Offsetting connectivity loss in rivers: Towards a no-net-loss approach for barrier planning

Abstract Biodiversity offsetting is a popular conservation tool to reduce the impact of human activities. This is especially relevant in freshwater ecosystems, under the increasing threat posed by the development of infrastructure to store freshwater or produce energy that break longitudinal connectivity and modify the structure and functioning of these systems. We demonstrate how to plan offset of connectivity loss in rivers derived from the construction of new barriers, by using the Tagus River (Iberian Peninsula) as a model. We simulate the construction of new barriers, measure the impact they would have on connectivity for each species individually, and identify an optimal set of existing barriers that should be removed to counterbalance the loss of connectivity caused for all species collectively. We found that loss in connectivity could be offset for most of species when a single new barrier was simulated at a time, by removing a small number of existing barriers. However, there was a group of species with very restricted ranges that could undergo irreversible loss of connectivity even when all existing barriers were made available as an offset option. The list of species that could not be offset and the cost of barrier removals increased as the number of new barriers simulated increased. The approach presented here could be used to plan offset actions for other types of impacts in freshwater systems or elsewhere, or to assess the vulnerability of particular species or processes to potential future impacts by identifying the boundaries of development that can be offset.