Changes in river corridor plant diversity over two decades: Lessons from the Adour River, SW France

Background/Question/Methods

Temperate riparian corridors include ones of the most species-rich plant communities. Aside species turnover due to natural dynamics, the biodiversity of these ecosystems is increasingly altered due to direct human pressure and recent global environmental changes. However, mid-term surveys carried out at the ecosystem scale in order to demonstrate such overall changes in community structure remain scarce.

In 1989, 1999 and 2009, we exhaustively investigated the flora along the riparian and aquatic corridor of the 350 km long Adour River, SW France. We analyzed species turnover rates, morphotypes, ecological strategic groups and species geographical status (natives versus introduced) to resume major changes in upstream-to-downstream gradients expressed by the 2,000 species identified.

Results/Conclusions

Surprisingly, total species richness remained comparable though time, suggesting a compositional stability of the riparian corridor flora. However, high species turnover rates depicted strong changes due to riparian species replacement by external, ruderal plants. The longitudinal trends of most biological indicators were consistent among dates, indicating the robustness of the ecological response of each functional compartment within the regional community. However, we observed overall significant quantitative shifts over time among spatial trends, and subtle local changes, both in relation to major mid-term environmental changes. Whilst riparian and aquatic stress-tolerant and competitive species tended to decrease in proportion, the contribution of ruderal and non-native species –including invasive ones– significantly increased. This can be explained by the alterations of riparian corridor physical permeability and of natural disturbance regime and by modifications in land use. These changes are also in agreement with independent predictions we made at the regional scale from climate change scenarios. This was more specifically the case for the increase in non-native species occurrence.

Addressing the concept of ecological resilience, we discuss the possible consequences of the settlement of such a novel biodiversity on temperate river system functioning. Possibly, newcomers could sustain riparian ecological functions and structure as far as a minimum natural disturbance is preserved. However, low beta diversity due to community homogeneization would have consequences on successional dynamics.