-11/12, 417-426.
Fried G, Girod C, Jacquot M, Dessaint F (2007) Répartition de la flore adventice à l’échelle d’un paysage agricole : analyse de la diversité des pleins champs et des bordures. Vingtième conférence du Columa. Journées internationales sur la lutte contre les mauvaises herbes. Dijon, FR, 2007-12-11/12, 346-355.