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Éco-Conception des ensembles bâtis et des infrastructures
Eco-design of buildings and infrastructure
Biodiversity offsetting: Certainty of the net loss but uncertainty of the netgain
Biodiversity offsetting is usually the last step in the mitigation hierarchy and aims to compensate for impacts ofdevelopment projects on biodiversity. It is supposed to contribute to the key environmental objective of“no netloss”of biodiversity by delivering gains equivalent to losses. We hypothesize that such gains can only be attainedthrough ecological restoration of degraded sites: the restored ecosystem should not only equal the original orreference ecosystem as usually assumed, but rather the original state of degradation of the ecosystem used foroffsetting should be of the same level as the impacted ecosystem after development. We built on this startingassumption to determine whether impacts and gains were considered equally in the offsetting measures of 24infrastructure projects, and to infer the potential gains in offset sites, based on an analysis of procedure andadministrative documents. The analysis showed that impacts were presented in much more detail than theoffsetting measures. In addition, out of 577 ha that was intended to offset areas being artificialized, only 3% ofthe area was artificial prior to offsetting work, i.e. delivering high potential gains, while 81% could be con-sidered semi-natural habitats, thus with lower potential gains. Little information on the ecological quality ofoffset sites was available. When described, their good quality was used as an argument to justify their selection,resulting in relatively uncertain gains in comparison to certain impacts. Our results suggest that includingmultiple comparisons of multiple ecosystem states is a way forward to better evaluate the equivalence betweengains and losses, and thus would ensure no net loss of biodiversity.