Dynamics of biodiversity in a changing space - The case of chalk grasslands in the lower section of the Seine Valley

Research has been carried out on the biodiversity of chalk grassland ecosystems in the Seine valley. We have recorded vegetation (plant communities, seed bank) and soil characteristics (macrofaunal communities and abiotic conditions) following several sampling procedures balled an temporal or spatial gradients. The aim is to test two research hypothesis concerning i) the different rythms of compartimental changes in a dynamic ecosystem, and ii) the spatial dimension of biodiversity. For the taxinomical groups studied, results show significant changes in biodiversity along successional gradients. However, these dynamics do not occur at the same rate, which suggests inertia between ecosystem components and differences in recorded dynamics. We underline that measures of biodiversity must take into account this temporal dimension in order to have a predictive value (notion of ecological organization). It is also suggested that the scale at which biodiversity is measured is not necessarily that of the studied ecosystem (notion of functional unit of a compartiment). (C) Elsevier, Paris.