There is no specific formula to determine how much habitat or how many populations are required to conserve or manage any particular target. But there is some common sense advice:
- Conservation goals should ideally be based on historical estimates of the distribution of the targets. Many marine ecosystems and species have declined greatly. For example, if 50% of the historical salt marsh distribution is known to have been lost in a region, than goals based on current salt marsh distributions should be doubled. However, most historical information on marine target distribution does not exist, or is only available at discrete sites. In these cases representative goals are based on current distributions.
- Conservation goals have typically been set in the 20-40% range based on current ecosystem distributions. This range assumes that conservation of these amounts will ensure a minimally viable representation of all the species that utilize these ecosystems
- Goals for individual species and aggregations also range from 20-40% of current distribution, but can be elevated towards 100 percent for endemic, rare, or extremely vulnerable features (e.g. sea turtle nesting sites).
- It is useful to run analyses with multiple goal levels to examine how the results change under scenarios with different goals.
- Sensitivity analyses are often conducted where multiple goal levels are set to examine how site selection changes under various scenarios (see Outputs and Results under Site Selection).
- From a management perspective many ecosystems and species should have goals set to 100%. For the purposes of identifying site priorities that are useful towards implementation goals under 100% are set.
Case Study: Carolinian Ecoregion
In the Carolinian ecoregion, expert advice was sought on setting goals for all the targets. Most goals were set at 30 percent of their current distributions or higher, because many of the targets have been substantially reduced in abundance or degraded in terms of function or quality.
For the reddish egret, a 100 percent goal was set, because there is only one recorded egret rookery in the ecoregion.
Restoration goals were also set, because the current distribution of some targets was already insufficient for their continued persistence. For shellfish ecosystems, goals were set at 30 percent of their potential distributions (i.e., classified as supporting shellfish by EPA).
More importantly, goals were varied to explore how different goals affected the overall results in terms of the numbers and locations of priority sites. The two figures illustrate the differences in number of priority areas chosen in Marxan when goals are cut in half. In this case the relationship is linear (~50% fewer sites are chosen), but that is not always the case.