Evaluating ponds as nature-based Solutions: effectiveness and eco-efficiency.

Nature-based Solutions are strategies inspired by nature that address major societal challenges, such as climate change and food security, by providing several socio-economic and environmental contributions. Despite widespread adoption, evaluating their impacts poses challenges within the field of environmental economics. In this study, we develop a framework for the comprehensive assessment of ponds as Nature-based Solutions. These water systems contribute to carbon storage, provide habitats for biodiversity, as well as leisure, business, and cultural opportunities. We assess them in terms of both effectiveness, which refers to their capacity to deliver benefits according to stakeholder expectations, and eco-efficiency, measuring input-output performance in environmental economic terms. First, we successfully reduce the heterogeneity of pond-specific contributions into comparable composite indicators with a novel application of the Benefit-of-the-Doubt method. Next, we combine Multi-Criteria Decision Analysis with stakeholder preferences to calculate ponds' effectiveness. The results show that these can be viable strategies for the delivery of locally relevant Ecosystem Services. Then, we investigate ponds' technical as well as ecological efficiencies by Data Envelopment Analysis, demonstrating that they can provide both socio-economic and environmental contributions in an efficient manner. Finally, we compare these complementary metrics and confirm the hypothesis that effective ponds can also provide eco-efficient Nature-based Solutions. Furthermore, we elaborate on the potential extensions of our analytical method to other strategies, highlighting how it could support decision makers taking context-specific, informed decisions on the best measures to implement.