Conservation planning across realms: Enhancing connectivity for multi‐realm species

Connectivity plays a key role in biodiversity conservation as it sustains ecological processes, such as migrations, important for the maintenance of populations. Connectivity is especially relevant for species that rely on different realms during their life cycle or use different realms daily or seasonally (multi‐realm species). However, efforts to address conservation across multiple realms have focused on identifying priority areas for conservation in a single realm (mostly marine) while accounting for threats propagating from other realms or single species needs. Here, we demonstrate how to identify priority areas for conservation across three different realms (freshwater–terrestrial, estuary and marine) for multiple species, including some multi‐realm species, that inhabit or move across the three realms, using the Tagus River (Western Iberian Peninsula), its estuary and nearby marine area as a case study. We integrated multiple types of connectivity in the selection of priority areas for conservation, each of them customised to depict important drivers of movement within each realm and across realms: longitudinal connectivity along rivers or multidimensional connectivity in the estuary and marine realms, guided by currents and depth similarity, respectively. We compared the allocation of priority areas and spatial connectivity achieved under three scenarios: no‐connectivity, within‐realm and cross‐realm connectivity scenarios. There were differences in the spatial allocation of priority areas across scenarios. The most remarkable difference laid on the connectivity achieved under each scenario, which experienced a threefold increase when considering connectivity across realms, compared to solutions that considered only connectivity within each realm independently. This improvement in connectivity was especially marked for species that occur across different realms. There were, however, trade‐offs between this improvement in connectivity and the increase in the number of planning units selected, especially in the estuary. Synthesis and applications. Addressing connectivity across realms deserves special attention when planning for the conservation of multi‐realm species to ensure the adequacy of conservation recommendations to respond to the needs of multi‐realm species. Given the potential trade‐offs between enhanced cross‐realm connectivity and total area needed or internal within‐realm connectivity, consideration of cross‐realm connectivity must be cautiously evaluated and integrated in multi‐realm conservation plans.