Addressing longitudinal connectivity in freshwater systematic conservation planning

Abstract

1. Freshwater conservation has received less attention than its terrestrial or marine counterparts, despite freshwater systems containing a considerable amount of the earth´s biodiversity. Given the accelerated rate of change and intensive human use that freshwater ecosystems are submitted to, it is urgent to devote some attention to them. The application of existing conservation planning tools - such as Marxan - to riverine planning needs some adaptations to account for the special nature of these systems. Connectivity plays a key role in freshwater ecosystems – threats are mediated along river corridors and the health of the entire catchmemt influences. This needs to be considered in conservation planning approaches. 2. The probability of occurrence, obtained from MARS-GLM models, of nine native freshwater fish species in a Mediterranean river basin was used as features to develop spatial conservation priorities. The priorities accounted for complementarity and spatial design issues. 3. To deal with the connected nature of rivers, we modified Marxan´s boundary length penalty, hence avoiding the selection of isolated planning units and forcing the inclusion of closer upstream areas. We introduced ‘virtual boundaries’ between non-headwater stream segments, and added distance-weighted penalties to the overall connectivity cost (CP) when stream segments upstream of the selected planning units are not selected. 4. This approach to prioritising connectivity rule is concordant with ecological theory, as it considers the natural and roughly exponential decay of upstream influences with distance. It allows accounting for the natural capacity of rivers to mitigate impacts when designing reserves. With a small emphasis on connectivity, Marxan prioritised natural corridors for longitudinal movements. In contrast, whole sub-basins were prioritised when connectivity was emphasized. Changing the relative emphasis on connectivity causes substantial changes in the spatial prioritisation; our conservation investment could move from one basin to another. 5. Our novel approach to dealing with directional connectivity enables managers in charge of freshwater systems to set ecologically meaningful spatial conservation priorities.