A spatial planning approach for the identification of critical habitat for threatened species.

The designation of critical habitat for the conservation of threatened species has long been recognized in the environmental legislation of different countries. However, translating vague legislation about critical habitat into practical real-world designation remains challenging because of its sensitivity to many context- and species-specific criteria and assumptions. We explored how spatial prioritization tools can help navigate such challenges and explicitly address sensitivities. Using a case study on the endangered little bustard (Tetrax tetrax) in Spain and the spatial prioritization tool Marxan, we tested and compared different critical habitat spatial designs across a series of scenarios for the little bustard at the national level. The scenarios accounted for habitat availability requirements over the species' annual cycle, the species' representativeness across the territory, the spatial connectivity of its habitat and populations, and potential cost constraints. This approach allowed us to quantify the sensitivity of critical habitat designations to how these criteria are quantified and integrated. Considering unoccupied habitat as critical habitat for the species generated larger, more spatially aggregated solutions that would likely be harder to implement than scenarios focusing conservation efforts on currently occupied habitat only. Considering the species' extirpation risks at individual planning units as a constraint to management success generated completely different solutions than scenarios assuming homogeneous extirpation risk across the landscape. The overall connectivity of identified critical habitats across the entire study area was double in scenarios that accounted for extirpation risk in individual planning units than that in scenarios that held extirpation risk constant across all units. Our approach, based on freely available software, can help guide conservation efforts by identifying new critical areas that maximize the effectiveness of conservation actions and can be used to assess the sensitivity and uncertainty of critical habitat designation to different criteria.