Regions are divided into subunits or strata to:
- Represent across the region within plans
- Represent unknown biodiversity (e.g., possible genetic variation in species or community level variation in ecosystems)
- Distribute sites to spread risk, i.e., avoid having local catastrophes affect all priority sites
- Create manageable units for data analysis
Types of Units
There are two types of analysis units: stratification and assessment units. Stratification units are subregions within the planning area, and assessment units are units that bin and aggregate all spatial information.
Stratification can be based on:
- Ecological criteria: e.g., depth, major currents, and salinity (e.g., estuarine/marine)
- Data quality and consistency: Where there are substantial differences in data quantity and quality (e.g. between states or countries ) it is reasonable to stratify by political boundaries. This form of stratification may also be helpful to partners who are more interested in their jurisdiction or programmatic boundaries (e.g. federal agency regions). In this regard setting up more than one stratification scheme may be necessary within the decision support system.
Assessment units are the smallest elements in which targets and the suitability factors are tracked. Attributing target, threat, and human use information to a finite set of assessment units makes subsequent decisions more manageable and reduces the relative complexity of the process.
There is no set planning unit size; it depends largely on the resolution of the data and the size of sites that will ultimately be ecologically meaningful in a given region. Consider the size of sites that are likely to be ecologically reasonable and ensure that at least several assessment units fit inside these areas (e.g., we may not want a single planning unit to cover an entire estuary).
- Smaller (and more) units are better when the data resolution will allow them.
- Hexagons are often useful because they have a consistent size and edge (boundary).
Case Study: Carolinian Ecoregion
Six subregions were identified and conservation goals had to be met not only overall but in every sub-region in which the target occurred. The first step was to establish northern, central and southern stratification units. Ecologically, the region’s estuaries can be classified into three broad types: the extensive, poorly flushed sounds of southeastern Virginia and North Carolina, the well-flushed bar-built and riverine estuaries of South Carolina and Georgia, and the poorly flushed bar-built estuaries of northeastern Florida. These stratification units follow well understood transition zones where different assemblages of species were found.
The ecoregion was then divided into inshore and offshore zones corresponding to the 50 meters isobath to account for variation in diversity across this gradient.
The ecoregion was divided into 11,903 hexagonal “assessment units,” each representing 1,500 hectares.