Including cross-system threats changed the selection of conservation priorities dramatically. The planning exercise that only included single-system threats identified 323 sites (161,500 ha) in the optimal or best Marxan solution that were at risk from cross-system threats. The difference between the sum solutions (outputs that incorporate within-scenario variability) shows that the total area avoided around the Columbia River plume by the cross-system threat scenario was much larger, encompassing 835,500 ha (1671 units). This was driven by the fact that 65{2a5d6d1706341671d74cd9e261e7084f344be5d0ac8e3cb469aaa53c623578a6} of the assessment units assigned with the highest ranking threats moved in space between the single and cross-system threat indices.
In addition to the spatial orientation of sites, the researchers also assessed target and spatial efficiency and the total optimization cost of each scenario. All three scenarios had similar target efficiency, meaning that regardless of the threat scenario assigned conservation goals were adequately met for each target. The no-threats scenario, however, had the highest spatial efficiency, selecting the fewest sites in the best solution. Spatial efficiency declined significantly when single-system threats were added, and again when cross-system threats were included, although the difference between single- and cross-system threat scenarios was relatively small. This efficiency pattern also held for total area in the best solution, with slightly less area in the single-system scenario (2,666,086 ha) than the cross-system scenario (2,669,569 ha).
The total cost of the best solution also changed among scenarios. The best solution from the cross-system threat scenario contained sites with the highest total cost, and as expected, sites chosen in the no-threat scenario had the lowest cost. The total cost of the cross-system threat best solution was also significantly higher than the single-system threat solution, but the relative difference was small.
Threats, Costs, and Best Solutions
Costs (a function of area and threat levels) of (a) system-specific and (b) cross-system threats in the Pacific Northwest coast ecoregion. Included in the cost scores are threats such as fishing in the marine realm, agriculture on land and agriculture as a marine threat.
Taken from Figure 3 in Tallis et al. (2008)
Best solutions from simultaneous MARXAN site-selection scenarios with (b) single-system threats, and (c) single-system and cross-system threats. Dark blue areas are sites chosen in the best MARXAN solution for each scenario. Many of the best sites chosen near the mouth of the Columbia River (red dot) were not chosen when cross-system threats were included.
Taken from Figure 4 in Tallis et al. (2008)
