Identifying River Corridors for the Implementation of the Network of Green Infrastructure in Spain

Abstract

1. Spatial–temporal connectivity plays a key role in freshwater ecosystems by maintaining processes such as the transfer of materials and energy, gene exchange, and migratory movements necessary for the maintenance of functional ecosystems. However, connectivity in these systems has undergone severe modifications over the last century, threatening the persistence of biodiversity and the ecosystem services they provide. The European Union (EU) acknowledges the value of freshwater ecosystems as important connectivity elements of the landscape and the need to recover their functionality, not only for freshwater biodiversity, in policy instruments such as the European Biodiversity Strategy for 2030 or the Green Infrastructure Strategy. Priority areas need to be designated and managed as corridors. However, given the widespread impacts to connectivity, balancing the functionality of corridors and socio-economic constraints will be key. 2. We demonstrate how to design a network of river corridors in Spain to connect populations of freshwater fish species, while minimising the impact of barriers that compromise the functionality of the corridor or make its restoration expensive. We integrated information on the spatial distribution of 40 fish species and more than 30.000 barriers along 80.000 km of rivers and streams to identify priority corridors that connect at least 50% of the populations for all species. We ran three different scenarios that depict alternative planning interests and constraints: (i) an unconstrained scenario, where all river reaches were equally available to be part of the corridor; (ii) a Natura 2000 scenario (N2K), where corridors connected protected areas; and (iii) a no dam allowed scenario (NDA), where we avoided selecting reaches with dams as part of the network of corridors. We measured four different indicators to compare scenarios: number of planning units selected, the number of dams included, the length of continuous units selected and the length of continuous units selected for each species individually. 3. We found that the optimal network of corridors always contained reaches with barriers. However, the network was more spatially continuous (22% and 26% more continuity) and was always less impacted by barriers (6.9 and 2.6 fewer barriers) under the unconstrained scenario than under the N2K and NDA scenarios. The network of corridors was free from dams only under the NDA scenario, although the average connectivity across all species was always lower than under the other two scenarios. 4. Our results demonstrate that the design and management of a coherent network of freshwater corridors in Spain will need to integrate reaches impacted by barriers. Securing the functionality of such a network by restoring the lost connectivity will pose a socio-economic challenge. Spatial planning can help address this challenge by identifying priority corridors that minimise restoration efforts. 5. The approach demonstrated here could be extended to other components of connectivity, such as lateral and vertical connectivity, as well as biodiversity and ecosystem services features to address other dimensions and functionality of freshwater corridors. However, implicit decisions that contextualise the planning exercise, exemplified by the differences across the three alternative scenarios we tested, lead to very different spatial priorities. Therefore, defining the planning objectives and constraints to deliver solutions that fit for purpose is critical.